The ringed planet : Cassini's voyage of discovery at Saturn [Second edition.] 9781643277141, 1643277146

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Cassini’s voyage of discovery at Saturn Joshua Colwell On September 15, 2017, the Cassini spacecraft sent its final transmission to the Earth as it entered the atmosphere of Saturn, ending its historic 13 year mission at the ringed planet. This book is a beautifully illustrated journey of discovery through the Saturn system. Cassini’s instruments have revealed never seen before details, including the only extraterrestrial lakes known in the solar system, and have provided unprecedented views of the rings, moons, and the planet itself. Results from Cassini’s dramatic Grand Finale of ring-grazing and planet-skimming orbits are included in this expanded and updated second edition. Saturn is the jewel of the solar system. The Cassini spacecraft has been exploring the ringed planet and its moons and rings since 2004 and has helped us solve many of its mysteries while generating a wealth of new questions. Cassini has observed the bizarre mountains of Iapetus, the geysers of Enceladus, the lakes of Titan, and the dynamic and evolving rings. Along the way, this book explores and explains the fundamental processes that shape not just the Saturn system, but planets and moons in general. Written for the general audience with an emphasis on the fundamental physics of planetary systems, The Ringed Planet is a fascinating exploration of the Saturn system that places Saturn in the context of the solar system as a whole. Cassini’s instruments have revealed Enceladus and Titan to have subsurface oceans of liquid water. Its cameras have returned stunning images of rings in turmoil, a tumbling moon, the only extraterrestrial lakes known in the solar system, a hexagon of clouds, some of the highest mountains in the solar system and much more. More than a journey of discovery at Saturn, The Ringed Planet is also an introduction to how planetary systems work. About the Author Dr. Joshua Colwell is a Planetary Scientist and Professor of Physics at the University of Central Florida with a Ph.D. in Astrophysical, Planetary and Atmospheric Sciences from the University of Colorado. His research interests are in the origin and evolution of the solar system with a particular emphasis on planet formation, planetary rings and interplanetary dust. He is a Co-Investigator on the Ultraviolet Imaging Spectrograph on the Cassini mission. He produces and hosts the astronomy podcast “Walkabout the Galaxy.”

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The RInged Planet: Cassini’s voyage of discovery at Saturn, 2nd Edition • Joshua Colwell

The Ringed Planet

The Ringed Planet (Second Edition) Cassini’s voyage of discovery at Saturn

The Ringed Planet (Second Edition) Cassini’s voyage of discovery at Saturn Joshua Colwell University of Central Florida, Florida, USA

Morgan & Claypool Publishers

Copyright ª 2019 Morgan & Claypool Publishers All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher, or as expressly permitted by law or under terms agreed with the appropriate rights organization. Multiple copying is permitted in accordance with the terms of licences issued by the Copyright Licensing Agency, the Copyright Clearance Centre and other reproduction rights organizations. Rights & Permissions To obtain permission to re-use copyrighted material from Morgan & Claypool Publishers, please contact [email protected]. Media content for this book is available from https://iopscience.iop.org/book/978-1-64327-714-1. ISBN ISBN ISBN

978-1-64327-714-1 (ebook) 978-1-64327-711-0 (print) 978-1-64327-712-7 (mobi)

DOI 10.1088/2053-2571/ab3783 Version: 20191101 IOP Concise Physics ISSN 2053-2571 (online) ISSN 2054-7307 (print) A Morgan & Claypool publication as part of IOP Concise Physics Published by Morgan & Claypool Publishers, 1210 Fifth Avenue, Suite 250, San Rafael, CA, 94901, USA IOP Publishing, Temple Circus, Temple Way, Bristol BS1 6HG, UK

To my mother, my wife, and my daughter, who see me through.

Contents Preface to the second edition

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Acknowledgments

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Author biography

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Introduction

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2

Seeing Saturn

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Further reading

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Why are there moons and rings?

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Further reading References

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A menagerie of moons

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9

The inner mid-sized icy moons Impacts and craters Synchronous rotation Icy satellite tectonics Interactions with the magnetosphere The curious case of Hyperion Iapetus: the two-tone moon Enceladus: Saturn’s old faithful The fabulous five: Cassini’s parting look at five ring-moons Further reading References

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The rings—a dance of gravity

5.1 5.2 5.3 5.4 5.5 5.6 5.7

Introduction The main rings Self-gravity wakes Satellite wakes and shepherding Density waves and bending waves in the rings The B ring, ballistic transport, and spokes The C ring and Cassini Division

4-1 4-1 4-3 4-9 4-11 4-13 4-17 4-21 4-29 4-35 4-38 4-38 5-1

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5-33 5-37 5-38 5-38

5.8 5.9

The F ring The dusty rings Further reading References

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Divining the age of the rings

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6.1 6.2 6.3 6.4 6.5 6.6 6.7

The problem The spreading of the rings The mass of the rings The brightness of the rings The erosion of the rings The composition of the rings The age and origin of the rings Further reading References

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Titan: the planet moon

7.1 7.2 7.3

The atmosphere Lakes, dunes and mountains Titan’s interior Further reading References

7-2 7-7 7-14 7-17 7-17

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Saturn’s hidden mysteries

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8.1 8.2 8.3

Origin and interior Atmosphere Magnetic field References

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Future exploration of the Saturn system

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9.1 9.2 9.3 9.4

Ocean worlds Rings Saturn Other worlds Further reading References

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Preface to the second edition One of the wonderful things about the Saturn system is that there is so much happening in it that it can almost be used as a review of planetary sciences in general. And that is also the challenge in writing a book about the system. Like the Cassini mission itself, this book explores Saturn, its rings, moons, and magnetosphere. That’s a lot of material to cover in a limited space. I emphasize the physical processes that have shaped the Saturn system, using some of the most dramatic and intriguing discoveries made by Cassini at Saturn as case studies to illustrate these processes. The book is written for a general audience, and it is also appropriate as an introduction to planetary sciences for students. Cassini’s observations of Saturn, spanning more than 13 years, have revolutionized our understanding of the planet, its complex ring system, and many of its moons, especially Enceladus and Titan. I have always been interested in why things are the way they are, and there are many astounding observations made by Cassini that demand an answer to the question ‘why is that?’. We have answers to that question for many of Cassini’s discoveries, and this book covers a diverse set, from the origins of dusty ring particles to the heart of the giant planet itself. At the same time, there are new questions revealed by Cassini that will require more research and future missions. I emphasize links between the phenomena at Saturn and planetary systems in general. And still this only scratches the surface. There are many more intriguing phenomena and discoveries that I could not include without having the book expand to several times its current length. I hope the reader is inspired to learn more about this fascinating planet and its family of rings and moons. The images in this book are also just a sampling of those returned by Cassini to illustrate the physics described in the book and to whet the appetite of the reader to discover more at the public repositories of all Cassini data. Cassini’s mission at Saturn ended with two daring sequences of orbits, known as the Ring Grazing Orbits, in early 2017 and the final set of orbits, known as the Proximal Orbits, that threaded the gap between the planet and the rings. This Grand Finale concluded with the spacecraft entering the atmosphere of the planet on 15 September 2017, exactly one month shy of the 20th anniversary of its launch. These unique orbits provided new views of the planet and its rings. In the time since Cassini’s dramatic end, scientists have assembled the jigsaw puzzle-pieces of data collected over the course of more than a decade from virtually every instrument on the spacecraft to try to answer one of the biggest questions about Saturn: how old are its rings? While they appear to be relative newcomers to the solar system at under 100 million years, there are still some intriguing puzzles that may upend this conclusion. More surprises were in store for Cassini scientists when they analyzed the same data to understand Saturn’s interior and magnetic field. These are some of the many revelations from Cassini’s Grand Finale that have been added to this expanded second edition of The Ringed Planet. Each chapter has been revised and updated based on the last dramatic year of Cassini’s mission, and a new chapter devoted to the origin of the rings has been

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added. In addition to providing new data and information on what we have learned from Cassini’s final year, every chapter has been revised to include more background, context, and improved and simpler descriptions and explanations. The data from Cassini will be analyzed for many years, and our understanding of the planet will certainly deepen and evolve. But we are now at a point where the broad strokes of the tapestry of Saturn have been revealed by Cassini. The goal of this expanded second edition is to illuminate that tapestry and to provide a guide for the reader for further exploration and investigation of this jewel of the solar system.

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Acknowledgments I am one of the lucky few who has worked on the Cassini mission continuously for almost three decades, and I owe that opportunity to Larry Esposito, my dissertation advisor and Principal Investigator of the Cassini Ultraviolet Imaging Spectrograph. I thank my colleagues on the Cassini UVIS team and the project in general for making the mission not only a scientific success but a wonderful working environment all these years. I was privileged to work with and learn from Joe Burns, Sebastien Charnoz, Jeff Cuzzi, Cécile Ferrari, Dick French, Matt Hedman, Essam Marouf, Carl Murray, Phil Nicholson, Stu Pilorz, Carolyn Porco, Mark Showalter, Frank Spahn, Linda Spilker, Matt Tiscareno, and the rest of the Cassini Rings Working Group. I thank all those scientists whose work has provided the substance for this book. Several more books could be filled with stories of discoveries that time and space did not permit me to include here. I am grateful to my Cassini colleagues, who number in the hundreds, but especially my UVIS colleagues. In particular I thank Nicole Albers, Michael Aye, Laura Bloom, Todd Bradley, Shawn Brooks, Scott Edgington, Candy Hansen, Amanda Hendrix, Greg Holsclaw, Uwe Keller, Bill McClintock, Wayne Pryor, Trina Ray, Ralf Reulke, Don Shemansky, Miodrag Sremcevic, Glen Stewart, Ian Stewart, and Bob West for their assistance, insights, and camaraderie over the years. I owe a special debt to the many hard-working Cassini Science Planning Engineers who designed observation sequences, including my own observations, especially Brad Wallis and Kelly Perry. I have learned much from work with my own students: Kevin Baillié, Tracy Becker, Richard Jerousek, and Akbar Whizin who have embarked on their own voyages of discovery, and current students Stephanie Eckert and Melody Rachael Green. Thanks to Mark Lewis for his beautiful simulations of the rings, Joe Spitale for providing a mosaic of the A ring edge, and Richard Jerousek for his figure of selfgravity wakes in stellar occultation data. I received invaluable feedback from my mother, Ann Colwell, who let me see the material through the eyes of a non-scientist. I cannot imagine having gone on this journey without the unwavering support, encouragement, and love of my wife, Anne-Marie Colwell.

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Credit: Lauren Schoepfer / ©University of Central Florida.

Dr Joshua Colwell is a Planetary Scientist and Pegasus Professor of Physics at the University of Central Florida. His research interests are in the origin and evolution of the solar system, planet formation, planetary rings, asteroids and comets. He worked on the Cassini mission from 1990 through the end of mission in 2017, and continues to analyze data from it. His experiments on planet formation, asteroids, and planetary rings have flown on the Space Shuttle, the International Space Station, suborbital rockets, and parabolic airplane flights. He is an avid runner, writer, and performer, and produces and hosts the astronomy podcast ‘Walkabout the Galaxy’.

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Chapter 1 Introduction

Saturn is not the largest planet in the solar system, but its spectacular ring system sets it apart from the rest. While the other large planets (Jupiter, Uranus, and Neptune) have ring systems of their own, they pale, literally and figuratively, in comparison to the broad and complex expanse of the rings of Saturn. One of the most dramatic discoveries of Cassini is the likelihood that Saturn itself may have been in the punyrings club a few hundred million years ago. As with all scientific investigations, that discovery just raises more questions: what happened to give Saturn its rings, and why? But the rings are just the first of the many intriguing features of the Saturn system. The diversity of its moons and the interactions among them, and between the moons and rings and Saturn itself, provide case studies in virtually every aspect of planetary sciences, including geophysics, fluid physics, celestial dynamics, plasma physics, and cosmochemistry. Cassini’s discoveries of liquid water oceans beneath the surfaces of two of Saturn’s moons, Titan and Enceladus, bring the Saturn system into the field of exobiology as well. Might those oceans harbor some form of life? Missions are already being imagined and designed that would help us answer that question. The international Cassini mission to Saturn had as its goal to study all the phenomena and bodies in the Saturn system, and this book is possible due to the vast amount of data collected by Cassini and the work done by thousands of scientists to analyze and interpret its findings. I had the privilege to be one of those thousands who worked on the Cassini mission, and the good fortune to have been one of a much smaller group that actively worked on the mission every year for nearly three decades. I began working on the Cassini mission as a post-doctoral researcher at the University of Colorado in 1990 for my PhD advisor Professor Larry Esposito, who had just been selected to provide the Ultraviolet Imaging Spectrograph, or UVIS1,

1

NASA, an agency whose name itself is an acronym, is infamous for the proliferation of acronyms for its missions, instruments, projects, and programs. The Cassini mission had a dictionary of acronyms and abbreviations that was many pages long.

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Figure 1.1. The Cassini spacecraft, nearly in flight configuration. The radioactive isotope thermoelectric generators (RTGs) have not yet been installed. The gold wrapping is thermal insulation to help the spacecraft regulate the temperature of its computers and instruments. Image Credit: NASA/JPL-Caltech.

for the mission. At that point Cassini existed only as a design, and that design evolved significantly before it was built and eventually launched on October 15, 1997. My graduate research had focused on the ring systems of Uranus and Neptune using data from the Voyager 2 spacecraft, and now I had the opportunity to be involved in a NASA ‘flagship mission’ to the most stunning ring system known. NASA’s interplanetary missions include focused missions with more singular science goals and more modest budgets, such as the asteroid sample return mission OSIRISRex, and the Mercury orbiter MESSENGER2. A flagship mission comes along only every decade or two and has a broad scientific mandate and a large suite of instruments to meet that mandate. Cassini (figure 1.1) arrived at Saturn on 1 July 2004, and since that time we have had the privilege to explore strange new worlds, moons with oceans hidden in their interiors that might harbor life, a dynamic ring system with moonlets forming and fragmenting before our eyes, and at the center of it all a planet with storms larger than the Earth that form with surprising regularity. The six-and-a-half years of Cassini’s flight through interplanetary space to Saturn were anything but dull for those of us working on the mission. The original nominal mission for Cassini had a duration of four years, and every scientist and science group wanted to ensure that those four years provided the best measurements of each aspect of the Saturn system. This nominal mission was the duration of

2

See footnote 1.

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exploration that had had nominal funding approved, assuming that everything functioned well and there were no budgetary catastrophes back on Earth. Frequently, though, NASA spacecraft outlast their designed life expectancy and, pending scientific merit and available funds, get to continue operating in what is called an extended mission. NASA’s twin Mars rovers, Spirit and Opportunity, had nominal missions of only 90 days, but each successfully explored the surface of Mars for many years. Nevertheless, it would be foolish to count on an extended mission, so everyone involved in the project was dedicated to getting the best and most useful observations possible during those first four years. The project was organized into ‘discipline working groups’, or DWGs, of scientists, each one focusing on the five aspects of the Saturn system that Cassini was to explore: the planet, Saturn’s large moon Titan, the rings, the magnetosphere, and the remaining icy moons. Each DWG had representation from each of the 12 instruments on Cassini, and the Titan group also included representation from the Huygens Probe team managed by the European Space Agency (ESA). Each DWG was led by an interdisciplinary scientist independent of the instrument teams whose job was to make sure that no science related to that particular discipline fell through the cracks. In a model of international scientific cooperation, Cassini transported the ESA-built Huygens probe to the Saturn system where it was deployed for a dramatic descent through Titan’s atmosphere and a soft landing on its surface (chapter 6). In order for the mission to succeed, Cassini would have to perform observations of dozens of moons, explore a magnetosphere filling a volume of space larger than the Sun, study a ring system that nearly vanishes when viewed edge-on, map the surface of a moon completely shrouded by haze, and study the atmosphere of the second-largest planet in the solar system without the pesky rings getting in the way. Mission designers at NASA’s Jet Propulsion Laboratory (JPL) spent years developing a variety of ‘tours’, or trajectories, for Cassini to follow during its four-year mission. These tours were designed to try to meet the demands of each DWG, each of which had numerous and sometimes incompatible demands from their various instrument representatives. The first tour design I worked with as a member of the Rings Discipline Working Group, or RWG, was named 92–01: it was the first tour designed and distributed to scientists in 1992. Cassini scientists spent many hours drafting strawman timelines for observations using 92–01. At that time we knew that the ultimate tour that Cassini would fly would likely be quite different from 92–01, but going through an observation planning exercise would help us develop a workable process once the final tour was selected. What we did not know or suspect at that time was that the spacecraft itself was about to undergo a radical design change that would have huge consequences for the way observations could be made. Cassini was originally planned to have a large scan platform that would hold most of the scientific instruments on a movable arm that could point at anything while the spacecraft antenna pointed at the Earth to send and receive data. Budget cuts forced a redesign in which all instruments were firmly bolted to the body of the spacecraft. This descope of the mission meant that the entire spacecraft would have 1-3

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to act as the scan platform, and that remote-sensing observations (those made by the various cameras on board) could not be carried out at the same time that Cassini was sending data to the Earth3. This made the planning of Cassini’s observation timeline that much more difficult. Not only was the amount of time available for scientific observations reduced by about a third to enable 8–9 h data downlinks each day, but suddenly the amount of time it would take to rotate the massive spacecraft from pointing at one observation target to the next became a critical factor in timeline planning. My early responsibilities for UVIS, and later for the project overall, involved developing software that would be used to help design and plan observations by providing visualizations of the Saturn system from the point of view of Cassini. This software was useful in evaluating the observation possibilities provided by a tour. Years of tour design ensued, with mission designers discovering ever more inventive and clever ways of using gravitational assists from Titan to enable Cassini to explore as much of the Saturn system as possible in just four years. Cassini spent almost all of its time falling around Saturn, just as the International Space Station (ISS) falls a full revolution around the Earth every 90 min, what we call an orbit. This length of time is determined by the size of the orbit and the mass of the planet. Anything orbiting the Earth just above our atmosphere will take about an hour and a half to complete an orbit. Saturn is much more massive and much larger than the Earth, and visiting its distant moons while avoiding collisions with its rings means much larger and longer-duration orbits than that of the ISS around the Earth. Designing a tour that flies by the most distant moons, spends time in the equatorial plane to provide unobstructed views of Saturn, flies by Titan at different geometries to allow cloud-penetrating radar to map most of the surface, and flies high over the poles of Saturn to allow views of the rings is a highly overconstrained problem. It takes time to get to all of those places and vantage points, and many of them are in motion. Complicating matters, certain instruments had key observations that required special orbits to allow observations of the Sun or the Earth as they passed behind the atmospheres of Saturn, or Titan, or the rings that needed to be made at a certain season in Saturn’s year. And all of this needed to be done using essentially no fuel, but rather taking advantage of gravitational encounters with moons to slingshot the spacecraft from one orbit to the next. Imagine that you want to visit your country by car, see all the main tourist spots, making sure you get there when the temperatures are just right and the Sun is shining and that your drive by provides the best photo opportunities. You have a limited amount of time and, oh, you cannot refill your tank of gas so you are going to have to coast most of the time. Put this in three dimensions with a remotecontrolled spacecraft a billion miles away, and it’s an inkling of the challenge faced by Cassini’s tour designers. The tour designers did an amazing job. Ultimately the Cassini mission was extended to more than 13 years in orbit around Saturn and more than 270 orbits, capturing views of rare events from special vantage points. Each time a new tour 3 A sister mission, the Comet Rendezvous Asteroid Flyby, or CRAF, was canceled altogether due to the same budget restrictions.

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Figure 1.2. The path of Cassini in ‘tour’ 92–01 as seen from above Saturn’s north pole. The units on the x- and y-axes are in Saturn radii. One Saturn radius is about 60 330 km at the equator.

was produced, the DWGs produced spreadsheets filled with green, yellow, and red cells indicating how well each candidate tour fared in meeting the science goals for that group. The tour ultimately decided upon for the initial four-year nominal mission was nicknamed T18–5TDJ4 (figure 1.3), for the fifth variant of the 18th Titan-class Tour4. A glance at the paths that Cassini would follow in these two tours (figures 1.2 and 1.3) makes it immediately clear that the later tour, using clever tricks of celestial mechanics, was able to explore much more of the Saturn system. Once the tour was decided upon, years of observation design and sequencing could begin. The tour was broken up into smaller units of time, a few days long, and these segments were assigned to different teams based on the primary observing target for that period of time. The time near a Titan flyby was assigned to the Titan Orbiter5 Science Team, or ‘TOST’. Other groups were identified as ‘Target Working Teams’ or TWTs and referred to, believe it or not, as the ‘twits’. I was assigned by Larry Esposito to be the representative of the UVIS instrument on the ‘Rings Twit’. While their duties and obligations were technically distinct, the twits and their 4 The TDJ4 addendum stands for ‘tour du jour number 4’ indicating that this is the fourth version of T18–5. Variations on the main tour were small and designated by various ‘tweaks’. 5 Orbiter here refers to Cassini, and the term is used to distinguish it from the Huygens lander teams. In addition to TOST and the Rings TWT (written this way, but always referred to as the ‘rings twit’), the project had SOST (the Satellites Orbiter Science Team), the Mag TWT (‘mag twit’, studying the magnetosphere), the Atmospheres TWT, and the XD TWT, or ‘cross-discipline twit’. The cross-discipline twit handled time periods when Cassini was far from any object and multiple targets-of-opportunity presented themselves, cutting across several scientific disciplines. My friend and colleague at JPL, Kelly Perry, who steered the XD TWT with a famously strong hand at the helm, fashioned herself ‘the Dominatrix of Cross Discipline’.

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Figure 1.3(a). The colored lines show the path of Cassini viewed from above Saturn’s north pole in the selected tour T18–5TDJ4. The colors indicate different phases of the mission. The two yellow circles at the center are Saturn and the outer edge of the rings. The white dashed lines are the orbits of Titan (inner) and Iapetus (outer). The initial orbits are in white, and the final orbits in red.

corresponding DWGs largely overlapped. I was on the RWG and the RTWT. I also was a frequent participant in the XDTWT. The twits met weekly for several years, and later biweekly, almost exclusively by teleconference, to decide what observations would take place during each and every minute of the time assigned to a ring segment. An example of the observation sequence for a single flyby of the moon Enceladus is shown in figure 1.4. The final sequence included the observations that would take place during Cassini’s plunge into Saturn’s atmosphere on 15 September 2017. Because of the lack of a scan platform and the data demands of imaging, during that final plunge the spacecraft was oriented so that its large radio antenna was pointed at the Earth. In that orientation the remote sensing instruments (cameras and spectrographs) were pointed away from the planet, looking up through the thin wisps of Saturn’s upper atmosphere. The final data transmitted to Earth showed spectra of the composition of Saturn’s atmosphere at the site of Cassini’s demise. This brought to a conclusion one of the most successful and ambitious planetary missions in history and one that has spanned my entire professional life.

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Figure 1.3(b). The T18–5TDJ4 tour seen from Saturn’s equatorial plane. Cassini arrived from the south due to the season at Saturn (white curves). The green orbits were in the equatorial plane and appear as a line here, seen edge-on. The white orbits show Cassini’s arrival at Saturn from the south and its initial orbits. The kink in the first, largest, white orbit is due to Cassini firing its main engine to raise its point of closest approach to Saturn so that it would not impact the rings.

A book about the Saturn system could easily fill several volumes. Indeed, the number of refereed scientific papers published based on results from the Cassini– Huygens mission was well over 4000 by its end. Discoveries will continue to be made with the wealth of data returned from Cassini for many years to come. The next mission to Saturn was selected by NASA in summer 2019 to be a lander to explore Saturn’s largest moon, Titan (chapter 9). Cassini has returned the gold standard of observations from within the Saturn system on the rings, the planet, many of its dozens of moons, and its magnetosphere that is unlikely to be bettered for decades. The Saturn system is dynamic in many ways: the rings, moons, and planet have changed before our eyes during the dozen years of exploration by Cassini. While this book is loosely organized by object or class of object (moons, rings, planet), it is a book about why things are the way they are, rather than just what things look like. Such is the scale and diversity of the Saturn system that virtually every important physical process in planetary science is at work or has left its mark

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Figure 1.4. Illustration of the observation plan for the flyby of Enceladus on Cassini’s 61st orbit of Saturn. The different panels show the maneuvers of Cassini and the views of the different instruments responsible for observations during those times. Movie Credit: NASA/JPL-Caltech. Movie available at https://iopscience.iop. org/book/978-1-64327-714-1.

somewhere in it. Each object in the Saturn system illustrates the laws of physics at work in unusual environments with sometimes surprising consequences. While this is a book about the Saturn system, it is also a primer on many processes in planetary sciences. We are learning not just about Saturn and its family, but about the history of our solar system as well as the processes that shape planetary systems around other stars.

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Chapter 2 Seeing Saturn

Seeing Saturn through even a small telescope can be startling: a mundane point of light in the night sky, brighter than most stars but otherwise unexceptional, suddenly becomes an alien world with a broad, bright ring around it. Galileo Galilei was the first person to have this experience, in 1610. It is difficult to imagine what emotions and thoughts he must have had when he first saw the ringed planet. Living in an age where satellite views of the Earth and telescopic views of planets and distant galaxies are routine, we cannot put ourselves in the shoes of those who believed the Earth to be literally the only world in the Universe, surrounded by several slightly larger spheres1 upon which the stars and other points of light were painted the way a child’s bedroom ceiling is decorated with glow-in-the-dark stickers. First we must acknowledge and appreciate the curiosity that caused Galileo to obtain a telescope and point it at the sky. After all, it was believed that all those points of light in the sky were simply that: points of light. Galileo embodied the human imperative to imagine the possibilities of something beyond the known. At that point in time, the heavens were considered to be known and essentially immutable. Thus the appearance of objects such as comets, temporary interlopers in the night sky (and occasionally even visible in daylight), were thought to portent all manner of evil things. Galileo’s observations of spots on the Sun and points of light dancing around the planet Jupiter, itself with noticeable markings, marked the end of ancient astronomy, which was one and the same as astrology, and the beginning of a new era of scientific observations of the sky. From that point forward, objects in the sky could be studied in detail, new objects discovered, and the nature of the wandering points of light could begin to be explained. Over the next two years of observations through his low-grade telescopes, among the first to be pointed at any celestial object, Galileo saw two large blobs on either 1 The spherical shape of the Earth, contrary to popular belief in the USA, was known for nearly 2000 years before Columbus’s voyage from Spain to the Caribbean islands.

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Figure 2.1. Christiaan Huygens’ geometric model of Saturn’s rings, the first to correctly explain the odd appearance of Saturn through the telescopes of the first half of the 17th century. The Sun is marked by the asterisk at the center, and the orbits of the Earth (E) and Saturn are shown. With Saturn’s rotation axis tilted (not pointing straight up in this picture), the rings appear edge-on twice each Saturn orbit (at points B and D), and they present their maximum opening at points A and C. The Earth’s axis is also tilted with respect to its orbit, and this is why we have seasons: in the summer the Sun is more directly overhead, while six months later the opposite hemisphere is more directly illuminated. From Huygens’ Systema Saturnium, 1659.

side of Saturn, which resembled giant ears on the planet more than anything else, gradually shrink until they disappeared. This was further evidence of the changing nature of objects in the sky, but also something completely unprecedented. Galileo originally thought that Saturn was a triple-planet. Then, to his amazement, he saw the two smaller orbs shrink and disappear. Before he could question his sanity, the extensions eventually reappeared. With the improvement of telescopes, the Dutch astronomer and physicist Christiaan Huygens deduced the geometric nature of Saturn’s odd shape in 1656. It was common at the time for scientists to publish anagrams that encoded their discoveries prior to releasing a book containing a full explanation. Huygens announced in an anagram (in Latin) that Saturn’s odd appearance could be explained by a thin flat ring circling the planet but not in contact with it. The changing appearance (by this time the ring had disappeared four times at 15-year intervals) was due to a tilt of the axis of the ring with respect to Saturn’s orbit around the Sun (figure 2.1). Kepler, at about the same time that Galileo was making his revolutionary observations, had provided a robust and simple mathematical model for the motion of the planets around the Sun that essentially confirmed the heliocentric picture of the solar system first put forward by Nicolas Copernicus. Kepler’s model explained that Saturn’s 29.5-year orbital period was a consequence of its great distance from the Sun, on average about nine and a half times further from the Sun the Earth, or 9.55 AU.2 Huygens recognized that if Saturn’s rotation axis were tilted with respect to the plane of its orbit around the Sun, similar to the tilt of the Earth’s axis that gives rise to our seasons, the rings would at times appear to us edge-on, and at other times closer to face-on. Thus, the rings disappear from our vantage point on the Earth twice each Saturn orbit, at the Saturnian equinoxes (the equivalent of our starts to the spring and fall seasons) when they appear to us edge-on (figure 2.1). At 2

1 AU = 149 000 000 km is the mean distance of the Earth from the Sun.

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Saturn’s northern and southern solstices (the equivalent of the starts of the summer and winter seasons on Earth), its rings are at their most open aspect to the Earth. Because Saturn is so much further from the Sun than the Earth, if you’re at Saturn the direction to the Sun is close to the direction to the Earth. This was beautifully and dramatically illustrated in pictures taken by the Cassini spacecraft of the Earth (chapter 9). So Saturn’s seasonal changes correspond very closely in time to when the rings appear edge-on or more open in their aspect from our vantage point on the Earth. Because Saturn’s rotation axis, which is also the axis of the rings, is tilted 26.7° with respect to its orbit, we are able to see the rings from an edge-on perspective (as seen from the Earth) up to a 26.7° tilt towards the Earth over the course of a Saturn year. It’s interesting to note that if Saturn’s pole were aligned with its orbit, discovery of the rings would have required much more sophisticated telescopes than Galileo’s and would have happened much later. Similarly, from our Earthly vantage point, we can never see the rings tilted toward us by more than that 26.7°. Only by sending a spacecraft to the planet can we gain the new perspectives like those provided by Cassini. The progress in telescope technology since the 17th century led to the discovery of structure within the ring and several moons. Huygens discovered Saturn’s largest moon, Titan, in 1655. Four smaller moons (Tethys, Dione, Rhea, and Iapetus) were discovered shortly thereafter by Giovanni Domenico Cassini. Cassini also discovered a major gap in the ring system now know as the Cassini Division. It is for him that the international Cassini mission to Saturn, whose observations have rewritten our understanding of Saturn and its place in the solar system, is named. Mimas and Enceladus were discovered by William Herschel in 1789, and two smaller moons were discovered in the 19th century. Mimas and Enceladus are next-door neighbors at Saturn, and comparable in size, but the similarities end there. Mimas is just what we would expect of a small icy moon: its surface is literally saturated with craters, with one crater so large that its formation came just shy of destroying Mimas entirely. Enceladus, on the other hand, only slightly larger than Mimas, is one of only a handful of geologically active objects in the solar system. It spews water vapor from a subsurface ocean out of a system of vents near its south pole, resulting in the ephemeral E ring of Saturn. It was not until 1966 that a 10th satellite of Saturn, Janus, was discovered by taking advantage of the reduced light from the rings during a Saturn equinox (a ring-plane crossing like that shown in figure 2.1 at points B and D). Saturn’s retinue of more than 80 moons is covered in chapter 4. Titan, a world that would be identified as a planet if it were orbiting the Sun instead of Saturn, is discussed in detail in chapter 7. The first spacecraft to visit Saturn were the Pioneer 11 and Voyager 1 and 2 explorers of the outer solar system in 1979, 1980, and 1981, respectively. These visits led to the discovery of details in the rings down to the resolution limit of the cameras and several more small moons. While the rings had previously been assumed to be essentially featureless, uniform disks of material leftover from the era of planet formation, the Voyager images showed the rings to be full of unexplained structure. The idea that the rings may be evolving and perhaps much younger than the solar system began to be taken seriously. The question of the age of the rings appears to 2-3

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Figure 2.2. The Jovian ring, seen in forward-scattered sunlight in a mosaic of images taken by the Galileo spacecraft on November 9, 1996, at a distance of 2.3 million km, when it was in orbit around Jupiter. The Sun is behind the disk of Jupiter, so only light shining through the upper atmosphere of the planet can be seen, as well as the dusty rings which are best observed in this geometry due to the small size of the particles (~1 micrometer) and the tenuous nature of the rings. From our vantage on the Earth, the rings are so transparent as to render them virtually invisible. A schematic of the rings and nearby moons is at lower right. Image Credit: NASA/JPL-Caltech/University of Arizona.

Figure 2.3. Saturn and its rings from Cassini, on 25 April 2016, from a distance of 3 million km and an elevation of 30° north of the ring plane. Saturn’s north pole is marked by a hexagonal feature in the clouds. The dark, apparently empty gap outside the bright B ring is the Cassini Division, and is in fact far from empty: it alone has more ring material than orbits any of the other planets. The rings cast a shadow on the planet near the five o’clock position, and Saturn returns the favor, casting its own shadow on the rings that varies over the course of its 29.5-year journey around the Sun. Image Credit: NASA/JPL-Caltech/SSI.

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Figure 2.4. The rings of Uranus were detected from Earth-based observations of a stellar occultation in 1977, in which the brightness of a star is measured as it passes behind another object. Nine narrow rings were detected this way as narrow dips in the star brightness on either side of Uranus, before Voyager 2 encountered Uranus in January 1986, providing the first images. Here the rings are seen in infrared images taken at the Keck telescope in July 2004 using adaptive optics. The outermost ε ring is most prominent, and is only about 50 km in width. Image Credit: Lawrence Sromovsky, Univ. Wisconsin-Madison, W M Keck Observatory.

have been settled by Cassini, and it took data collected by many instruments over the full 13-year lifteime of the mission to resolve the question3. By the time Cassini arrived at Saturn in 2004, 31 moons had been discovered, most of them of the pipsqueak variety (less than 50 km in diameter). Dedicated observing campaigns from Earth together with Cassini’s cameras have doubled that count, with all of the new moons falling in the tiny moonlet category. Indeed, as we will see, many of the objects discovered by Cassini blur the line between moon and ring particle. Other advances in observational astronomy led to the discovery of rings around Uranus in the 1970s and around Neptune in the 1980s. Jupiter’s ring was discovered in images taken by the Voyager 1 spacecraft in 1979. The ring systems of these other planets, while fascinating in their own right, literally pale in comparison to the broad expanse of Saturn’s rings (figures 2.2–2.5). As we will see in chapters 5 and 6, however, determining just how much material there is in a planetary ring is not straightforward, and knowing the mass of the rings, not just their surace area, is a critical piece of information for understanding their origin and evolution. While Saturn’s rings outshine those of its nearest planetary neighbor, the Saturnian system of moons is oddly deficient compared to Jupiter’s. The four Galilean satellites 3 Spoiler alert: the preponderance of evidence points to the rings being less than 100 million years old, far younger than Saturn itself.

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Figure 2.5. Voyager 2 acquired this image as it encountered Neptune in August of 1989 looking back toward the Sun to accentuate the appearance of the narrow dusty rings, similar to the viewing geometry of figure 2.2. The overexposed limb of Neptune is visible at lower right. In Neptune’s outermost ring, material mysteriously clumps into three arcs. Earth-based images taken since the Voyager 2 flyby show that the arcs have evolved and rearranged their orientation. Image credit: NASA/JPL-Caltech/Univ. of Arizona.

of Jupiter, Io, Europa, Ganymede, and Callisto, comprise an orderly set of worlds that makes the Jupiter system look like a miniature solar system. Io, the innermost of Jupiter’s moons, is the densest, while the most distant, Callisto, is the iciest and least dense, a trend in composition that mirrors the trend seen in the inner planets of the solar system. Mercury, the innermost planet, is iron rich, while the Earth is less dense, with more rock compared to iron, and a modest complement of water and other molecules (nitrogen in the atmosphere, and CO2, for example). These are too lightweight to be bound to a small planet such as Mercury at the high temperatures close to the Sun. When Jupiter and Saturn formed, temperatures in their neonatal system of moons and rings would also have been higher close to the central body (a planet in this case, rather than the Sun) so it is not surprising to see ice-poor Io close to Jupiter and ice-rich Callisto further away. Saturn, however, unlike Jupiter, has only a single giant moon, Titan. Titan is second only to Ganymede in size, and the largest in the solar system if one measures to its cloudtops instead of the solid surface. Aside from Titan, the remaining moons of Saturn are middling in size, at best. The differences between the two satellite systems hold clues to both the conditions when the planets were forming and also the interactions between moons and rings. One thing is certain, however, and that is that neither Jupiter nor Saturn formed in isolation. The individual peculiarities of each system are clues to the formation of the planets, 2-6

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reshuffled through billions of years of dynamical and geochemical evolution, but detailed and distinct enough to place strong constraints on the story of the formation of our solar system. The Cassini spacecraft (figure 2.6), which arrived at Saturn in the summer of 2004, collected evidence from this scene of planetary origins through nearly half of a Saturn year. When it was launched Cassini was the largest spacecraft ever sent to the outer solar system. It was designed to explore all aspects of the Saturn system, including the rings, moons, and magnetosphere, as well as the planet itself. To carry out this exploration, Cassini was equipped with a dozen scientific instruments to see Saturn and its environs from very short wavelength extreme ultraviolet light through the visible and infrared to radio waves with wavelengths as long as 13 cm. Cassini carried the Huygens probe (figure 2.7), mounted on its side, to be deployed once they arrived at Saturn for Huygens to descend through the atmosphere of Saturn’s largest moon, Titan. The Huygens probe was built and managed by the ESA, and Cassini was built and managed by NASA. There was international participation in the science instruments and investigations for both spacecraft. Huygens carried another

Figure 2.6. The Cassini spacecraft is the largest interplanetary probe to date, measuring 6.8 m in length. The large cone-shaped object on the right is the protective atmospheric aeroshell for the European-managed Huygens probe. The 4 m dish antenna is both a communications device and a scientific instrument in its own right. Some of the optical cameras are visible at the left. Image Credit: NASA/JPL-Caltech.

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Figure 2.7. The Huygens probe undergoes inspections in this image. The can-shaped probe is visible on the right, with the heat shield attached. The backshell (left) is detached to enable access to the top of the probe. Image Credit: European Space Agency.

half dozen instruments to the surface of Titan, but its primary mission was to study the atmosphere as it descended. Cassini’s four optical remote sensing cameras (figure 2.8) captured the ultraviolet glow of Saturn’s aurora and the feeble thermal emission from ‘warm’ spots on Enceladus that are nearly 200 °C below zero. The main imaging system recorded stunning views of alien vistas at unprecendented detail, revealing the knobby southpolar terrain of Enceladus and mile-high snowpiles at the edge of the B ring. Its radar penetrated the clouds and haze of Titan’s thick atmosphere to catch the glint off the surface of its methane lakes. It had detectors to measure the charged particles, atoms, and dust particles as it flies through the system, hear the radio emission from lightning on Saturn, and literally sniff the plumes of Enceladus and taste the dust of the E ring. Originally planned for a four-year mission at Saturn, the wealth of information returned by Cassini and the excellent performance of the spacecraft and its scientific instruments enabled it to continue exploring the ringed planet for nearly two full Saturn seasons. Its demise on 15 September 2017, exactly one month shy of its 20th anniversary in space, and the dramatic series of orbits leading up to its final fateful plunge, yielded critical information about Saturn, its interior, and its rings. This Grand Finale is discussed in chapter 9. The Cassini mission has been an inspiring story of international scientific collaboration, with participation from 17 countries to answer some of the fundamental questions about our solar system. Amazingly, rings that were once thought to be static ruins leftover from Saturn’s formation 4.5 billion years ago have changed before our eyes. Moons swap orbits and launch waves that march across the rings, 2-8

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Figure 2.8. This computer rendering shows the remote sensing pallet of Cassini where the four optical instrument systems were mounted. The narrow angle camera (or NAC, which provides the highest resolution images) and wide angle camera (WAC) are part of the Imaging Science Subsystem. Below the WAC is the Composite Infrared Spectrometer (CIRS), and above the NAC is the Visible and Infrared Mapping Spectrometer (VIMS). The Ultraviolet Imaging Spectrograph (UVIS) is mounted to the inside of the L-shaped pallet, between the ISS cameras and the main spacecraft body. Image Credit: NASA/JPL-Caltech.

while elsewhere moons form and break apart within the rings themselves. Cassini has discovered new moonlets that may be nothing more than sandpiles in space, as well as geysers erupting from Enceladus, a moon roughly the size of the state of Colorado. Lakes and clouds on Titan come and go with the seasons, but are composed of methane rather than water. Storms flare up in the planet’s atmosphere, and ripples in the rings hint at massive invisible storms or other anomalies deep within the planet. Cassini’s Grand Finale orbits showed more surprises about the deep interior of the planet, distinguishing it from Jupiter’s more orderly struture. Even the rotation of the planet itself seems to change, an illusion caused by variations in its magnetic field. This dynamic system, still evolving after billions of years, is a keystone in the architecture of our solar system, and its story, dramatically unveiled by observations from Cassini, helps us discover our own history and understand the formation of planets throughout the Galaxy.

Further reading Following the conclusion of Cassini’s initial four year mission at Saturn two volumes of review articles on the Saturn system and on Titan were produced (Dougherty et al 2009, Brown et al 2009). These are comprehensive technical overviews of the system as of 2009. Detailed reviews of the instruments on the Cassini orbiter and the Huygens Titan probe were published in the journal Space 2-9

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Science Reviews and collected in three volumes in 2003–2004 (Russell 2003, 2004a, 2004b).

References Brown R, Lebreton J-P and Waite H J (ed) 2009 Titan from Cassini–Huygens (Berlin: Springer) Dougherty M, Esposito L and Krimigis S (ed) 2009 Saturn from Cassini–Huygens (Berlin: Springer) Russell C T (ed) 2003 The Cassini–Huygens mission: overview, objectives and Huygens instrumentarium Space Sci. Rev. 104 1–640 Russell C T (ed) 2004a The Cassini–Huygens mission: orbiter in situ investigations Space Sci. Rev. 114 1–518 Russell C T (ed) 2004b The Cassini–Huygens mission: orbiter remote sensing investigations Space Sci. Rev. 115 1–497

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IOP Concise Physics

The Ringed Planet (Second Edition) Cassini’s voyage of discovery at Saturn Joshua Colwell

Chapter 3 Why are there moons and rings?

In order to understand the origin of Saturn’s moons and rings we must first explore why rings continue to exist at all. If we could get close enough to Saturn’s rings we would be able to see individual ring particles. We believe these would look something like snowballs, ranging in size from about a centimeter, or the size of a small marble, to a few meters across. In some locations these particles would be clustered together, forming aggregates that could stretch for more than 100 m in length. For the moment, though, such an observation remains tantalizingly just beyond our grasp. Larger clumps, whose origins are not as well understood, have been seen by Cassini and are discussed in chapter 5. Nevertheless, we have learned a lot about the sizes and compositions of these particles from their collective behavior. Each ring particle is, in a sense, a moonlet of Saturn. A ring particle orbits Saturn just as a moon does, with two important distinctions: they are typically much smaller (less than 10 m) than anything we would identify as a moon, and they are orbiting within a crowd of other ring particles that are frequently jostling against them, sometimes even sticking together for prolonged periods of time. There are, however, at least dozens of objects within the rings that, were they not, we would likely designate as moons. And there are small moons that, if we placed them within the rings, we might identify as anomalously large ring particles. In this chapter we will see that the reason for the existence of rings has to do with their proximity to Saturn. Rings exist close to the planet while further away we find individual, isolated moons. There is a transition region from rings to moons where both co-exist. Saturn’s complicated F ring is bordered on either side by the small moons Prometheus and Pandora. The gravitational pull between ring particles tries to make them accrete, or stick together. In the absence of any competing processes this gravitational pull would result in the rings accreting into a handful of moons. But, close to Saturn, the effects of the gravitational pull from Saturn itself inhibit this accretion as explained below. We begin with a brief survey of the many objects

doi:10.1088/2053-2571/ab3783ch3

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orbiting Saturn, some within the rings, some quite distant, and some themselves the sources of faint rings. Taking a census of these objects, there are 53 objects orbiting Saturn that have been given names as moons of Saturn, from Aegaeon, a scant 250 m in diameter, which orbits within (and contributes to) the ghostly G ring (figure 3.1), to Ymir, a small distant moon on a retrograde (backwards) orbit. Nine additional objects have not been officially named by the International Astronomical Union, but eight of these nine, like Ymir, are small objects orbiting far from Saturn on retrograde orbits. The planets and most moons in the solar system travel in the same direction in their orbital motions, and with few exceptions they rotate in that same direction. If we could take a journey in a spaceship far out of the plane of the planets’ orbits and look back at the solar system from far above the Earth’s north pole, we would see the rotation of the Earth, and the orbital motion of most moons, all planets, and all asteroids to be in the counterclockwise direction. This sense of orbital motion and rotation is called prograde. It is a relic of the original motions of all the gas and dust in the cloud that collapsed to form the solar system. Retrograde moons orbit in the opposite sense (clockwise, as viewed from above Saturn’s north pole). This indicates that they were captured by Saturn from interplanetary space and that they were not part of the original disk of material that Saturn formed from. Moons can also be captured into prograde orbits, and indeed some of Saturn’s moons seem to fit this description as well. While their orbits are prograde, their orbits are highly tilted relative to Saturn’s equatorial plane (also the plane of the rings) and tend to be more oval and less circular. These moons come in groups that share similar orbits, suggesting that they are fragments from a single

Figure 3.1. These three images taken at 5 min intervals by Cassini on 11 June 2004, show the faint G ring and the tiny moon Aegeon. The background streaks are stars, blurred by the long exposure necessary to capture the faint ring. Micrometeoroid impacts onto Aegeon knock dust off the surface which eventually spreads into the G ring around Saturn. Image Credit: NASA/JPL-Caltech/SSI.

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Figure 3.2. Cassini image taken on 26 July 2009, of the outer edge of Saturn’s B ring showing an object (bright point near center) casting a shadow on the rings. From the extent of the shadow (36 km) and the position of the Sun, S/2009 S1, as it is provisionally known, is protruding 150 m above the rings (and presumably below it as well), but the width of the shadow indicates it is much wider than it is tall. This non-spherical shape is what would be expected for an object that accreted within the rings. The mottled appearance at the edge of the ring is due to clumping of ring particles. The two bright spots in the Huygens gap to the right of the ring edge are background stars, and the narrow band at the right edge of the image is the Huygens ringlet. Image Credit: NASA/JPL-Caltech/SSI.

object that was broken apart by a collision after being captured into orbit around Saturn. There are three such collisional groupings of moons at Saturn, which comprise virtually all of Saturn’s irregular satellites. The ninth provisional moon of Saturn is not on a retrograde orbit, but it is an object within the rings themselves (figure 3.2). Due to its location and its small size (less than 200 m out of the plane of the rings), it is unlikely it will be observed from the ground, so confirmation of its continued existence will await some future mission to Saturn. The captured moons, like the two moons of Mars, Phobos and Deimos, are asteroids or comets that formed elsewhere in the solar system along with all the other small objects that eventually became parts of the planets themselves. Over time their orbits evolved primarily due to gravitational perturbations from the planets. When an object orbiting the Sun passes close enough to a planet, that planet’s gravity can temporarily overwhelm the gravitational pull from the Sun and alter its orbit. Even at great distances, small gravitational perturbations from a planet can build up over time, adding to each other orbit after orbit, and result in large changes in an object’s orbit. The asteroid belt, for instance, is heavily sculpted by the effects of Jupiter’s gravity even at a distance of 2–3 AU.1 If an asteroid or comet comes close enough to a planet, it may become captured into a new orbit around it. Frequently, but not always, these objects are on retrograde orbits, and they are almost always on very 1

The Earth’s average distance from the Sun is 1 AU, Jupiter’s is 5.2 AU, and Saturn’s is 9.55 AU.

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Figure 3.3. Saturn’s largest ‘irregular’ satellite, Phoebe, was visited once by Cassini on 11 June 2004. left: arrival view. Right: departure view. The white streaks near the rim of the large crater are evidence for landslides of material toward the crater floor, exposing brighter icy material. Image Credit: NASA/JPL-Caltech/SSI.

distant orbits from the planets. Although most of Saturn’s captured moons are small (all but one is less than 40 km across), they dominate the planet’s moon tally. Fully 58 of Saturn’s 82 moons, including eight of the provisional ones, are irregular, captured satellites. Phoebe, at over 200 km in diameter, is by far the largest of the captured moons (figure 3.3). Moons that orbit on nearly circular orbits in the prograde direction and in the same plane as the rings are called regular satellites. Jupiter and Saturn have many captured moons in part because they are much further from the Sun than the Earth, and also because their gravitational pull is much greater than the Earth’s. At greater distances from the Sun, the Sun’s gravitational pull is weaker, so there are large regions of space around Jupiter and Saturn where the planets’ gravity dominates the Sun’s. These regions are called the planets’ Hill or Roche spheres after the two astronomers who initially defined their sizes. With its smaller mass and much closer proximity to the Sun, the Earth has a harder time capturing moons than the giant planets. Although heavily cratered, Phoebe is nearly spherical in shape with dimensions of 219 by 204 km. This is a bit smaller than the largest size a natural moon can be without its gravity forcing it into a spherical shape. Rocky or icy objects with typical densities of ice and rock will naturally deform into a spherical shape2 under the influence of their own gravity when they are larger than about 300 km in diameter. Phoebe’s nearly spherical shape may thus be partly due to chance, though it may have been somewhat larger (and more spherical) when it first formed. Like most of 2 More generally, bodies deform into an ellipsoidal shape due to the effects of rotation which tend to flatten objects, and tidal forces, discussed below, which tend to stretch them.

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the irregular satellites, Phoebe is on a retrograde orbit around Saturn. It was likely captured by Saturn’s gravity after migrating inward from the Kuiper Belt, the region of the solar system beyond the orbit of Neptune3. In addition to its orbit pointing to a capture origin for Phoebe, its surface characteristics are quite distinct from those of the bright inner moons of Saturn which are predominantly water ice (chapter 4). The reflectivity of the surface is on average only about 6%, about the same value for black acrylic paint or fresh asphalt. We would perceive Phoebe with the naked eye as black, but some craters reveal ice beneath the dark surface deposits (figure 3.3). Cassini measurements show the presence of CO2 ice on the surface of Phoebe which points to an origin in the cold, outer solar system rather than in the asteroid belt, for example, where temperatures are too high for CO2 ice to form and remain for billions of years. Some of the impacts that formed these large craters, in addition to deforming the shape of the moon, may have knocked chunks of material off Phoebe that we now see as small moons of Saturn on orbits similar to that of Phoebe. Two other moons have been observed within the rings: Pan, discovered by Mark Showalter in images taken by Voyager 2, and Daphnis, discovered in images taken by Cassini cameras (figure 3.4). Each of these moons creates a gap in the rings, and both gaps are in the outer portion of the main ring system. As we will see in chapter 5, the interactions between moons and ring particles have some surprising and counter-intuitive results, one of which is that a moon’s gravity effectively repels, rather than attracts, the particles. In this way moons such as Pan and Daphnis open up and maintain gaps in the rings (figure 3.5). In spite of extensive imaging campaigns by Cassini to locate moons in other gaps in the ring system, none has been discovered. Since nature abhors a vacuum, gaps in the rings require some explanation: over time we would expect the ring particles to gradually diffuse into the gap, filling it in. There are many more objects in the rings that are too small to open full gaps, but leave characteristic propeller-shaped holes in the rings. These so-called propeller objects may be fragments of larger moonlets, or they may be large collections of ring particles that managed stick together to form mini-moonlets (figure 3.6). This brings us to the question of how moons form: why sometimes do particles stick together to form a larger object, eventually accreting enough material to be a moon, alone in its orbit, while the countless particles of Saturn’s rings, colliding thousands of times each year for millions of years, stubbornly refuse to accrete into moons? The gravitational force that would bind meter-sized ring particles to each other is incredibly weak: the weight of one such ring particle lying on another is about the same as the weight of a post-it note on the Earth. If those particles were alone in the Universe, even that feeble force would be enough to bind them together. But the ring particles are not alone. They orbit Saturn at close proximity, and particles in smaller orbits travel faster than those in larger orbits as a natural consequence of how the strength of gravity between two objects depends on their separation. This is why Saturn, which is almost 9.6 times further from the Sun than 3 Pluto is the most famous denizen of the Kuiper Belt, and its orbital location within that larger population is the source of the debate about whether or not it should be considered a ‘planet’.

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Figure 3.4. The moon Daphnis, upper left, has a mean radius of 3.8 km and is orbiting in the Keeler Gap, just interior to the outer edge of Saturn’s A ring (at lower left). The gravitational influence of Daphnis on the ring particles orbiting Saturn on either side of the Keeler Gap results in a wavy pattern. The waves are asymmetrical around Daphnis’s position because the particles closer to Saturn (upper right) are orbiting faster than Daphnis and have just lapped the moon and are showing the effects of its gravitational perturbations. Meanwhile, particles at the outer edge of the Keeler Gap and to the lower right of Daphnis were last lapped by Daphnis many orbits ago, so those perturbations have been damped out by collisions between the ring particles. Above and to the left of Daphnis, the effects of its gravity can be seen in wispy, wavy features as Daphnis has just lapped those particles. See also chapter 5. While there are more than a dozen gaps in Saturn’s rings, Daphnis is one of only two moons detected in any of these gaps. Image Credit: NASA/JPL-Caltech/SSI.

the Earth, takes almost 30 years to orbit the Sun4. Two neighboring ring particles orbit Saturn at slightly different speeds, and that difference is enough to overcome that featherweight, or rather post-it-note-weight, gravitational attraction between the particles. Instead of sticking together, they slowly drift apart. The reason that there are both rings and moons at Saturn and the other giant planets is that this difference in orbital speed is larger when the orbits are closer to Saturn. At a large enough distance from Saturn the difference in orbital speeds becomes small enough that the gravitational force between particles can overcome it, and the particles stick to each other. Thus, we see moons far from a planet, and rings close to it. At Saturn, the outer reaches of the rings are at the transition where accretion can take place, and we see a mix of rings and moons (figures 3.4 and 3.6).

4 If it orbited at the same speed as the Earth, it would take 9.6 years to orbit the Sun because its orbit is 9.6 times larger.

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Figure 3.5. This striking image of the outer edge of Saturn’s A ring shows the moon Pan in the center of the Encke gap, casting a shadow on the neighboring ring material (the thin dark line extending to the right of the moon). This image was taken by Cassini on 2 May 2009, a few months before the rings were edge-on to the Sun. The shadow cast by Pan here is like the long shadows cast just before sunset on the Earth. When the shadow is much larger than the object, it makes it easier to measure the dimension of the object from precise knowledge of the geometry of the image. Image Credit: NASA/JPL-Caltech/SSI.

Figure 3.6. This image shows a disturbance in the rings caused by a small moonlet orbiting within the rings, invisible at the resolution of this image. The object at the center of the propeller-shaped enhancement in ring brightness at the center of the image is at most a few hundred meters across, large enough to clear away ring particles from its immediate vicinity, but not large enough to open a complete gap, like Daphnis does in the Keeler Gap (figure 3.4). Image Credit: NASA/JPL-Caltech/SSI.

The region close to a planet (or star) where accretion is inhibited is called the Roche zone, and its precise size depends on the density of the particles and their relative sizes, but it is usually about 2.3 times the size of the planet (figure 3.7). With the exception of faint, tenuous rings made up of dust particles, all planetary rings lie within their planet’s Roche zone. The difference in orbital speeds for objects in different orbits is related to what produces tides on the Earth: they are both the result of how the force of gravity between two objects diminishes as the distance between them increases. This difference in the strength of gravity between two objects across the size of the objects is called the tidal force. The Earth’s oceans, being large bodies of liquid, are distorted by a small amount by this tidal force from

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Figure 3.7. Far from a planet, the tidal force is weak, and a moon or comet is not distorted much because the gravitational force exerted on the moon varies by only a small amount across it. When the separation between the objects is smaller, the difference in the gravitational force across the moon is larger, and there is a larger tidal distortion of it. Within a certain distance of the planet, the Roche limit, an object with no internal tensile strength will break apart.

the moon, and as the Earth rotates beneath the moon (what we perceive as the moon rising and setting), the bulge in the shape of the ocean slowly washes up and down the beach, giving us high and low tide. We ourselves, however, also live within the Earth’s Roche zone, and not only are we not torn apart by the tidal force, we do not even perceive it. It is important only when compared to the even weaker gravitational force between two small objects, but it is negligible compared to the molecular forces that hold our bodies together. Place two boulders side by side on the surface of the Earth and they will happily stay there, held in place by the combination of the Earth’s gravity and the force of the ground pushing up on them. But place those two boulders in orbit, so that gravity – between the boulders and the Earth and between the boulders themselves – is the only force acting on them, and they will gradually drift apart in their orbits, with the inner one eventually circling around the Earth to lap the outer one. Thus, moons can exist inside the Roche zone if they are solid objects; it’s just difficult for them to form there. But take a moon that formed outside the Roche zone and move it inward and, unless it is either very large or structurally very weak (or both), it will remain a moon inside the Roche zone. Conversely, take a bunch of ring particles within the Roche zone and have them move outward. As the tidal force weakens dramatically with increasing distance from Saturn, these outward drifting ring particles could begin to clump together due to their gravity and grow into a moon. This very process has been proposed as a possible formation method for many of Saturn’s inner moons (Charnoz et al 2010). Rings and moons are intimately related to each other, with each perhaps serving as progenitors of the other.

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This leads to two basic ideas for how the rings can form: they are left over from the formation of the planet itself, or they are the debris left over from the destruction of some large object such as a moon or a wayward comet that found its way inside the Roche zone. Both hypotheses present serious challenges for the case of Saturn’s rings due to their sheer size. Pictures of the rings show how much area they cover, but tell us little about how massive they are, just as a distant picture of a large, opaque sheet could be a picture of tissue paper or of a 10 m thick slab of lead. Pictures show us the surface, like the façades of buildings on a movie set, without revealing how much substance lies beneath. Fortunately, we have other means of estimating the masses of rings, and while there is still considerable uncertainty, the mass of Saturn’s ring system is likely at least 1000 times more than the mass of all the other rings in the solar system combined. And this has made explaining their existence roughly 1000 times more complicated. The small amounts of material in the ring systems of the other planets can be accounted for by the fragmentation of small moons when hit by a comet, for example, or the gradual erosion of moons by the constant hail of micrometeoroids in interplanetary space. Indeed, the latter process is what produces the G ring of Saturn (figure 3.1), but this ring is just a wispy companion to the massive, bright main rings. The rates of these impacts and the number and sizes of moons available fit nicely with the rings observed at Uranus, Neptune, and Jupiter. But to create Saturn’s giant ring system, a moon roughly half the mass of Saturn’s moon Mimas (figure 3.8) must have been completely fragmented into ring particles, and since nothing is ever 100% efficient, the progenitor must have been larger still. And since Saturn’s ring particles are almost pure water ice, the progenitor must have been as well, or only the outer icy layers of an even larger object with a rocky core were stripped off to form the rings. But, as we’ve seen with the census of Saturn’s many moons, larger objects are less common than small objects. Furthermore, they are more difficult to break up through catastrophic impact. Although they present a larger target area to interloping comets that might smash them to smithereens, their greater size also results in a greater gravitational binding energy: all that mass is holding the thing together even more powerfully than the physical strength of the ice and rock it is made of. And the creation of the rings by the tidal disruption and capture of a comet would require an unusually large comet. Calculations and numerical simulations of these processes indicate that it is very unlikely to have occurred after the era of planet formation itself ended some 4.5 billion years ago. There is a model, dubbed the ‘Nice model’ after the observatory in Nice, France (though it is also a nice model), that posits a major shake-up of the solar system, leading to a large increase in the number of major impacts about 3.8–3.9 billion years ago. This epoch, sometimes called the Late Heavy Bombardment, or LHB, could have produced the impacts or capture scenarios necessary to create Saturn’s rings after the era of planet formation. After that time, however, it becomes increasingly difficult to create a scenario for the formation of Saturn’s rings that doesn’t require a very unlikely event, and even the existence of the LHB itself remains controversial. The difficulty of forming the rings after the rate of large collisions settled down, whether that was 3.8 or 4.5 billion years ago, would not be 3-9

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Figure 3.8. Mimas, 396 km across, is the seventh largest moon of Saturn. This figure is a mosaic created from six images taken on 13 February 2010, from a distance of 50 000 km in visible light with Cassini’s narrow-angle camera. The images were re-projected to provide this global perspective. The large Herschel crater at right is 130 km in diameter and gives Mimas the nickname ‘the Death Star moon’. If the object that hit Mimas to make this crater had been somewhat larger it would have destroyed the moon. Image Credit: NASA/JPL-Caltech/SSI.

such a big deal if not for a long list of clues indicating that Saturn’s rings date from only about 0.1 billion (100 million) years ago. We’ll return to these in chapter 6. The evidence compiled by Cassini makes the list of clues particularly compelling for a youthful ring system. The formation of Saturn mimicked, on a smaller scale, the formation of the solar system. The planet formed at the center of a disk of gas and solid particles. The solid particles orbit the planet according to the law of gravity, with the orbital speed decreasing with increasing distance from Saturn. The gas, on the other hand, is like an extended atmosphere of the planet and moves at a different speed. This difference in speeds results in friction between the solid particles and the gas, which causes the solid particles to spiral in toward the growing planet. As they migrated inward, particles were able to accrete to form moons as long as they were outside the Roche zone. Interlopers from interplanetary space were captured in the cloud of gas and continued to grow there by collisions with other particles in this sub-nebula disk. A few tens of millions of years after the initial formation of the Sun the intensity of the solar wind, the normal flow of electrons and protons away from the Sun, increased dramatically and blew the remaining gases out of the solar system, including the subnebular disks of Saturn and the other giant planets. This effectively ended the formation of the planets and stopped the migration of proto-moons inward towards

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Saturn. Just how the collection of moons Saturn had at that time compares to the present distribution is not entirely clear. It is possible that there were many more small moons that over the eons have been disrupted to create rings. It is also possible that a primordial ring system itself gave birth to some of the small icy moons. We’ll return to this intriguing possibility in chapter 6. The origins of Saturn’s moons may be as diverse as the moons themselves. In the next chapter we turn our attention to the icy worlds at Saturn as they are now.

Further reading The “Nice model” was originally proposed in a series of papers (Gomes et al 2005, Tsiganis et al 2005, Morbidelli et al 2005) by researchers working at the observatory in Nice, France. A model for formation of moons from the rings was proposed by Sébastien Charnoz and colleagues (2010).

References Charnoz S et al 2010 Nature 465 752 Gomes R et al 2005 Nature 435 466 Morbidelli A et al 2005 Nature 435 462 Tsiganis K et al 2005 Nature 435 459

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IOP Concise Physics

The Ringed Planet (Second Edition) Cassini’s voyage of discovery at Saturn Joshua Colwell

Chapter 4 A menagerie of moons

Saturn’s moons fall into a number of different categories, as we’ve seen in chapter 3. Titan, the largest moon, would be a planet were it orbiting the Sun instead of Saturn, and we’ll discuss it in detail in chapter 7. The remaining, smaller, regular1 moons exhibit a variety of unique and surprising features, each of which traces back to a physical process in the system. In this chapter we describe the processes that have shaped and altered the moons, looking at specific examples that highlight these processes. Since the majority of Saturn’s moons are small moons on distant orbits from Saturn, Cassini did not visit them.

4.1 The inner mid-sized icy moons The four large satellites of Jupiter, the Galilean satellites, fit well in a model of moon formation that mimics the formation of the planets as a whole, but on a smaller scale centered on a planet instead of the Sun. Saturn’s satellite system presents no such regular grouping that can be readily explained that way. The largest moon, Titan, may well have formed analogously to the Galilean satellites. Saturn formed in what is sometimes called a sub-nebula of the proto-planetary nebula, the disk-shaped cloud of gas and dust around the forming Sun. Accretion – the formation of larger objects from smaller objects sticking together through gravity – should have led to satellite formation in the Saturnian sub-nebula. That’s probably what led to the formation of Titan. In other words, it formed more or less the way the Earth did, but around Saturn instead of around the Sun. There are important differences between the planetary nebula and the sub-nebula, chiefly that the sub-nebula has a flow of material into it from the rest of the solar system, and this flow can vary in rate and composition with time, leading to different conditions for satellite formation. Also, even on a relative scale, moons are much closer to their planets than planets are to 1 Regular moons are those on prograde orbits that are nearly circular and close to Saturn’s equatorial plane. The irregular moons are likely captured from interplanetary space, as discussed in chapter 3.

doi:10.1088/2053-2571/ab3783ch4

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the Sun. This means that the forming satellites move across a greater fraction of their nebula than the planets do as their orbits evolve, and this can affect their formation or if they even manage to survive instead of getting swallowed up by the planet. The remaining regular moons may have a variety of origin stories. Some of the smaller ones may even have been born in Saturn’s rings well after the planet formed. We will visit Titan, a world larger than the planet Mercury, in chapter 7. Next in size from Titan is a group of regular moons referred to as the mid-sized icy satellites: Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, and Iapetus. Enceladus, Hyperion, and Iapetus have remarkable characteristics that are discussed in detail later in this chapter. We begin our discussion of moons in this chapter, for some context, with the remaining four regular mid-sized moons. Their surfaces are marked primarily by craters with varying degrees of tectonic features due to ancient stresses associated with tides and the original freezing of the moon from its initial partially molten state. These moons, more or less, meet our expectations for medium-sized icy moons in the outer solar system: they are giant balls of rock and ice bearing the scars of eons of bombardment with not much else going on. Proceeding outward from Saturn these four moons are Mimas, Tethys, Dione, and finally Rhea, the second-largest of Saturn’s moons (figure 4.1). Each of these moons is large enough for its gravity to deform it into a roughly spherical shape, but not so large that they have not long ago lost the heat leftover

Figure 4.1. Saturn, its main rings, and the orbits of the inner mid-sized icy moons are shown to scale in this schematic.

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from their formation. This heat of formation, the relic of the kinetic energy of all the pieces that collided with each other to make the moon in the first place, together with the energy released from the decay of any radioactive materials in the moon, is what drives geological activity, such as volcanoes and plate tectonics on the Earth. It is more than a little remarkable that the Earth is a geologically active planet today because its interior is still warm from its formation 4.5 billion years ago2. Larger objects retain their heat longer than smaller ones because, although they have a larger surface to radiate heat to space, their stored energy in their interiors is larger still. A baked potato stays hot longer than peas on your dinner plate, and babies catch a chill when we might be comfortable, for the same reason. What we might expect, then, for these four moons is an appearance similar to our own moon: heavily cratered with perhaps some relics of ancient geologic activity. For the most part, that is what we see.

4.2 Impacts and craters All objects in the solar system are hit by leftover debris from the period of planet formation. We refer to these objects now, generically, as either comets if they are icy objects in the outer solar system, or asteroids if they are primarily rocky or metallic objects in the inner solar system. An impact by a 10 km asteroid 65 million years ago ended the Cretaceous period and the existence of most species on Earth, but the crater from that impact is now buried beneath the ocean floor off the Yucatan peninsula. So, while that was a relatively large and recent (compared to the age of the planets) impact, the visible surface traces of it have long since been erased by tectonic and erosional processes on the Earth. Moons without an atmosphere don’t have erosion to cover up their impact craters, and generally any geological activity ceased long ago. Their surfaces thus provide an important record of their bombardment history. This history, in turn, is linked to the population of asteroids and comets in the solar system and the evolution of that population over the age of the solar system. The cratered terrains of these icy moons are witness plates to the history of Saturn’s corner of the solar system (figure 4.2). There has been extensive research on the relationship between the size of a crater and the properties of the impact that produced it3. The mass and speed of the impactor are the two most important properties, but the densities of both the impactor and the target surface also play important roles. When we observe the surface of a moon such as Mimas or Dione (figure 4.2) we see a distribution of crater sizes, with smaller craters more prevalent than larger ones. This reflects primarily the greater abundance of smaller objects in the solar system than larger objects. For 2

It is a bit sobering to note that in a billion years or so the Earth will have cooled off enough that the geological activity will shut down the cycling of carbon between the atmosphere and the interior and the surface will become uninhabitable to life as we know it. We owe our existence to this lingering heat of formation. 3 Many of these studies are based on studies of craters produced by bombs. This is a valid application of those data because an interplanetary impactor delivers energy in as concentrated a form as a conventional explosive.

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Figure 4.2. This view of ancient cratered terrain on Saturn’s moon Dione was taken from a distance of 537 km and has a resolution of 32 m/pixel. The high-resolution inset on the center-left of the image has a resolution of 3 m/pixel. Small craters greatly outnumber larger craters, some of which are now only faintly visible as partial circular depressions. Image Credit: NASA/JPL-Caltech/SSI.

each factor of 10 increase in the size of a comet or asteroid there is roughly a factor of 300 decrease in their abundance. If you smash a rock with a hammer repeatedly you will find a similar result: many more small pieces than large pieces. There are significant deviations from this general rule based on the population of objects (e.g., the main asteroid belt compared to the vast reservoir of comets beyond Neptune known as the Kuiper Belt, or the more distant Oort Cloud reservoir of comets), and this relationship does not hold over all sizes. Nevertheless, if all other things were equal, we might expect the same distribution of crater sizes on a moon of Saturn, namely 300 1 km craters for each 10 km crater. Of course all other things are far from equal. Craters are formed on a moon when an interplanetary impactor happens to hit it. These impactors may be icy or rocky, and they come in a range of sizes, from the size of a dust particle only one-hundredth the thickness of a human hair up to objects tens of km across. Smaller particles are more abundant than larger ones. The small particles pulverize and turn over the surface layers of whatever moon (or ring particle) they happen to strike in a process quaintly called ‘impact gardening’, but they don’t leave visible craters. Objects only a centimeter in size, however, will leave a bowl-shaped (and bowl-sized) depression on the surface. When these objects hit the Earth, unless they are large enough to survive being vaporized from friction with the atmosphere (a few tens of meters across, at least), they burn-up high in the sky,

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creating a meteor trail. But when they hit an object with no atmosphere, such as our moon, or Saturn’s moon Rhea, for example, they smash into the surface at speeds of several km s−1 up to tens of km s−1. For comparison, the speed of sound in air is 343 m s−1, and commercial jets travel at about 250 m s−1. With no atmosphere to filter out smaller impactors or provide erosion to cover up older ones, the surfaces of moons can take on a weary, battered appearance, with newer craters covering up and modifying the appearance of older ones (figure 4.3). The amount of energy delivered to the surface by an impacting interplanetary meteoroid is like a bomb going off on the surface. The energy of motion of a moving object, its kinetic energy, depends on how massive the object is and how fast it is moving. Double the mass of the object and its kinetic energy doubles. But double the impact speed and the kinetic energy quadruples. So we can calculate how fast a package of explosives would need to move in order to have the same impact kinetic energy as the energy contained in the explosive. Impacts faster than 2.89 km s−1 (which is the case for most cratering impacts onto moons in the Saturn system) produce more energy than if the entire impactor were converted to TNT and detonated. Usually these impactors excavate a bowl-shaped depression (the crater), while sending a shock wave into the interior of the moon and sending a spray of high-speed ejecta (vaporized and particulate debris produced in the impact) out in a conical fan from the crater rim. Cratering impacts on large objects, such as Rhea, produce ejecta that falls back down on the surface producing a pattern of rays emanating from the crater (figure 4.4). This is because the ejecta travels at a much slower speed than the impactor, and the gravitational pull of the moon prevents this slow-moving material

Figure 4.3. This view of Dione taken on 17 August 2015, by Cassini’s wide angle camera, reveals a surface battered by eons of impacts, with smaller fresher craters imprinted on an already heavily cratered terrain. The resolution in this view is about 45 m/pixel. Image credit: NASA/JPL-Caltech/SSI.

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Figure 4.4. This image of Rhea shows the crater Inktomi and its prominent butterfly-shaped pattern of bright ejecta rays. The large impact basin Tirawa can be seen at upper left. Image Credit: NASA/JPL-Caltech/SSI.

from escaping to space. The impactor thus leaves a characteristic scar on the surface of its target that is determined largely by the combination of the mass and speed of the impactor. The ejecta, however, has two complicating effects if we wish to use these scars to learn about the objects that created them. First, it buries and obscures older, small craters. Ejecta particles can also create new ‘secondary craters’ when it falls back down onto the surface. Each of these processes complicates the effort to relate the distribution of crater sizes to the impact history of a moon. We know something about the current size distribution of impactors in the Saturn system from astronomical observations of asteroids and comets, but our knowledge is limited by our ability to see small objects at great distances from the Sun4. A useful rule of thumb is that the diameter of a crater is about ten times the diameter of the object that made it. So a 10 km crater is produced by the impact of a 1 km object. A primary source of impactors for the Saturn system is the Kuiper Belt whose inner 4

Observations of small distant objects are even more difficult than one might expect: they are difficult to see because of their great distance from us, but even more so because of their great distance from the Sun, which means they are only very faintly illuminated. Even at Saturn, relatively nearby compared to comets in the Kuiper Belt, the illumination from the Sun is on average only 1.1% of what we have on Earth. Finally, many objects in the outer solar system are extremely dark, reflecting only 5%–10% of incident sunlight.

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Figure 4.5. Had the impactor that made the Herschel crater on Mimas been somewhat larger, it might have completely shattered the moon. Herschel is 130 km in diameter. The sunlit part of Mimas in this view is on the leading hemisphere. NASA/JPL-Caltech/SSI.

edge is more than three times further from the Sun than Saturn itself. We have only a rough idea of the population and distribution of sizes small comets in the Kuiper Belt because we typically can only see them when they happen to migrate closer to the Sun (and the Earth)5. Most of the objects we have discovered in the Kuiper Belt are so large that they would have destroyed, rather than cratered, the icy moons of Saturn on impact. An example of a nearly destructive impact on a moon can be seen at Mimas, nicknamed the Death Star moon for its singular Herschel crater (figure 4.5). The fragmentation of moons is an idea we will return to with the origin of Saturn’s rings in chapter 6. We have a more complete picture of the size distribution of asteroids, the rocky counterparts of comets and Kuiper Belt objects that reside in the inner solar system largely between the orbits of Mars and Jupiter. We have also learned a lot about the size distribution of impactors in the ancient solar system from observations of the distributions of craters on the Earth’s moon. Critically, for the moon, the Apollo 5 This migration occurs as a result of gravitational perturbations from the planets, other objects in the Kuiper Belt, and even other stars in the Galaxy.

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astronauts returned rock samples whose ages have been accurately measured in laboratories here on Earth using radioactive dating techniques. We lack such an absolute calibration for chronology of the Saturn system, because we don’t have physical samples in hand to carry out radioactive dating. Nevertheless, the combination of crater counts on other worlds and observations of comets and asteroids throughout the solar system provides some reasonable estimates for the population of interplanetary impactors passing through the Saturn system. Critically, as we’ll see in chapter 6, Cassini directly measured the smallest of these impactors as they hit Cassini’s Cosmic Dust Analyzer (CDA). Those measurements help us tell the history of the rings, but fortunately for the Cassini mission those dust-sized impactors are too small to create the kind of impact craters we see on the moons. One of the discoveries of the Voyager missions, confirmed by Cassini, is that the impactor population is inconsistent with the distribution of craters on Saturn’s moons. Here’s how we know this. Potential impactors (comets and asteroids) in interplanetary space are on orbits around the Sun, just like Saturn is. If an impactor comes close to Saturn it will be attracted by the planet’s gravitational pull and pass even closer to it than it would if Saturn had no gravity. Saturn’s gravity thus increases the number of impactors that pass within a certain distance of the planet, say the outer edge of the rings or the orbit of one of its moons. Saturn’s gravity acts like a lens, bringing the paths of impactors closer together just as a magnifying glass causes rays of light to converge on a point. This gravitational focusing of the interplanetary impactor population is larger close to Saturn, and fades away at large distances. Thus we would expect a greater flux of impacts from comets and asteroids on the inner moons such as Mimas and Rhea than the outer moons such as Iapetus and Hyperion. Saturn’s gravity also accelerates these impactors so that they hit their targets at a faster speed than they would if it had no gravity. This effect is also strongest for the inner moons close to Saturn. To hit them, in effect, a comet must fall further toward Saturn than it would if it hit an outer moon, and as it falls toward Saturn it picks up speed. As a result of this acceleration of the impacting objects and the gravitational focusing effect, we expect both more numerous and more energetic impacts on the inner moons. This should produce denser crater fields and larger craters on the inner moons. Instead we see the greatest number of large impact basins, five that are more than 300 km across, on Iapetus, the most distant of the mid-sized icy moons, while we see only a total of seven such basins on the remaining four heavily cratered mid-sized moons combined. One such basin is shown in figure 4.6. Based on our knowledge of the interplanetary impactor flux and the effects of gravitational focusing we would expect none at all on Iapetus, and roughly only one per moon for the remaining satellites. If the number of large craters on Iapetus is inconsistent with our understanding of the population of impactors orbiting the Sun, then we are led to conclude that the objects that made these craters were not part of that Sun-orbiting (heliocentric) population. Instead they are likely to have been made by objects that hit Iapetus from within the Saturn system itself. The impactors were not comets orbiting the Sun, but instead were moons of Saturn. The abundance of large impact basins is but 4-8

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Figure 4.6. This view of Tethys, taken by Cassini on 11 April 2015, shows the large Odysseus impact basin at right. This crater has a diameter of 450 km, nearly half the diameter of Tethys itself, and is one of the largest impact basins on any of the Saturnian moons. Tethys has a heavily cratered terrain with almost no signs of geological activity. Careful counting of all the craters in images such as this provide clues to the identity of the objects that hit the moons and the history of Saturn and the solar system. Image Credit: NASA/JPL-Caltech/SSI.

one of several oddities about Iapetus, and we’ll return to this moon in detail later in the chapter.

4.3 Synchronous rotation Another important contributor to the speed of an impact is the motion of the moon itself in its orbit around Saturn. Most moons in the solar system, including our own as well as almost all the moons of Saturn, rotate6 synchronously with their orbital motion. Synchronous rotation means that the time it takes for the moon to rotate once is equal to the time it takes for it to complete one orbit of its planet. Since the 6 Tidal evolution does not come to a stop when the moon settles into synchronous rotation. Because the planet is not synchronously rotating with the moon (except for Pluto and its large moon Charon which are each tidally locked, like dancers holding hands and swirling around a common center) tidal dissipation of energy continues, gradually slowing the rotation of the planet and also changing the orbit of the moon. Earth’s moon is gradually receding from the Earth while the Earth’s day is gradually getting longer, and at some amazing time in the future (in a couple of hundred million years, give or take) we will finally have a 25 h day.

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time to rotate determines the length of a day–night cycle, when a moon is in synchronous rotation, the length of a day on the moon is equal to its orbital time. This is why, from the Earth, we always see the same face of the moon. This synchronization of a moon’s spin period with its orbital period is not a coincidence. The same tidal force that gives us ocean tides on Earth and prevents ring particles from accreting into moons at Saturn (chapter 3) produces this synchronous rotation. Such moons are tidally locked into synchronous rotation. The tidal force stretches the moon, ever so slightly. Because it takes energy to stretch a moon (try it sometime if you don’t believe me), the tidal deformation energy must come from some other source of energy in the planet–moon system. The first source of energy that is depleted is the energy of the rotation of the moon. So, gradually, a moon that is rotating more rapidly than synchronous rotation will gradually slow down as its rotational energy is converted into the thermal energy associated with the tidal stretching. Eventually it will settle into synchronous rotation and there is no more stretching of the moon: it stays stretched in one particular direction and this requires no more energy. After this synchronization no further changes in the deformation take place (figure 4.7). Some energy also comes from the planet’s rotation, but the planet is much more massive than the moon, so it experiences a much smaller change. Moons that are tidally locked into synchronous rotation not only have one hemisphere always facing the planet and one facing away, they must also therefore have a ‘leading hemisphere’ that is always facing in the direction of its orbital motion, and a ‘trailing hemisphere’ facing in the opposite direction. Impactors that hit the leading hemisphere of a moon hit it at a much greater speed than those that hit the trailing hemisphere because the orbital velocity of the moon adds to whatever

Figure 4.7. The blue moon is on a circular orbit. Tidal forces have stretched it, and it becomes tidally locked into synchronous rotation so that it keeps the same face always pointing toward the planet (green). The red moon is on a highly eccentric orbit (exaggerated here for clarity). At point A it is moving slower and it is easier for the moon to maintain its orientation as it orbits the planet. At points B and C, when it is closer to the planet, it is moving faster (arrow lengths indicate speed) so it must rotate faster to maintain its orientation to the planet. This changing orbital speed introduces a wobble in the moon’s rotation.

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velocity the impactor had. Those that hit the trailing hemisphere must catch up to the moon, in a sense, and hit the surface more gently and produce a smaller crater for the same size impactor. Impactors flying through the Saturn system from interplanetary space hit the leading hemispheres of moons more frequently and violently than they hit the trailing hemispheres, leading to larger craters on the leading hemisphere. But this is not what is seen on Saturn’s moons. The lack of a clear asymmetry in the crater distributions between the leading and trailing hemispheres together with the mismatch in the size distribution of craters with the size distributions of asteroids and comets suggests that many of the craters were not produced by impactors that were orbiting the Sun but rather by objects that were also orbiting Saturn. This lack of asymmetry in crater sizes between the leading and trailing hemispheres is another piece of evidence that many of the craters on Saturn’s moons were made by impacts by other moons or fragments of other moons. The system of satellites at Saturn has had a violent and dramatic history. The number of moons has likely changed significantly, with new moons captured into orbit and then broken apart either by impacts from comets or collisions with other moons in the system. The irregular satellites come in groups, with the moons in a group having similar orbits indicating that they are fragments of a single larger progenitor, broken apart by collision with another moon or an interplanetary interloper. Some regular satellites certainly suffered the same fate. The debris from these collisions in turn rained down upon the larger moons. Such a cataclysm may explain the existence of the ring system itself, as we will see in chapter 6.

4.4 Icy satellite tectonics There are also some intriguing pieces of evidence of past geological activity on some of the mid-sized icy satellites. Aside from cratering, the surfaces of moons can be altered by tectonic stresses. These can be caused by changes in the amount of tidal stretching of a moon as its spin state and orbit evolve (figure 4.8) and by changes in the volume of the moon as it cools and freezes. Moons don’t settle perfectly into synchronous rotation. Various perturbations cause the orbits of moons to be slightly non-circular or eccentric, and this means that the speed of the moon in its orbit is not a constant (because orbital speed depends on distance from the central object). This in turn means that the speed of rotation of the moon is not constant. As a result, the moon always has to do a little bit of catch up to get back into synchronous rotation, producing a wobbling motion, back and forth, of the rotation called libration. The libration of the Earth’s moon lets us get a peek around the edges to a little bit of the far side of the moon each month. Tidal stresses occur during this libration, and they can produce cracks and fissures. The rate of libration also depends on what the interior of the moon is like, much the way a boiled egg spins differently than a raw egg. Cassini was able to make precise enough observations of the libration of some of Saturn’s moons to constrain the state of their interiors. This is true of Titan (chapter 7) and Enceladus (below), both of which have liquid water oceans revealed in part by their libration patterns.

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Figure 4.8. One of the many stunning ‘photo-op’ images taken by Cassini, this 2011 image shows Dione’s ‘wispy terrain’ and the rings in the distance with the heavily cratered surface of Rhea in the foreground at a resolution of 358 m/ pixel. Larger craters have a central peak formed when the surface, molten by the impact, splashes back upwards. A similar phenomenon can be seen by dropping a cheerio in some milk. Image Credit: NASA/JPL-Caltech/SSI.

Even tiny Mimas has a libration that indicates there may be a deep subsurface ocean. Very large impacting events can also cause global fractures on the surface of a moon. There is also the possibility of ‘cryovolcanism’, or the eruption and flooding of the surface by water from a liquid reservoir beneath the surface. With the exception of Enceladus, which we’ll discuss in detail below, the other icy moons have long since frozen, so their tectonic features are billions of years old. This is confirmed by the density of craters on top of the tectonic features, though we cannot place precise ages on features without a sample for absolute radiometric dating. There is another important consequence of the tidal interaction between moon and planet discussed above (figure 4.7). The orbit of the moon gradually evolves as well, becoming larger for moons whose orbits are longer than a Saturn day7. As the orbit expands, the strength of the tidal deformation decreases, resulting in changes to the 7 All known moons of Saturn have orbits longer than a Saturn day. The Earth’s moon is also undergoing this tidal orbital evolution, currently moving away from the Earth at rate of 3.8 cm yr–1. The rate is much faster when the moon is closer to the planet. Mars’s moon Phobos orbits in less than a Mars day, and as a result its orbit is decaying. Phobos will one day crash onto Mars, probably within the next 100 million years.

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Figure 4.9. A series of tectonic linear features can be seen almost bending around the large crater with a central peak in the bottom third of this image of Dione taken on 16 June 2015, from a distance of 77 000 km. These same fractures can be seen in a very different perspective and illumination in figure 4.10 at the far right edge. The rings can be seen at lower left peaking out from behind the moon. NASA/JPL-Caltech/SSI.

stress patterns on the moon. This is likely the explanation of the system of fractures on Dione (figure 4.10). Features of unknown origin on the icy moon Tethys appear tectonic, if only because they are linear, but whether they are associated with cracks in the surface that are too small to see or are a result of some impact scouring of the surface to reveal ice with different composition will require more observations from a future mission (figure 4.11). They bear a suggestive similarity to arcs on Jupiter’s moon Europa that are more like the tidally created cracks seen on Dione in figure 4.10.

4.5 Interactions with the magnetosphere Cassini’s cameras extend to wavelengths far beyond what the human eye can see in the familiar visible-light images shown above. Seen in the far-infrared portion of the spectrum, where light we observe is not reflected sunlight but is instead emitted by the surfaces due to the heat of the moon, Mimas and Tethys exhibit a striking ‘Pac-Man’ appearance (figure 4.12). Here, bright yellow features are warmer than dark blue features, and the false-color rendering of these images with yellow indicating higher 4-13

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Figure 4.10. Chasmata or ‘wispy terrain’ on Dione are bright icy cliffs likely due to stresses resulting from changing tidal deformation as the orbit of Dione evolved over time. Image Credit: NASA/JPL-Caltech/SSI.

temperatures than blue features results in an uncanny resemblance to the video arcade game of the 1980s. The appearance of these temperature maps was completely unexpected. Without an internal heat source, the surface should be warmest where it receives the most direct sunlight, and cool gradually away from that point. The Pac-Man temperature patterns are striking in that they are independent of the position of the Sun, and the boundary between cold and warm temperatures is an abrupt, sharp V-shaped feature. Either there is something heating up the warmer regions, or there is something different about the surface that causes one region to cool much more quickly than the other. The orientation of Pac-Man provides a clue: the cool regions are on the leading hemispheres of the moons. This suggests that it may be related to something hitting that side of the moon and altering it so that it doesn’t heat up as much in the daytime. That something altering the leading hemispheres of these moons turns out to be energetic electrons in Saturn’s magnetosphere. In addition to being pelted by comets, asteroids, or other moons, the moons are also bombarded by charged particles. Like Earth, Saturn has a magnetic field that blocks and diverts the electrons and protons flowing outward from the Sun, the so-called solar wind. The magnetic field traps these charged particles, as well as those liberated from the upper atmosphere of Saturn and Titan by high-energy solar 4-14

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Figure 4.11. This highly color-enhanced view of Tethys shows narrow (a few km wide) red arcs that are several hundred km in length. The yellow coloration on the left is due to alteration of the surface by charged particles in Saturn’s magnetosphere. The image resolution is about 700 m/pixel, and the region shown is about 490 km wide. Image credit: NASA/JPL-Caltech/SSI.

photons, and atoms knocked off the surfaces of the moons and subsequently ionized by sunlight. These negatively charged electrons and positively charged ions spiral around the magnetic field and ‘bounce’ back and forth across the equatorial plane of Saturn as they do so due to the electromagnetic force. This force causes charged particles to follow curved paths perpendicular to the direction of the magnetic field. Understanding this complicated gyrating motion frequently requires some complicated mental gymnastics (figure 4.13). The magnetic field rotates with Saturn itself, which has a day that is about 10 h long. The moons take longer than 10 h to orbit Saturn, so the magnetic field is moving faster than the moons are moving in their orbits. It sweeps by the moons from behind their trailing hemispheres. If the electrons and protons were stuck to the magnetic field, this would mean they bombard the trailing hemispheres of the moons. The slower-moving or ‘cold’ protons and electrons do just that. Their additional motion is tiny compared to the motion they get from being carried along by the magnetic field, so they preferentially bombard the trailing hemispheres of the satellites as the magnetic field sweeps by the moons. The more energetic a charged particle is, however, the larger is its radius of gyration around magnetic field lines (figure 4.13). While an energetic electron is traveling along its spiraling path, its distance from Saturn can vary by a large

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Figure 4.12. Mimas (left) and Tethys (right) seen in infrared light from Cassini’s Composite Infrared Spectrometer (CIRS) instrument (top) and visible light (bottom). In the upper images, yellow corresponds to the highest temperatures, and blue regions are about 20° C cooler. Image Credit: NASA/JPL-Caltech/GSFC/SWRI/SSI.

amount, so the strength and geometry of the local magnetic field it is moving through also changes. As a result, it experiences a varying magnetic force that has the net effect of causing it to drift around Saturn in the opposite direction of the rotation of the magnetic field. This only happens for energetic (fast-moving) electrons. Unlike the slow-moving charged particles, these energetic electrons preferentially hit the leading hemisphere of a moon. Low-energy electrons and ions move on much smaller, tighter spirals that don’t experience enough variation in the magnetic force to produce a significant drift. The motion of positively charged particles, meanwhile, is in the opposite sense from the electrons, so they drift around Saturn in the prograde direction, moving past the trailing hemisphere of the moon even faster than they would in the absence of a drift. This complex dance of charged particles is likely the cause of the Pac-Man features on Mimas and Tethys. By bombarding the leading hemispheres of these moons, high-energy electrons physically alter the icy grains on their surfaces, compacting a relatively porous layer of particles into hard-packed ice. This hardpacked surface takes longer to heat up in the daytime because it conducts heat from the Sun into the interior of the moon more efficiently than a fluffy porous surface does. This property of a surface is called its thermal inertia and is why reflective concrete is relatively cool in the midday sun, while equally reflective dry beach sand 4-16

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Figure 4.13. This schematic of Saturn’s magnetic field shows the solar wind compressing the magnetic field on the left, temporarily leaving Titan outside Saturn’s magnetosphere. Electrons gyrate around magnetic field lines and ‘bounce’ up and down along the lines between the north and south polar regions. They are carried around with the magnetic field which rotates with Saturn (green). The size of the spiral (not shown) and the speed of the drift depend on the particle’s energy. High-energy electrons have large gyrations which result in an additional ‘drift’ in the opposite sense of the rotation of the magnetic field (red). Based on an image from NASA/JPL-Caltech.

can be painfully hot on the feet8. As a result, the temperatures in the mouths of the Pac-Man features remain relatively constant while the rest of the moons’ surfaces have large daily fluctuation in temperatures, producing the distributions of temperatures seen in figure 4.12.

4.6 The curious case of Hyperion A quick glance at Hyperion is enough to tell us that there is something peculiar about this moon (figure 4.14). Like the other small moons, it is not spherical in shape. Its small size means its gravity is too weak to deform the object into a spherical shape. It is the appearance of its craters that make it stand out. Hyperion looks so much like a loofah, in fact, that the paper from the Cassini imaging team describing Hyperion’s craters in the prestigious scientific journal Nature is titled ‘Hyperion’s sponge-like appearance’ (Thomas et al 2007). Like many of Saturn’s moons that orbit near the rings, Hyperion’s density is quite low, only about 0.54 g cm−3 (water has a density of 1.0 g cm−3), though Hyperion is on a very distant orbit from Saturn. Those so-called ring moons, however, are much smaller, and owe their low densities to the accumulation of thick coatings of loosely packed small particles on the surface. Hyperion’s surface does not appear to have such a coating. Phoebe, which is slightly smaller than Hyperion, has a density of 1.6 g cm−3 (and a shape that 8 Dark pavement such as asphalt has a high thermal inertia like concrete (meaning it conducts heat well), but it is hot because it is dark and absorbs more sunlight.

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Figure 4.14. This false-color view of Saturn’s moon Hyperion was obtained during Cassini’s close flyby on 26 September 2005. The spectrum of Hyperion is reddish, though the moon appears gray to the eye. Color differences are enhanced here to highlight what could be compositional differences between cliffs and crater bottoms. This is a composite made from images taken through infrared, green, and ultraviolet filters. The resolution is 362 m/pixel, and the moon is 360 km across in the long dimension. Image Credit: NASA/JPL-Caltech/SSI.

is much closer to a sphere). By making reasonable assumptions about the fraction of ice and rock in the moon, the low density of Hyperion means that its porosity is greater than 40%. That is, almost half of Hyperion is empty space, like a sponge. This low density means that it has a small mass for an object of its size, so its weak gravity is not able to prevent most impact ejecta particles from simply escaping the moon altogether, never to return. When a crater is formed on Hyperion, it loses most of the ejecta to space. This means that not only is there no production of secondary craters from ejecta falling back onto the surface but also that the surface remains free of the coating of debris from other impacts. On most objects this ejecta coating obscures and eventually covers up older craters (figures 4.2 and 4.3). Craters on Hyperion thus maintain a sharper appearance, giving them the appearance of geological youth. Compared to Phoebe (figure 4.15), whose surface is worn and softened by the accumulation of ejecta from its craters, Hyperion’s surface looks crisp, clean, and freshly carved as though someone had taken a giant ice cream scoop to it (figure 4.14). Because most craters are bowl-shaped, with roughly the same depth-to-diameter ratio, smaller craters are more quickly buried under fresh

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Figure 4.15. This image of Phoebe is at roughly twice the resolution of the image of Hyperion in figure 4.14. In spite of the higher resolution (190 m/pixel), Phoebe’s surface appears softer than Hyperion’s due to the presence of worn-down craters. This is the result of older craters being gradually covered up by ejecta from more recent craters, a process that is less efficient on Hyperion due to its high porosity. Image Credit: NASA/JPL-Caltech/SSI.

ejecta than larger craters. Further, because it is so porous, impactors tend to burrow into the surface, creating slightly deeper craters for the same size impactor on Hyperion than on larger moons. Whether Hyperion’s high porosity results in the production of less ejecta, faster ejecta, or both, the lack of ejecta coating the surface and the difference in crater morphology due to its high porosity give it an abundance of mid-sized craters compared to other moons. Another peculiar aspect of Hyperion is how it spins as it orbits Saturn. Hyperion is the only regular (not captured) moon of Saturn (or of any planet) that is not tidally locked. Its rotation is chaotic which simply means that its orientation in space cannot be predicted ahead of time. In contrast, the other moons maintain a nearly fixed rotation axis whose small wobbles (librations) are periodic and predictable. Hyperion’s chaotic rotation is due to the proximity of its orbit to that of Titan, its oblong shape (figure 4.14), the high eccentricity (deviation from a perfect circle) of its orbit (figure 4.7), and that it is in an orbital resonance with Titan. Hyperion’s eccentricity is 0.12, meaning that on each orbit of Saturn its closest point to the planet is 12% closer than the average distance, and the furthest distance is 12% further than the average distance. While 12% doesn’t sound like a huge amount, it’s much larger than that of the other regular satellites, and because it is 12% in both directions, the distance between Hyperion and Saturn varies by about 24% each orbit. A moon on a circular orbit can easily settle into a tidally locked synchronous rotation as shown in figure 4.7, where the spin rate and orbital rate are both constant and equal to each other. But if the moon is on a highly eccentric orbit, its orbital rate

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Figure 4.16. An oblong object has three principal axes of rotation. Spins around axes A and B are stable, but around axis C the spin is unstable.

of motion around Saturn varies with its distance from it each orbit. The tidal locking mechanism, which tries to make the spin rate and orbital rate synchronize, takes much longer than one orbit to change the spin rate. In a sense, Hyperion is constantly struggling to change its rotation rate to match its ever changing orbital rate. While this results in small librations for moons with a small eccentricity, for Hypertion the result is a complicated and chaotic tumbling of the moon. The highly oblong shape of Hyperion, unusual for an object of its size, plays an important role as well. The spin of oblong objects is surprisingly complicated. There is a simple experiment you can perform that illustrates this. For this experiment, take an object that has different lengths in each dimension, such as this book9, whose length, width, and depth are very different (figure 4.16). If you toss this object in the air with a spin so that it rotates around its short axis (A) or its long axis (B), the direction of the axis of rotation remains constant: the rotation is stable. But toss the book in the air with a spin around the intermediate axis (C), and it tumbles. Try it; it’s weird. The reason for Hyperion’s large eccentricity, which in turn leads to its struggles to maintain a constant rate of rotation, is the strong gravitational tug it receives from neighboring Titan. The gravitational perturbations from Titan that Hyperion experiences are large, due to the former’s large mass. There is also an orbital resonance between Hyperion and Titan: Titan orbits Saturn four times in the time Hyperion orbits three times. This means Hyperion and Titan encounter each other at the same phase of Hyperion’s inward and outward journey on its eccentric orbit so that the gravitational perturbations build up over time rather than canceling each other out. Orbital resonances play an important role in sculpting the architecture of the solar system. There are many in play within the system of Saturn’s moons and rings as well that produce a variety of interesting phenomena from Hyperion’s rotation to waves in the rings and geysers on Enceladus as we’ll see below.

9

If you’re reading this on a cellphone or tablet, the experiment will still work, but take care not to damage your device! If you use a physical book, fasten it shut with a rubber band. If you’re reading it on a computer, find something else to throw in the air.

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Figure 4.17. The two hemispheres of Iapetus have very different reflectivities. The leading hemisphere, on the left, is covered by a dark material, except near the poles, while the trailing hemisphere (right) is much brighter. Image Credit: NASA/JPL-Caltech/SSI.

4.7 Iapetus: the two-tone moon Iapetus is the second-largest, after Rhea, of the mid-sized icy satellites of Saturn. Its orbit is much larger than that of the other regular satellites, however, and this is likely responsible for its distinctive appearance. In spite of its great distance from Saturn (59 RS, where 1 RS is the radius of Saturn, or about 60 300 km), Iapetus, like the inner moons, is tidally locked in synchronous rotation around Saturn10. Like the inner icy moons discussed above (but unlike the chaotically rotating Hyperion), Iapetus has a leading and trailing hemisphere. The combination of the synchronous rotation with Iapetus’s large orbit gives rise to a unique and distinctive feature. The leading hemisphere of Iapetus is among the darkest surfaces in the solar system, reflecting just 5% of incident sunlight, while the trailing hemisphere is among the brightest in the solar system, reflecting 50% of incident sunlight (figure 4.17). For comparison, our moon has, on average, a reflectivity (or albedo) of about 14%. One can imagine that the dark surface is the result of Iapetus plowing through some space dirt in its orbital journey around Saturn, its leading hemisphere getting coated with dirt like a car’s windshield driven through a cloud of bugs. The trailing hemisphere, shielded from this polluting material, remains bright, pristine water ice. This relatively simple hypothesis poses many questions, and the answers are not so simple. What is the source of this dark material? Why is not coating the other moons of Saturn? Is there enough of it to explain the depth of the dark terrain (which can be measured by the appearance of small bright craters on the dark side)? How and why

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The Earth’s moon orbits at an average distance of 60 REarth from the Earth, but Saturn is 9.5 times the size of the Earth, so while the relative separations are similar, Iapetus’s orbit is about 9.3 times the size of our moon’s orbit.

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Figure 4.18. The faint, but enormous, dust ring produced by impacts onto the moon Phoebe was revealed in images taken by Anne Verbiscer and colleagues using the NASA Spitzer Space Telescope. The dot at right indicates the size of Saturn and the main rings to scale with the dust ring which is too faint to observe without long exposures and careful image processing. Image Credit: NASA/JPL-Caltech/Anne Verbiscer (Univ. Virginia).

does the dark material wrap around to part of the trailing hemisphere, and why are the polar regions of Iapetus bright, even on the leading hemisphere? The answers to some of these questions lie with yet another of Saturn’s moons: Phoebe, the largest of the captured irregular satellites we learned about in chapter 3. Phoebe is the source of what is, by sheer volume, the largest planetary ring in the solar system. This ring is so faint and so large that it escaped detection until 2009 when Anne Verbiscer at the University of Virginia and her colleagues used data from the NASA Spitzer Space Telescope to make a large mosaic of the region where they expected the ring to lie using the infrared region of the spectrum where the ring was expected to be relatively bright (figure 4.18) (Verbiscer et al 2009). It’s difficult to see this ring with Cassini’s instruments because it is an enormous faint cloud of dust, and Cassini is too close to it to be able to get the big picture that shows the slight contrast between the ring and empty space. It’s difficult to get a good picture of a cloud if you are inside it, or even right next to it, especially if the cloud is so tenuous that it blocks only two millionths of 1% of the light passing through it. This ring is similar to the G ring in that it is made of dust particles knocked off of a moon by micrometeoroid impacts. In the case of the G ring, the tiny moon Aegeon is the source (chapter 3, figure 4.1), and the resulting ring is faint but also relatively small in extent. The Phoebe ring, on the other hand, is vertically (in the north–south direction) 20 times as large as Saturn itself, and extends well over 100 times the radius of Saturn from its inner edge to its outer edge. The dust from Phoebe fills such a large space because it is launched from Phoebe itself, and Phoebe is on a very eccentric (elliptical) and inclined (tilted) orbit around 4-22

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Figure 4.19. This 2007 mosaic of Cassini images of Iapetus was assembled by the Imaging Science team led by Carolyn Porco at the Space Science Institute in Boulder, Colorado. Bright and dark areas are intermixed at the transition between the dark hemisphere at right and the bright hemisphere at the left, but there are no gray areas. In the transition region the floors of craters are darker than the crater walls. Bright peaks can be seen in the centers of some of the larger craters. Image Credit: NASA/JPL-Caltech/SSI.

Saturn, with a mean distance from it of almost 13 million km, more than 3.5 times the size of Iapetus’s orbit. Cassini was only able to visit Phoebe once, during its initial approach to Saturn. It never again ventured that far away from Saturn. As Phoebe orbits Saturn it makes large excursions north and south of Saturn’s equatorial plane, and its distance from Saturn ranges from 11 to 15 million km. All along the way, small particles are liberated from its surface by the faint but continuous bombardment of interplanetary micrometeoroids, the small particles liberated from comets and asteroids when they collide with each other and when comets come close to the Sun and evaporate. Phoebe is large enough to provide a significant target area to these interplanetary impactors, but not so large that its gravity prevents the ejected dust particles from escaping the moon entirely. These particles occupy the space traversed by Phoebe in its orbital journey, and the smallest ones gradually spiral inward toward Saturn due to the effects of the weak but persistent force of sunlight impinging on the particles. Here is where Phoebe connects to Iapetus. Iapetus plows through the Phoebe ring particles as it orbits Saturn. Phoebe, and therefore the dust knocked off it to form the Phoebe ring, orbits Saturn in the retrograde direction, while Iapetus is a regular, prograde satellite. The Phoebe ring thus creates a steady rain, or hail, of dust particles onto the leading hemisphere of Iapetus, darkening its icy surface. The other, smaller outer irregular satellites also contribute small amounts to this inwardly migrating population of dust particles. An immediate test of whether this dust is what we see on the dark, leading hemisphere of Iapetus is to compare the 4-23

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spectral properties of the dark face of Iapetus with the spectrum of Phoebe11, the source of most of the dust. It turns out that the spectrum of Phoebe is not a good match for the dark hemisphere of Iapetus. Furthermore, the sweepup of dust would lead to a gradual transition from the leading hemisphere to the trailing hemisphere: we would expect to see some terrain with an intermediate brightness between the dark face and the bright face. Instead, the transition shows a mottling of bright and dark areas, but nothing in between (figure 4.19). So although Iapetus sweeps up the dark material from Phoebe and the other outer moons, something else is going on to create the dramatic dark/bright asymmetry. In 2010, John Spencer of the Southwest Research Institute in Boulder, Colorado, and Tillman Denk published a paper that provides an explanation for the distribution of bright and dark terrain on Iapetus (Spencer & Denk 2010). Because Iapetus is in synchronous rotation with its orbit, but also on a very large orbit, both its orbital period and its day are long: 79.3 Earth days. This is the longest day of any Saturnian moon. Moons with orbits much larger than that of Iapetus do not rotate synchronously because they are too far from Saturn for tidal locking to take place. Iapetus’s long day means the dayside has a long time to heat up, and the nightside has a long time to cool off, so there is a relatively large 40 K temperature difference between noon and midnight on Iapetus. The difference is only relatively large, because the surface is still extremely cold, by Earthly standards, everywhere on Iapetus at all times12. If the dust from the Phoebe ring produces even a slight darkening of the leading hemisphere, then during daytime on that hemisphere it gets even warmer because it absorbs more sunlight. This increase in temperature and the long duration of the day is enough to increase the loss rate of water ice from the leading hemisphere due to sublimation13. The sublimated water molecules do not escape Iapetus entirely, but instead land somewhere else on the surface. If that new location is warm (such as on the dark leading hemisphere) it is likely to hop off the surface again. It is likely to continue hopping until it lands on a cooler surface. Surfaces without dark material reflect more sunlight and are therefore cooler. So water molecules preferentially migrate from the leading hemisphere, due to its initial coating of material from the Phoebe ring, to the trailing hemisphere. This leaves behind non-icy, dark material on the leading hemisphere, and provides a fresh, bright coating of water ice on the trailing hemisphere. The cold poles are also bright for the same reason. The same process happens even on very small spatial scales. The bottoms of craters tend to get to higher temperatures than the top of a hill, for example, because the crater acts as a trap for heat. The temperature of a surface is determined by a balance between the energy into the surface and the energy loss from the surface.

11 The reflectance spectrum of an object is a measure of how much light is reflected by that object as a function of the wavelength of light. It is a powerful diagnostic of the composition of a material as well as physical properties such as porosity and the size of grains on the surface. 12 140° C below zero on the ‘warm’ dayside. 13 Sublimation is the process of evaporation from the solid phase to the vapor phase, such as when ice cubes are removed from the freezer and give off steam.

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Figure 4.20. The bottoms of larger craters near the transition between the bright and dark hemispheres on Iapetus are as dark as the dark hemisphere because they reach higher temperatures in the daytime than flat terrain. Image Credit: NASA/JPL-Caltech/SSI.

Absorption of sunlight is a primary heating mechanism, and this heat is lost through conduction into the interior of the moon, through sublimation of ice, and through radiation of infrared light out to space. If this thermal radiation is prevented from escaping to space, whether through absorption by greenhouse gases in an atmosphere, as on Earth, Mars, and Venus, or by physical barriers such as the walls of a crater, some heat is returned to the surface and the temperature rises. Indeed, in the transition region of Iapetus, crater floors are dark, while the peaks of mountains in the centers of those same craters are bright (figures 4.19 and 4.20). So the dark material on Iapetus is not primarily from the Phoebe dust ring, but it owes its existence to that ring. A slight darkening of the leading hemisphere by Phoebe dust triggered a positive feedback loop where bright ice sublimated away from the leading hemisphere and coated the trailing hemisphere, making the leading hemisphere darker due to a lack of water ice, and the trailing hemisphere brighter due to an overabundance of it. This phenomenon was first observed and explained on the icy moons of Jupiter which are much warmer than the moons of Saturn due to Jupiter’s smaller distance from the Sun. Spencer did not expect the process would operate at Iapetus because 4-25

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Figure 4.21. This view of the dark, leading hemisphere of Iapetus shows a portion of the 20 km high ridge that extends along its equator. Image Credit: NASA/JPL-Caltech/SSI.

water sublimation is so slow at the lower temperatures there. Only the very long day of Iapetus, allowing daytime temperatures to creep a bit higher, and the broad initial coating of the leading hemisphere by Phoebe ring dust, make the thermal segregation process work. The inner moons of Saturn are on smaller, faster orbits and therefore have shorter days, which prevents their daysides from heating up as much as that of Iapetus. The two-tone nature of Iapetus is not the only oddity of this moon. Its equator features a prominent ridge that gives the impression that the moon was assembled rather hastily by smashing the top half into the bottom half with a little too much force (figure 4.17). Unlike the leading–trailing asymmetry, which had been noticed by Giovanni Cassini in 1671 when he first discovered Iapetus, the ridge was only discovered in 2004 by the spacecraft bearing his name. The flanks of part of the ridge were observed by Voyager 2 in 1981 and dubbed the ‘Voyager Mountains’, but the full extent of the ridge was unknown because most of the ridge is in the dark terrain. Voyager did not have the sensitivity to make out the ridge in the dark terrain (figure 4.21). Cassini’s cameras were able to obtain long exposures of the dark hemisphere, revealing the detail seen in figure 4.21 in spite of the low reflectivity of the surface. These long exposures were enabled by the sophisticated target-tracking capabilities of Cassini. The ridge is not continuous around the moon, lying primarily in the dark, leading hemisphere, but there are a number of tall (10 km), isolated mountains on the equator on the bright trailing hemisphere. The altitude of the ridge above the surrounding plains is about 20 km, making it taller than all but two other known mountains in the solar system: Olympus Mons on Mars at 21.9 km from base to peak, and the central peak of an enormous crater on the asteroid Ceres, also about 22 km high. 4-26

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It is not unusual for moons and planets to have a larger equatorial radius than polar radius. Spinning objects, even those made of rock and ice, deform slightly into an oblate shape, with a shorter polar diameter than the equatorial diameter. The faster the spin rate and the more deformable the planet, the larger the oblateness. The Earth is 43 km larger across the equator than from pole to pole for this reason. Saturn, spinning much faster and a more deformable planet altogether, is a whopping 10%, or 6000 km, smaller along its rotational axis than across the equatorial plane. Iapetus shows signs of just such a spin-related flattening, but the equatorial ridge is a narrow feature above and beyond the moon’s oblateness. The oblateness of Iapetus can be explained if the moon originally had a much faster rotation rate, a day perhaps as short as 16 h, causing the moon to flatten by about 4.6%. When moons and planets first form, they are warm due to the energy released from all the material that forms them coming together as well as energy released from radioactive materials naturally present in the solar system. Over time this energy is lost to space in the form of radiation, but when Iapetus first formed it would have been warmer and partially molten in its interior, allowing an initially rapid spin to deform it into an oblate (flattened) spheroid. As it cooled, it froze into this oblate state. As its rotation rate slowed due to tidal locking, it retained its flattened profile. In order to produce the ridge from an early, rapid rotation, Iapetus must have formed quickly and cooled quickly. This could be the result of early heating from a radioactive isotope of aluminum that decays quickly, allowing the moon to freeze-in a prominent equatorial bulge while it was still rotating quickly. A problem with this scenario, though, is why Iapetus alone, of all the moons in the solar system, has such an equatorial ridge. Explaining its uniqueness in this regard requires invoking special circumstances for its formation, such as an impact that changes the rate of rotation more quickly than tidal de-spinning (Kuchta et al 2015). One alternative explanation for the ridge relies on another unusual aspect of Iapetus: the large size of its orbit. The strength of the force of gravity between a planet and its moon is inversely proportional to the square of the distance separating the two, and proportional to the mass of the moon. It is also proportional to the mass of the planet. Consider two otherwise identical moons, with one on an orbit twice as large as the other. The moon on the larger orbit feels only one-quarter the gravitational pull from the planet as the moon on the smaller orbit. Moons orbit their respective planets rather than orbit the Sun because they are much closer to their planets than they are to the Sun. This proximity to the planet wins out over the Sun’s much greater mass than the planet’s mass. Similarly, there is a region around a moon where its gravitational influence dominates the gravitational influence of the planet. The region around any object where its gravity is the dominant gravitational force is called the Hill sphere, named after 19th century American astronomer George William Hill. Large, massive objects have large Hill spheres, and objects that are very far from the body they orbit also have large Hill spheres. Our moon orbits safely within the Earth’s Hill sphere. Were it about four times further from the Earth, however, it would be outside it, and the gravitational influence of the Sun would win out over that of the Earth, eventually causing the moon to enter its own independent orbit around the Sun. The large mass 4-27

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of Saturn and the other giant planets together with their great distance from the Sun gives them large Hill spheres. This is why they were able to capture large collections of irregular satellites from interplanetary space while of all the terrestrial planets there are only two captured moons at Mars, which borders the asteroid belt. By virtue of its great distance from Saturn and relatively large size for such a distant moon, Iapetus itself has a large Hill sphere. This means that there is a relatively large region of space around Iapetus in which it can harbor its own satellites, moons of a moon as it were, or indeed even a system of rings around the moon. If Iapetus were hit by another moon of Saturn or an interplanetary interloper, the resulting debris may have formed a ring around Iapetus. Following an impact onto Iapetus, its spin would be tilted by the impact to the same direction as the orbiting debris from the impact. The ring would thus orbit above Iapetus’s equator. But such a ring would be short-lived due to perturbations from the Sun and Saturn as well as interactions between the particles in the ring. Those hypothesized Iapetian ring particles would have rained down on the moon, piling up to form what we now see as a cratered equatorial ridge (Levison et al 2011). Lending support to this hypothesis is the unusual density of craters on Iapetus. Its great distance from Saturn means that it should suffer fewer impacts, and impacts at lower speeds which produce smaller craters, than its sibling moons on smaller orbits. In reality it has more large impact basins than the inner moons, an observation that is difficult to reconcile with a population of impactors from outside the Saturn system (asteroids and comets). The clustering of irregular satellites into groups of small moons sharing similar orbits argues for an evolving population of moons, growing when Saturn captures an interplanetary interloper or when an interloper hits and shatters a moon. Iapetus, with its combination of a large orbit and large size, is uniquely positioned in the Saturn system to capture some of this debris within its Hill sphere and eventually have its surface battered by the infalling material. We noted above that the dark material on Iapetus has a different spectrum than Phoebe, indicating that while the Phoebe ring dust may have triggered the sublimation and condensation transport of ice from the leading hemisphere to the trailing hemisphere, the dark material itself is not predominantly from Phoebe. Other spectral measurements of the moons and rings suggest that the dark material, seen also on the other moons, comes from outside the Saturn system. The source may be those same captured comets that produced a ring around Iapetus before eventually falling onto the surface and, to a lesser extent, bombarded the other inner moons. The top foot or two of material on Iapetus’s dark side may be a collection of carbon-rich cometary gunk that formed much further from the Sun. It has now been conveniently collected on Iapetus for some future mission of exploration to collect and analyze to help us better understand the history of our solar system and, in particular, the outer solar system. While Iapetus’s large distance from Saturn and relatively large size may make it the ideal candidate among Saturn’s moons to have a ring of its own, many of the other moons could have had material orbiting them at some point in the past. With Saturn capturing material from interplanetary space and with a system of rings potentially supplying material to the inner moon region, the other mid-sized moons could have 4-28

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Figure 4.22. Three views of the equatorial region of Rhea. Left: the blue color is a result of combining images taken through infrared, green, and ultraviolet filters, where blue indicates cleaner water ice. Center: the ratio of infrared to ultraviolet wavelengths here emphasizes the strength of the blue signal seen in the left image, with dark spots corresponding to the bluer regions. Right: colors here indicate topography, with purple indicating low-lying areas, and reddish-pink at altitudes up to 6 km above the crater floors. Image credit: NASA/JPL-Caltech/SSI/LPI.

temporarily captured their own rings. Some evidence for a crashing ring onto the moon Rhea lies in its so-called blue pearls. These are bluish markings on high elevations along Rhea’s equator, right where in-falling ring particles would be expected to hit (figure 4.22). The bluish color (only apparent when the image is processed to enhance color differences) is due to cleaner ice, suggesting that it has been exposed relatively recently, perhaps as a result of a string of recent impacts at those locations.

4.8 Enceladus: Saturn’s old faithful The first unambiguous evidence that something was happening at Enceladus came from Cassini’s magnetometer, an instrument that measures the strength and direction of the magnetic field as Cassini flies through it. When Cassini flew by Enceladus for the first time in February 2005 it measured a large deflection in Saturn’s magnetic field. The suggested explanation by Michele Dougherty and colleagues on the magnetometer team was that there was an extended atmosphere of some kind near Enceladus (Dougherty et al 2006). The discovery was compelling enough to motivate the project managers to approve a change to Cassini’s trajectory to bring the next flyby of Enceladus, only a few months later, much closer (166 km instead of 1000 km) to its surface14. That next flyby, in July 2005, confirmed that water vapor is erupting from the south polar region of Enceladus, making it the first and only known geologically active icy body in the solar system. The discovery of Enceladus’s geysers is a perfect example of the value of having multiple instruments studying a complex system. It is like the ancient Indian parable of the blind men encountering an elephant: each one comes to a different conclusion based on the part of the animal he touches. The Cassini magnetometer could tell that 14

Trajectory changes are not easy to make. Cassini was almost always on a ballistic (meaning no rocket propulsion being used) trajectory, falling around Saturn from one satellite encounter to another. It required Herculean efforts by the project to design, test, and implement this trajectory change on such short notice.

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Figure 4.23. This temperature map of Enceladus shows a warm region at the moon’s south pole. The expected warm spot corresponds to the point with the Sun most directly overhead. While the temperatures are still quite low, higher-resolution images by the CIRS instrument show that the hot regions are confined to narrow ‘tiger stripe’ fissures crossing the south pole. Image Credit: NASA/JPL-Caltech/GSFC.

there was some sort of electrically conducting ‘barrier’ to Saturn’s magnetic field in the vicinity of Enceladus and that the barrier was larger than the moon itself. An atmosphere of water vapor would be consistent with this measurement. The CIRS measured temperatures on Enceladus and found that four fissures crossing the south polar terrain were much warmer than the surrounding terrain, and too warm to explain by solar heating (figure 4.23) (Spencer et al 2005). UVIS observed the spectrum of a star as it passed behind the south polar region of Enceladus in a stellar occultation. The starlight dimmed near the south pole, and the spectrum of the light that got through to UVIS matched laboratory spectra of water vapor (Hansen et al 2005). The Ion and Neutral Mass Spectrometer (INMS) directly sampled water molecule products as it flew by Cassini (Waite et al 2005). The CDA detected enhanced numbers of microscopic solid particles hitting it during the flyby (Spahn et al 2005). Finally, Cassini images revealed the active geysers themselves (figure 4.24) (Porco et al 2005). Something is heating the interior of Enceladus enough to boil water vapor off into space through fissures over the south pole (figure 4.25). Based on the amount of water vapor being produced (determined from the UVIS stellar occultation measurements), the speed of the vapor (determined from images and occultations), and measurements of Enceladus’s gravity by tracking Cassini’s radio signals, we know that the source of the geysers must be liquid water beneath the south pole. Precise measurements of how Enceladus rotates as it orbits Saturn show that the wobble of the surface due to changing tidal forces (figure 4.7) is too large if the moon were solid from surface to core. Instead, the surface must be decoupled from the interior by a global subsurface ocean of liquid water. Models indicate that the ocean is a few tens of kilometers in depth, and that it is buried beneath an ice shell of variable thickness, ranging from perhaps just 1 or 2 km at the south pole to 20–30 km at the equator, and perhaps 10 km at the north pole. 4-30

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Figure 4.24. Cassini’s cameras capture the geysers of water vapor and ice crystals erupting from the ‘tiger stripe’ fissures the cross the south pole of Enceladus. This image demonstrates the discrete sources for the geysers. The shadow of the moon partially blocks the lower portion of the jets from the more distant fissures over the horizon. Image Credit: NASA/JPL-Caltech/SSI.

This ocean is the source of the geysers, but it raises lots of questions for the history of Enceladus. It is difficult to maintain such an ocean in equilibrium with the ice above it without a large source of energy. The uneven ice surface above the ocean should flow laterally to reach a uniform thickness. The water in the ocean may either be freezing out onto this ice shell ceiling, or the ice shell might be melting into the ocean. Another question is why there are geysers only at the south pole. A commonly invoked but not entirely satisfying explanation is that an impactor punched through the ice shell in the past, resulting in a local thinning. But if the liquid water is in a global subsurface ocean, why are the geysers only at the south pole? The active zone is centered quite precisely on Enceladus’s south pole. And why is Enceladus active at all, of all of Saturn’s moons? The answers to both questions are at least partly due to tidal interactions with Saturn. If there is a warm blob somewhere inside Enceladus, it would be extremely unlikely for it to be perfectly centered, or perfectly uniformly distributed, in the moon’s interior. Wherever random chance places it, it causes material to expand and rise toward the surface, in much the same way that there are volcanic upwellings on the Earth. If there is a large impact that thins the ice shell at one location, it also affects the mass distribution of the moon. This gives it an oblong distribution of mass even while the outer shape remains essentially spherical. Once it has that oblong distribution, tidal flexing of the moon will cause it to tip over until the low-density blob is on its rotation axis, meaning the thin part of the ice shell will end up at either the north or south pole (Nimmo & Pappalardo 2006). A flip of the coin, or moon in this case, 4-31

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Figure 4.25. This view of the active south polar region of Enceladus shows geyser activity from two of Enceladus’s ‘tiger stripe’ features: Baghdad and Damascus. The resolution is about 70 m/pixel. NASA/JPL-Caltech/SSI.

placed Enceladus’s hot spot at the south pole (figure 4.26). The recent results showing that the subsurface ocean is global means that the south pole appears hotter because the subsurface water is close enough to the surface that it can erupt as geysers, carrying the heat of the warm water with it. The water beneath the north pole, and the rest of Enceladus, is also warm, but the heat it gives the thicker overlying ice shell is not enough to significantly raise the temperature of the surface of the moon. But what provides the energy to maintain a large subsurface ocean on a tiny moon of Saturn? There is one other highly active moon in the solar system: the innermost Galilean satellite of Jupiter, Io15. Io’s surface is free of craters due to continuous eruptions from sulfur volcanoes. The heat source for Io’s volcanoes is tidal flexing. Before a moon settles into synchronous rotation, tidal stretching continually changes its shape as the long dimension points at the planet, and the moon’s rotation causes different pieces of real estate to move onto the long dimension. It takes energy to stretch a moon this way, and that energy comes 15 Voyager 2 discovered plumes in the tenuous atmosphere of Neptune’s large moon, Triton. Triton has lots of similarities to Pluto, and it would be a very interesting place to visit again!

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Figure 4.26. This mosaic of Cassini images is centered on Enceladus’s south pole, marked by four prominent fissures nicknamed ‘tiger stripes’. The geysers emanate from these fissures. Surrounding the south pole is a complex pattern of fractures produced by tidal stresses and the upward pressure of the warm water beneath the south pole. Image Credit: NASA/JPL-Caltech/SSI.

from the rotation of the moon until it settles into synchronous rotation (figure 4.7). In the case of Io, it keeps flexing even though it is in synchronous rotation because its orbit is perturbed in a regular fashion by the neighboring moons Europa and Ganymede. Io orbits Jupiter exactly twice for each orbit of Europa, and Europa orbits Jupiter exactly twice for each orbit of Ganymede. These strong orbital resonances mean that the Io is pulled onto an eccentric (non-circular) orbit by the gravity of the other moons, and this results in a changing amount of tidal stretching of the moon as its distance to Jupiter varies over the course of an orbit. It is amazing that what may seem like a small perturbation can have such dramatic consequences, but the existence of volcanic activity on Io was famously predicted by Stan Peale, Pat Cassen, and R. T. Reynolds in a paper published just months before volcanoes were discovered by Voyager 1 (Peale et al 1979). They calculated that the heat generated by the tidal flexing due to its slightly eccentric orbit would be great enough to power volcanic activity. If Io is a textbook case for tidal heating, Enceladus is anything but. Although Enceladus is in a 2:1 resonance with Dione (it orbits Saturn twice for each of Dione’s orbits), it is not clear that this can provide the amount of heat observed flowing from Enceladus’s south pole. Worse, the energy required to maintain a global ocean is

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much greater even than what is inferred from the high temperatures we see at the south pole. Heating from the decay of any radioactive material present in Enceladus is almost certainly much weaker than tidal heating due to Enceladus’s small size and correspondingly small reservoir of radioactive materials, so tidal flexing remains the favored mechanism. Also, the activity of the geysers seems to vary with Enceladus’s position in its orbit, increasing and decreasing with the small changes in the moon’s distance from Saturn consistent with the predictions of tidal flexing controlling the amount of gas production. However, the tides affect not just the amount of heat, but also the configuration of the cracks at the south pole which get pulled and stretched over the course of an orbit. The water vapor blasting through these cracks may have larger or smaller paths to open space as the fissuers flex over the course of an orbit. One difficulty is that the amount of power generated by tidal flexing depends on the physical properties of the interior of the body being flexed. It turns out that if Enceladus already has a global ocean in its interior then tidal flexing may be able to generate enough power to keep the ocean liquid. But if you start with a solid interior, tidal flexing, at least at the current rate, is insufficient to melt the ice to produce the ocean. Furthermore, the current value of the eccentricity of Enceladus does not seem to be in equilibrium with the perturbations from Dione and the dissipation of energy from the south pole. The full story of the origin of Enceladus’s ocean, how old it is, whether it is freezing out or not, and what source of energy maintains it, is still being discovered and may require observations from future missions dedicated to studying this intriguing moon. The presence of liquid water is a bellweather for habitable environments. Liquids enable much more rapid mixing of chemicals and more rapid chemical reactions necessary for life. It is far from the only ingredient necessary, of course. The raw ingredients for life in the form of heavier elements and more complex molecules, energy to drive chemical reactions, and physical mechanisms to confine and concentrate molecules are also necessary. Some of these other ingredients are present at least at some level at Enceladus. Measurements of the composition of dust and gas in the plumes by Cassini have indicated the presence of some complex carbon-bearing molecules, indicating that some chemical reactions are taking place in the ocean. There are very few environments in our solar system that are even potentially habitable, and we now know that this tiny moon of Saturn is one of them. The geysers have consequences far beyond Enceladus itself. The water vapor carries small amounts of silicate particles along with it from Enceladus’s rocky core and condenses into grains of solid ice after it leaves the fissures. Much of this material reimpacts Enceladus, coating the south polar region like a fresh snowfall. The southern hemisphere is also tectonically deformed by the upward pressure of the warm water, causing a network of fractures. The southern hemisphere as a consequence is completely free of craters, while the northern hemisphere has a more conventional cratered surface (figure 4.27). But much of the material escapes Enceladus’s gravity and goes into independent orbits around Saturn, forming its tenuous E ring. As is the case with the material in the Phoebe dust ring, the E ring particles impact the neighboring icy moons of Saturn, as well as the main rings. Clearly Enceladus has had a complicated history that involved episodes of greater heating in the past. It is possible that its ocean is a remnant from strong radiogenic 4-34

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Figure 4.27. This global view of Enceladus shows the dichotomy between the southern hemisphere, which is almost entirely free of impact craters, and the more heavily cratered northern hemisphere. Upwelling of ice near the south pole as well as fallout of icy grains from the geysers resurfaces the southern hemisphere, quickly destroying or covering up craters. Image Credit: NASA/JPL-Caltech/SSI.

heating shortly after its formation, but higher eccentricities in the past would likely be necessary to maintain the liquid ocean over the age of the solar system. Such high eccentricities could be produced as the moons’ orbits tidally evolve, and they pass into and out of resonances with each other. Mimas ended up on a very different evolutionary path from its neighbor perhaps due to subtle differences in orbital evolution that prevented it from ever getting hot enough to create or maintain a liquid ocean. We may not be able to determine a unique answer for the history of Enceladus, but we have learned from Cassini that it is a potential abode for life. Its history is intertwined with the other moons through resonances, and with Saturn’s mighty ring system as we will see in the next chapter.

4.9 The fabulous five: Cassini’s parting look at five ring-moons Cassini’s final year included two sequences of orbits that brought the spacecraft progressively closer to Saturn before its fateful plunge on 15 September 2017. The first of these sequences was called the ring-grazing orbits from December 2016 to 4-35

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Figure 4.28. What is going on with Pan (right)? This moon’s bizarre appearance makes it my favorite object in the solar system. The blurred appearance of Atlas’s equatorial ridge is not an image artifact: the material is really that smoothly and evenly deposited. Moons are shown to scale. Image credit: NASA/JPL-Caltech/SSI.

April 2017. In these orbits the spacecraft crossed the plane of Saturn’s rings just outside the outer edge of the dense, main rings, providing it with an opportunity to view not only the rings at high resolution but also the small moons in and near the outer regions of the rings. The particular timing of Cassini’s trajectory in the ring-grazing orbits allowed it to make high-resolution measurements of five of these moons in what mission scientists dubbed ‘the fabulous five’ flybys. Two of those moons, Pan and Daphnis (figure 4.28), orbit within Saturn’s A ring, and all five orbit much closer to Saturn than Cassini passed on these orbits, so the spacecraft could not get as close to them as it did when making the close targeted flybys that allow the stunning closeups seen, for example, in figures 4.2 and 4.25. The other three of the fab five are also closely linked to the rings: Pandora (figure 4.29) is one of the shepherd satellites of the F ring (chapter 5), Atlas (figure 4.28) orbits just beyond the edge of the main rings, and Epimetheus (figure 4.30) is one of a pair of moons that creates a series of strong waves in the rings (chapter 5). The ‘fab five’ show, to varying degrees, the effects of a moon living with rings. Pan, Daphnis, and Atlas show pronounced equatorial ridges of ring particles that have landed on the moons. Pan and Daphnis, while tiny by satellite standards, tower many km above and below the rings which are only about 10 m thick. So the ring particles land preferentially on the equators of these moons. Pandora and Epimetheus orbit in the outer dusty regions of the rings that are both more tenuous and more vertically extended than the main rings. They have accreted a more uniform coating of dust, giving them a somewhat softened appearance. We will explore the relationship between the rings and these nearby ring moons in more detail in chapter 5. 4-36

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Figure 4.29. Pandora, 84 km across, was imaged by Cassini on 18 December 2016, during the ring-grazing orbits with a resolution of 240 m/pixel. The moon orbits just outside Saturn’s narrow and dusty F ring and has collected dust from it, filling in its craters and giving it a soft appearance. Image credit: NASA/JPL-Caltech/SSI.

Figure 4.30. Epimetheus is seen here with Saturn in the background at a resolution of 520 m/pixel. Although it is heavily cratered, this moon, which is about 113 km in diameter, has a soft appearance due to dust from the nearby rings filling in craters. Image credit: NASA/JPL-Caltech/SSI.

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Further reading The review chapters by Dones et al (Icy satellites of Saturn: impact cratering and age determination p 613), Jaumann et al (Icy satellites: geological evolution and surface processes p 637) and Spencer et al (Enceladus: an active cryovolcanic satellite p 683) in Dougherty et al (2009), include discussions of many of the processes discussed in this chapter as well as references to prior works pre-dating Cassini. The following is a small selection of further technical research papers on Saturn’s moons: Denk & Mottola (2019), Tajeddine et al (2014), Waite et al (2017) and Buratti et al (2019).

References Buratti B J et al 2019 Science 364 eaat2349 Denk T and Mottola S 2019 Icarus 322 80–102 Dones L et al 2009 Icy satellites of Saturn: impact cratering and age determination Saturn from Cassini–Huygens (Berlin: Springer) pp 613–35 Dougherty M K et al 2006 Science 311 1406–9 Dougherty M K, Esposito L and Krimigis L (ed) 2009 Saturn from Cassini–Huygens (Berlin: Springer) Hansen C J et al 2005 Science 311 1422–5 Jaumann R et al 2009 Icy satellites: geological evolution and surface processes Saturn from Cassini–Huygens (Berlin: Springer) pp 637–81 Kuchta M et al 2015 Icarus 252 454–65 Levison H et al 2011 Icarus 214 773–8 Nimmo F and Pappalardo R T 2006 Nature 441 614–6 Peale S J, Cassen P and Reynolds R T 1979 Science 203 892–4 Porco C C et al 2005 Science 311 1393–401 Spahn F et al 2005 Science 311 1416–8 Spencer J R and Denk T 2010 Science 327 432 Spencer J R et al 2005 Science 311 1401–5 Spencer J R et al 2009 Enceladus: an active cryovolcanic satellite Saturn from Cassini–Huygens (Berlin: Springer) pp 683–724 Tajeddine R et al 2014 Science 346 322–4 Thomas P C et al 2007 Nature 448 50–3 Verbiscer A J, Skrutskie M F and Hamilton D P 2009 Nature 461 1098 Waite J H et al 2005 Science 311 1419–22 Waite J H et al 2017 Science 356 155–9

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The Ringed Planet (Second Edition) Cassini’s voyage of discovery at Saturn Joshua Colwell

Chapter 5 The rings—a dance of gravity

5.1 Introduction While the moons of Saturn are graced with poetic names from Greek mythology (and other mythologies, once the number of satellites proliferated), its glorious rings are stuck with the letters A, B, and C. Perhaps it was ignorance about what the rings actually are that led early astronomers to be hesitant to fix them with a more noble and definitive name such as Titan or Tethys. Designating a set of things by letters from the alphabet smacks of uncertainty and impermanence. And we must admit that a considerable degree of uncertainty remains about some of the fundamental questions of Saturn’s rings: how did they form? How old are they? How long will they last? What did they look like 1, 10, or 100 million years ago? What will they look like 10 million years from now? Was Saturn merely lucky to win the planetary rings jackpot, or is there something about its place in the solar system that makes it the best place for a broad, massive set of rings to set up shop? We will explore the answers to these questions in chapter 6 thanks to a broad set of intriguing measurements made by Cassini during its Grand Finale. But before we attempt to understand the origin of Saturn’s rings, we must first understand just what they are. The broad expanses, tightly wound spirals, mountains of snowballs, wispy tendrils, and many other complex structures that make up Saturn’s rings are the focus of this chapter. Prior to the Voyager flybys of the early 1980s it was thought that the rings were the remnant debris from the formation of Saturn and a relatively quiescent and perhaps uninteresting system, evolving slowly over the eons. The Voyagers showed a wealth of structure within the ring system that immediately upended this view. A handful of broad rings that could be counted (or lettered in this case), on one hand, now came into focus as myriad structures as small as the Voyager cameras could see. These newly discovered waves, ringlets, and other narrow features, and the rings’ nearly pristine icy composition, suggested a recent origin and changes happening on

doi:10.1088/2053-2571/ab3783ch5

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timescales of years, not millennia, let alone millions or billions of years. Untangling these mysteries has been one of the primary goals of the Cassini mission1.

5.2 The main rings Before we tackle these questions and the phenomena that shape the rings, let’s take a tour of the system. Figure 5.1 provides an overview of the ring system with a view similar to what one would get from a terrestrial telescope with the exception of Saturn’s prominent shadow cast across the rings. From our Earthly vantage point, the Sun is shining on Saturn from almost the same direction as our viewpoint. From the Earth, therefore, the shadow of the planet on the rings is behind the planet. Figure 5.2 illustrates the regions of the main rings. The Cassini Division between the A and B rings resembles the C ring in color and density of material. The broad B ring is the brightest in reflected sunlight because it is the densest ring and is completely opaque in some areas. We know from how they reflect, absorb, and scatter sunlight that the particles are composed mostly of water ice. While we have never observed an individual ring particle, the small embedded moons Pan and Daphnis discussed in chapter 4 may in some ways be like giant ring particles. Those ring moons are only about half as dense as water ice. By studying clumps and waves in the rings, discussed in more detail below, we have strong evidence that the ring particles also have a very low density, like fluffy snowballs more than ice cubes. For a cloud of countless particles such as the rings, a useful measure to describe them is their transparency, measured as a percentage of light incident on the rings that makes it through to the other side. Figure 5.3 shows the transparency of the

Figure 5.1. Saturn, its rings, and a few small moons are seen in this image taken by Cassini shortly prior to its Saturn Orbit Insertion (SOI) maneuver in which it fired its main engine to lose enough energy to be captured by Saturn’s gravity. Image Credit: NASA/JPL-Caltech/SSI. 1

Results on Saturn’s rings from the initial phase of Cassini’s mission together with technical discussions and references for the processes discussed in this chapter are reviewed in the five rings chapters in Saturn from Cassini-Huygens (Dougherty et al 2009) by Colwell et al, Schmidt et al, Cuzzi et al, Horanyi et al, and Charnoz et al.

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Figure 5.2. The main rings (A, B, C, and Cassini Division) and the F ring. The faint D ring lies interior to the C ring and is not visible in the geometry and lighting of these images. Even fainter E and G rings lie beyond the F ring. Images: NASA/JPL-Caltech/SSI.

Figure 5.3. The normal transparency of the rings from a stellar occultation observed by Cassini’s Ultraviolet Imaging Spectrograph, where in this context ‘normal’ means it is the transparency that would be measured from a line of sight perpendicular to the plane of the rings. The curve goes to 1 at gaps in the rings (100% transparent) and dips to 0 in the opaque central region of the B ring.

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rings from an observation of a bright star (Beta Centauri or Hadar) as the rings passed in front of it from the point of view of Cassini. The main ring regions seen in the images can also be identified in this plot, but we also see that there is a wealth of structure in each ring region. This type of observation, called an occultation, was also made by sending radio transmissions from Cassini through the rings to the Earth at different radio frequencies and observing that transmitted radio signal with radio telescopes on the Earth. The rings are more or less transparent to different radio frequencies depending on how large or small the ring particles are. Very small particles are virtually invisible to the long wavelengths of low-frequency radio signals, while high-frequency radio waves (which have a shorter wavelength) are absorbed and scattered by small and large particles. From these measurements we know that the particles in the main rings range in size from about 1 cm to several meters, though identifying the size of the largest particles is complicated by their tendency to clump together. In chapter 3 we saw that the rings owe their existence to the tidal force exerted on the ring particles by nearby Saturn: while the weak gravitational attraction of the particles to each other tries to assemble them into larger objects, they race around Saturn at different rates and are thus torn away from each other by the planet’s gravity. This so-called tidal force is very strongly dependent on distance from Saturn, and beyond the Roche limit discussed in chapter 3, particles are able to stick together. The Roche limit itself, however, is not a singular boundary. For example, inside the Roche limit it is possible for a small particle to gravitationally stick to a larger ring particle, while two large particles might be torn apart by tides. This is because a ring particle, like a moon or planet, has its own tiny gravitational sphere of influence, or Hill sphere (see also chapter 4). The closer the ring particle is to Saturn, the smaller is its Hill sphere. At the Roche limit, the Hill sphere is larger than the particle, but not large enough to fit another comparably sized particle inside it. A small particle, however, can fit within the Hill sphere. At a certain distance from Saturn, well inside its Roche limit, the size of a ring particle’s Hill sphere shrinks to become smaller than the ring particle itself. At those locations Saturn’s tidal force is too strong to allow even a pebble to rest on the surface of a larger ring particle by gravity alone. Down on such a ring particle is up toward Saturn, not toward the particle. The location of this point depends on the densities of the particles, but it’s well within Saturn’s main rings. The A ring, particularly near its outer edge, however, is close enough to Saturn’s Roche limit to allow some particles to gravitationally stick together. This limited accretion in the outer rings shapes, quite literally, the nearby moons (sometimes called ring moons) and the rings themselves. In spite of its name, the Hill sphere is not spherical. For objects that are very close to Saturn, the Hill sphere is flattened: particles are pulled by Saturn’s gravity off the north or south poles of a moon more readily than off the equator. This can be seen quite dramatically with the saucer-shaped moons Atlas, which orbits just outside the A ring edge (figure 4.28 of chapter 4), and Pan, which orbits within the A ring itself (figure 5.4). Further from Saturn the Hill sphere of a moon becomes less flattened (figures 5.5 and 5.6), and the accretion of material onto the moon takes on a more egg-like shape until, even 5-4

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Figure 5.4. Two close views of Pan, which orbits in the Encke Gap near the outer edge of Saturn’s rings, from the ring-grazing orbits in Cassini’s final year. Left: the northern hemisphere is shown at a resolution of 147 m/pixel. Right: the southern hemisphere is shown at a resolution of 224 m/pixel. Both views are of the trailing hemisphere of Pan. That is, it is moving away from us in its orbit around Saturn, and the prominent ridge of accreted ring particles lies on the moon’s equator. The diameter of Pan is 34 km across the equator, but only 21 km from north pole to south pole. Its density is half that of water ice. Image Credit: NASA/JPL-Caltech/SSI.

Figure 5.5. This stunning view of Prometheus, which orbits between the F and A rings, shows its potato shape in the dim illumination of Saturnshine from the left. The surface has a softened appearance due to accretion of material from the F ring which covers and fills in craters. The image resolution is 200 m/pixel. Image Credit: NASA/JPL-Caltech/SSI.

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Figure 5.6. It is difficult to believe that this is a moon from its apparently perfectly smooth, egg-like appearance. Like Prometheus and the other small, inner moons, Methone accretes material from the nearby rings, but its small size means that it quickly fills its Hill sphere, which has an oblong shape this close to Saturn. Methone is only 3 km across. The resolution of this image is 27 m/pixel. No craters can be seen at this resolution, indicating that the moon has accreted material to cover up any impact scars. Image Credit: NASA/ JPL-Caltech/SSI.

further from the planet, the Hill sphere becomes much larger than the moon in all dimensions and does not affect its shape at all.

5.3 Self-gravity wakes Within the A ring itself, ring particles continually collide with each other, but at a snail’s pace2. The reason the collisions are so gentle is that the particles have been colliding with each other for so long. In the main rings a particle collides with a neighbor several times per orbit, and the orbits are only about 10 h long. These collisions are inelastic, meaning that some of the mechanical energy of motion of the particles is converted to heat by compressing or crushing of the surface. Quite rapidly, the relative motion between the particles damps down to a minimum. That minimum collision speed is set by two things: the gravitational stirring of the ring particles themselves, and the small relative velocity due to particles being on slightly different orbits around Saturn. To see how small these collision speeds are, let’s consider a relatively large ring particle with a radius of 1 m. Individual ring particles may be as much as 10 times larger than this in some ring regions though, as we will see below, even defining what 2

This is literally true. Although species of snail have a range of speeds, they actually overlap quite neatly with the range of collision speeds in unperturbed regions of Saturn’s rings. In areas where the ring particles are stirred up for one reason or another, collision speeds may be almost as fast as a walking tortoise’s pace.

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is meant by ‘individual ring particle’ is not as straightforward as one might imagine. Two such particles on neighboring orbits separated by 2 m will have a grazing collision as the inner particle, with its slightly faster orbital speed, laps the outer particle. Since the orbital speeds are precisely calculable from Kepler’s laws of orbital motion and the mass of Saturn, we can calculate the speed that the inner particle passes by, or nudges, the outer particle. This speed varies a little from the inner edge of the rings to the outer edge, but at the center of the ring system at an orbital radius of 110 000 km, the collision speed due to this Kepler shear would be 0.2 mm s–1 (that’s moving about twice the thickness of a human hair each second). The gravity of the ring particles also provides some small collision velocity. If we imagine our two ring particles are alone in the Universe, without the complicating effect of nearby Saturn, the gravitational interaction between them would make them fall toward each other until they hit at a speed known as their escape velocity. For our 1 m ring particle made of porous ice, this is about 6 mm s–1, much faster than the minimum set by Kepler shear. Elsewhere in the rings, as we’ll see below, gravitational nudges from moons can lead to higher collision speeds. The competition between the particles falling toward each other due to their mutual gravitational attraction and the particles falling away from each other due to the difference in the gravitational force they feel from Saturn (the tidal force) can produce ephemeral clumps of particles dubbed self-gravity wakes3. A group of particles in space, left to their own devices, will form a spherical clump. But in orbit around Saturn the particles closer to it orbit more quickly, so the clump gets sheared out into an elongated clump that is canted by about 25° from the direction of orbital motion. Computer simulations of the rings, where the motions and interactions of millions of individual ring particles are calculated, show the formation and evolution of these self-gravity wakes (figure 5.7) (Salo 1995; Lewis & Stewart 2009). Because these clumps are elongated and have a characteristic orientation determined by how quickly the gravitational force changes with distance from Saturn, they affect the appearance of the rings from different viewing geometries. You can see this effect by holding your hand, fingers spread, in front of you, with your fingers pointing away from you and up at a 45° angle. You can easily see past your fingers through the spaces in between them. If you now rotate your hand left or right, your fingers block the spaces in between them from your view. Your hand has become more opaque from the vantage point of your eyes. And so it is with the rings (figure 5.8). Although Cassini’s cameras cannot resolve individual self-gravity wakes, UVIS had a detector that could measure the brightness of a star 1000 times each second as the rings passed in front of, or occulted, the star (figures 5.3 and 5.9). Typical speeds of the rings in front of the star as seen from Cassini were about 10 km s–1, meaning that UVIS could make measurements of the transparency of the rings with a 3

This is a terrible name because they are not wakes at all and, worse, there actually are things in the rings that truly are wakes, as we’ll see later in the chapter. I have done research on these structures and managed to get the name changed to the terrible ‘self-gravity wakes’ from the slightly more terrible ‘gravitational wakes’ that prevailed in the literature prior to Cassini.

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Figure 5.7. This computer simulation of particles in Saturn’s rings shows the formation of ephemeral clumps called self-gravity wakes. Particles are orbiting left to right, and Saturn is down in this view. The lower panel is a zoomed-in view of the highlighted region in the upper panel. The simulation is of a very small patch of the rings. Particles closer to Saturn (at the bottom of the simulation) orbit the planet faster and so move ahead of (to the right) the slower particles further from Saturn (at the top of the simulation). Collisions and gravity between the particles cause them to form clumps, but the orbital shear stretches the clumps out. Simulation courtesy of Mark C Lewis, Trinity University. Movie available at https://iopscience.iop.org/book/978-1-64327-714-1.

resolution of about 10 m. These occultations provide one-dimensional slices of the rings at this high resolution, but they are affected by smear due to the orbital motion of the ring particles themselves, which is generally much faster than the motion of the footprint of the star on the rings. We were able to find some stars that passed behind the rings in such a way that the stellar spot moved slowly relative to the ring particles at certain locations. These particle-tracking occultations provide us with direct measurements of the self-gravity wakes whose collective effects have been seen in numerous observations, such as in figure 5.8, but are too small to see individually (figure 5.10).

5.4 Satellite wakes and shepherding The other kind of wake in the rings truly is a wake, like that produced by a boat moving through water. The moons Pan and Daphins in the Encke and Keeler gaps, respectively, near the outer edge of the A ring produce satellite wakes in the ring on either side of their gaps. These wakes, unlike that of a boat, are asymmetric for the following reason. Ring particles on the inner edge of the gap orbit faster than the moon, while those on the outer edge orbit more slowly. From a vantage point on 5-8

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Figure 5.8. This view of the main rings was taken by Cassini from only 5° above the ring plane. From this geometry the effect of self-gravity wakes is quite apparent: the rings on the far side of the planet in this view are much brighter than those on the near side, where the central region of the rings is completely dark. This is due to the different orientation of the elongated self-gravity wakes with respect to the Sun and Cassini with longitude around the rings. Image credit: NASA/JPL-Caltech/SSI.

the moon, ring particles on either side of the moon travel in opposite directions. The wake is produced by gravitational perturbations on the ring particles by the moon, so downstream from it, and hence the wake direction is in opposite directions on either side of the moon. We are all familiar with gravity as an attractive force, but the consequences of the gravitational interaction between two objects, say a moon and a ring particle, orbiting Saturn can be counterintuitive. Let’s place ourselves on the moon Pan and consider the passage of our faster, neighboring ring particles on the inner edge of the Encke Gap some 160 km away. At that distance, the ring particles orbit Saturn in 13.820 h, while Pan takes about 90 s longer to complete one revolution of the planet. The ring particles drift past Pan at a rate of 10 m s–1. Pan itself has a diameter of about 30 km, so it takes almost an hour for a ring particle at the edge of the gap to pass Pan, and 2.66 yr before that same particle will have gone all the way around to lap Pan again. This time between successive encounters is called the synodic period. In that time, the particle will have orbited Saturn about 1685 times (and Pan will have orbited it exactly one fewer time). That’s a lot of orbits, and in that time in the crowded A ring, that particle will 5-9

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Figure 5.9. This schematic shows Saturn and its rings and inner moons against a backdrop of bright stars, some of which pass behind the rings from the point of view of Cassini. By observing the brightness of a star during such an event, called a stellar occultation, we obtain a very high-resolution measurement of the structure of the ring along the occultation path. When the path closely matches the orbital motion of the ring particles around Saturn, the resolution can be as fine as a single large ring particle (figure 5.10). Movie available at https://iopscience.iop.org/book/978-1-64327-714-1.

Figure 5.10. The brightness of the star β Persei is occasionally reduced to zero for long stretches as the star is completely occulted or blocked by opaque self-gravity wakes. In between the self-gravity wakes (the largest here is from about 450 to 580 m on the horizontal axis) the stellar signal fluctuates by large amounts due to the varying numbers of small ring particles blocking the star.

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have hundreds or thousands of collisions with its neighbors, and its motion will completely forget the last time it drifted by Pan. When it does drift by Pan, it and all of its immediate neighbors will be gravitationally attracted by Pan in a coherent way, deviating ever so slightly from their circular orbits. The eccentricity of an orbit is a measure of how much it deviates from a perfect circle. Ring particle orbits are nearly perfect circles, but the encounter with Pan gives the particle a small eccentricity. Moving closer to Pan (and further from Saturn), the particle’s orbital motion slows a little so that as it completes its passage of Pan it is moving away from it more slowly than it was approaching it at the beginning of the encounter. This asymmetry between the time spent catching up to Pan and the time pulling away from it produces the counterintuitive result that the net effect of the encounter is for Pan to repel the particle rather than attract it! Here’s how that happens. When the particle is catching up to Pan, the latter is pulling it forward, increasing the orbital energy of the particle. When the particle is moving past Pan, the latter is pulling back on it, reducing the orbital energy of the particle. Because the second phase of the encounter lasts longer and because the particle is a little closer to Pan, on average, during the second phase (because it has been pulled toward Pan during the first phase), the second phase wins and the net effect is for the particle to lose orbital energy and end up on a smaller orbit, closer to Saturn (and further from Pan) than before the encounter (figure 5.11). Everything reverses for a particle on the outer edge of the gap, which orbits more slowly than Pan, so the net effect of that encounter is to put the particle on a more energetic and higher orbit, again further from Pan. Thus Pan pushes the ring particles away from itself and creates a gap in the rings. This entire interaction takes

Figure 5.11. This schematic diagram of the interaction between a ring particle and a moon is in a reference frame orbiting with the moon. The ring particle in this case is closer to Saturn and so orbits (counterclockwise) more quickly than the moon and moves right to left in this view. The moon pulls the particle closer to it, giving the particle an eccentricity. After the particle passes the moon, it continues to pull on the particle, slowing it down. The net result is that the particle is pushed away from the moon.

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Figure 5.12. This Cassini image of the outer portion of the A ring shows Pan at the center of the Encke gap. Most of the dark circular features are spiral density waves. There are tilted features like the wake of a boat on either side of the Encke gap due to perturbations from Pan (above and trailing Pan on the outer edge, and below and leading Pan on the inner edge). The narrower Keeler gap is very close to the outer edge of the ring. Image Credit: NASA/JPL-Caltech/SSI.

a full orbit of the ring particle, during which time it is given a small eccentricity by Pan, and afterwards, collisions between the ring particles jostle them back onto parallel circular orbits again so that at the next encounter with Pan, more than two and a half years later, the process repeats (figure 5.12). Because there are a large number of particles following very similar perturbed paths in the region of the rings, the moon creates a wavy satellite wake on either edge of the gap. Daphnis does the same thing in the Keeler gap (figures 5.13 and 5.14). Prior to Cassini’s arrival at Saturn we anticipated that many more moons would be discovered in other gaps in the rings, but extensive searches have turned up nothing. Newton’s third law of motion explains that Pan can’t push on the ring particles without the particles pushing back on it. If it were not within a gap in the rings, it would slowly be pushed away from the edge. As it turns out, the outer edge of the A ring itself interacts with the moons Janus and Epimetheus in just this way. The outer edge of the A ring is in an orbital resonance with these two moons which themselves are in a special orbital relationship with each other: they essentially share the same 5-12

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Figure 5.13. The best image of Daphnis obtained by Cassini shows the scalloping of the edge of the Keeler gap. A narrow ringlet threads the narrow space between the moon and the edge of the gap, acting as a tracer of the dynamics of the moon, Saturn, and ring particles. Image Credit: NASA/JPL-Caltech/SSI.

orbit and are called co-orbital satellites. The resonant interaction between the moons and the ring particles at the edge keep the particles from drifting away from each other. The resonance acts like a bookend to the rings. As with Pan and the Encke gap, Janus and Epimetheus push on the particles at the outer edge of the A ring, maintaining an abrupt ring boundary, and the particles push back on the moons, causing them to slowly drift away. As they do so, the resonance location also moves away from Saturn, and the ring edge moves outward to remain at the resonance location. Without this ring–satellite interaction, collisions between the ring particles would cause the ring to rapidly spread out into a broader diffuse ring. At this 7:6 resonance, the approximate relation holds that particles at the ring edge orbit seven times for each six orbits of Janus. As a result, the pattern of the perturbations on the ring edge has seven lobes (figure 5.15).

5.5 Density waves and bending waves in the rings We saw in chapter 4 that orbital resonances between objects can enhance the effects of gravitational interactions between the objects in resonance by repeating small perturbations in phase with each other over the course of many separate 5-13

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Figure 5.14. This Cassini image shows Daphnis at the center of the Keeler gap. The inner edge of the gap is towards the top of this image and orbital motion is right to left. Particles at the inner edge of the gap orbit faster than Daphnis, so particles to the left of Daphnis at the inner edge in this image have just drifted by Daphnis, and the perturbations to their orbits is evident from the scallopled appearance of the gap edge there. At the outer edge, particles orbit slower than Daphnis, so it is particles to the right of Daphnis in the image that have just encountered the moon. The particles that are about to encounter the moon (to the right of Daphnis on the inner edge and to the left at the outer edge) have not had an encounter with Daphnis in years. In that time, collisions have damped out any eccentricities so the edge is smooth and circular. Image Credit: NASA/JPL-Caltech/SSI.

Figure 5.15. This mosaic shows most of the seven-lobe structure at the outer edge of the A ring from multiple Cassini images due to the influence of the Janus 7:6 Lindblad resonance at the ring edge. Near the bottom of the image the edge of the Keeler gap, home to the moon Daphnis, shows many more edge ripples due to the close proximity of the moon to the edge. The amplitude of this edge wave is comparable to the 35 km width of the Keeler gap. Image Credit: NASA/JPL-Caltech/SSI. Mosaic and image processing by Joseph Spitale, Planetary Science Institute.

interactions. The simplest type of orbital resonance is called a mean motion resonance and is one in which the ratio of the orbital periods of the two objects is equal to the ratio of two whole numbers, usually small numbers, and usually different from each other by only 1 or 2. So, for example, a 2:1 mean motion resonance would involve one object orbiting Saturn exactly twice in the time it takes the other object to orbit exactly once. The strongest resonances are those in which the difference between the numbers is small. A 5:4 resonance is a first-order resonance because 5 minus 4 equals 1, and it is stronger than a 5:2 resonance, for example. The latter, a third-order resonance, is weaker because the higher order is due to the much greater separation, and hence weaker gravitational pull, between the two objects. A dramatic consequence of resonances in Saturn’s rings is the presence of dozens of waves, the largest of which is shown in figure 5.16. 5-14

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Figure 5.16. The largest density wave in Saturn’s rings is shown in this high-resolution image taken during Cassini’s Grand Finale orbits, on 4 June 2017, from a distance of 76 000 km, revealing the rings at a resolution of 530 m/pixel. The grooved appearance is due to the tightly wound spiral wave wrapping around Saturn dozens of times. This wave is the Janus 2:1 density wave in the inner B ring and extends well over 400 km across the rings. The resonance location is near the right edge of the image, and the grooves get closer to each other on the left as the wave propagates in that direction. The changing wavelength carries useful information about the mass of the rings. If you look closely you may notice some irregularities in the pattern. These are because this is actually the superposition of waves due to resonances with Janus and with Epimetheus on nearly identical orbits. Image credit: NASA/JPL-Caltech/SSI.

Resonances get a little more interesting due to Saturn’s flattened shape. The mass within Saturn is not distributed in perfect spherical symmetry (chapter 8). One consequence of this is that objects orbiting Saturn have not one but three different natural orbital periods. The one we are most familiar with is the time for a particle to complete 360° of orbital motion. The average angular speed (measured in degrees or radians per second) of an object to complete this orbital period is called the mean motion. An object on an eccentric orbit has a changing distance from Saturn over the course of one orbit, moving from pericenter at its closest point to Saturn and back to apocenter at its furthest. One complete cycle from pericenter to apocenter and back to pericenter again takes a different amount of time to complete than the time to complete 360° of motion around the planet. The time to complete this cycle is called the epicyclic period. Furthermore, if the object is on an inclined

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orbit, one that is tilted relative to the equator of Saturn, it undergoes a third periodic cycle above and below the equatorial plane (also called the ring plane). This period for vertical motion is different than the other two. When describing resonances we usually work with the angular rates of motion, also called angular frequencies, rather than the periods. The frequencies are just the inverse of the periods. For example, the Earth’s mean motion period of 1 yr means that it travels 360° in about 365.25 days, so its mean motion is about 1° per day. The frequencies for particles in Saturn’s rings are much faster, measuring about 600° to over 1000° per day, depending on the location within the rings, reflecting their much shorter orbital periods of less than a day. The key point where resonances are concerned is that at every distance from Saturn there are three natural frequencies for objects to orbit, be they ring particles or moons: the mean motion, the epicyclic (radial) frequency, and the vertical frequency. Resonances can exist with any of these frequencies, and the kind of resonance determines what sort of phenomenon will be produced. An important type of resonance in Saturn’s rings is called a Lindblad resonance after Swedish astronomer Bertil Lindblad. In this type of resonance, gravitational perturbations from a moon are in resonance with the radial frequency of a ring particle. This excites or enhances the eccentricities of all particles at the location of the resonance. Each time the moon passes by the ring particle, the gravitational pull it exerts on the particle is at the same phase in the particle’s natural radial motion as determined by Saturn’s gravitational field. It is a very subtle phenomenon with some dramatic consequences. The co-orbital moons Janus and Epimetheus are at the 7:6 Lindblad resonance with the outer edge of the A ring. Because the resonance is with the natural radial (in and out) motion of the ring particles it produces the sevenlobed radial distortion of the edge of the ring seen in figure 5.15. Lindblad resonances are also responsible for producing dozens of spiral density waves in the rings (figure 5.12). These waves are the same type that give spiral galaxies such as the Milky Way their spiral arms, but in the rings the waves are wrapped around Saturn much more tightly. Each moon can have Lindblad resonances in the rings, but only those moons that are close to the rings have resonances in them that are strong enough to produce observable density waves. While Pan is actually within the rings, it is fairly small so the density waves it produces can only be detected through careful image processing techniques. The strongest resonances are those due to Janus, Mimas, and two moons straddling the F ring, Prometheus and Pandora (figure 5.17). These waves are identified by the number of spiral arms, which is determined by the number of orbits a ring particle completes before it gets its next resonant gravitational kick from the moon. This number of kicks is denoted by the letter m for no good reason, and the number of spiral arms is equal to m+1 (figure 5.18)4. The perturbations to particles’ eccentricities near the resonance produce alternating regions of more and less dense regions of the rings. The spiral density wave is 4 Detailed treatment of resonances in planetary rings and planetary systems in general can be found in Murray and Dermott (1999).

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Figure 5.17. All the bright features visible in this image of the A ring in the vicinity of the Encke gap (the dark band with the narrow, irregular ringlet at its center) are waves created at resonances with moons. The two waves indicated by the arrows are both due to 5:3 resonances with Mimas (shown at roughly twice its size relative to the rings), but one is a resonance with the radial frequency of the ring particles (green arrow) and the other is with the vertical frequency (red arrow). The region in the box is shown in figure 5.20. Images: NASA/ JPL-Caltech/SSI.

produced by a combination of the periodic forcing of particle eccentricities due to the resonant gravitational perturbations from a moon coupled with the gravitational attraction of the ring particles on their neighbors. The resonant perturbations produce a non-axisymmetric distribution of the ring particles, like the multi-lobed ring edge seen in figure 5.15 and schematically shown in figure 5.18. With this noncircular distribution of ring density, the ring’s own internal gravity causes the wave to distort. The spiral pattern is created because the radial motions of the particles have different frequencies (speeds) at different distances from the resonance. Once that pattern is created, the ring is no longer circularly symmetric, so ring particles feel a slight gravitational tug forward or backward in their orbital motion due to the neighboring spiral arms. This tightens the spiral up the further one moves away from resonance. The mass of the spiral arms of the wave acts on the orbits of the particles in such a way as to tighten the winding of the spiral arms. The more massive the rings, the tighter the winding. Figure 5.16 shows a classic example of this tightening of a spiral density wave in the rings, and figure 5.19 shows how this winding up of the wave occurs. This provides us with a very powerful tool to measure the mass of the ring, at least at the location of the wave. Prior to the end of Cassini’s mission, when the mass of the rings as a whole was measured by its perturbations on the motion of Cassini itself, this was the only tool we had to measure the mass of the rings. It is quite valuable and more precise than the measurements made of the whole 5-17

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Figure 5.18. This schematic shows the paths of particles on an unperturbed (m = 0) orbit and two different perturbed orbits in a reference frame rotating at the rate of the perturbation. In such a rotating frame a particle makes m radial excursions before the pattern repeats. The duration of each full radial cycle is one orbit. So the m = 7 path shown here is seven full orbits for the particle.

ring mass in the Grand Finale, but it only tells us the mass where there happen to be density waves. Density waves are most densely distributed in the A ring, but many have been discovered in the C ring and Cassini Division, with only a handful in the central, dense B ring. The spiral arms in the density waves are wrapped around Saturn so tightly that adjacent peaks in the wave appear nearly circular (figure 5.20). Like any wave, it is the pattern that moves through the medium, and not the particles themselves. The ring particles find themselves moving from a wave crest, where particles are more closely packed, to a wave trough where they are less tightly packed, just like a buoy on the water or a floating bather in the ocean bobbing up and down from crest to trough as a wave passes by without actually moving in the direction that the wave moves. Ring particles move from crest to trough and back again several times each orbit, depending on the value of m. So, as we’ve seen, the Lindblad resonances occur at locations in the rings where the ring particles’ natural frequencies for radial motions are periodically tickled by a moon so that the perturbations add up over time. The resulting density waves are

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Figure 5.19. These schematic diagrams illustrate how a spiral density wave is formed from the perturbed paths of ring particles near a Lindblad resonance. An m = 5 Lindblad resonance puts particles on a five-lobed pattern seen in a reference frame rotating with the perturbing moon (a). The epicyclic or radial frequencies of ring particles are different at different distances from Saturn, however, so the five-lobed pattern is rotated a little as one moves away from the resonance location. This results in the particle paths forming a spiral pattern with m = 5 arms (b). The gravitational influence of the particles in those spiral arms rotates the orbits back a little, resulting in a spiral density wave where the wavelength (the distance between two spiral arms) decreases with increasing distance from the resonance (c).

like a compression wave in a slinky. Vertical resonances occur at locations in the rings where the ring particles’ natural frequencies for vertical motions are periodically tickled by a moon. These are less common than Lindblad resonances because they require the perturbing moon to be on an orbit that is inclined to the rings, and the details of how these resonances work make them intrinsically more widely spaced and less strong. Without going into those details, I’ll just note that there is no such thing as a first-order vertical resonance, so the strongest vertical resonances are second order, and thus produce smaller perturbations on the ring particle orbits. Mimas is on an inclined orbit and is a pretty massive moon, so its 5:3 vertical resonance produces a prominent bending wave in the A ring. This wave is a warping of the ring rather than a compression of particle orbits. It propagates in the opposite direction of a density wave (figure 5.20), and because the vertical and radial frequencies of the particles are different, the 5:3 density wave and 5:3 bending wave occur at different locations in the rings (figure 5.17). All of these waves in the A ring give us a pretty good measure of the mass in that ring. Assuming that our theories of how density waves work are correct, a 1 m2 chunk of the A ring (which would be 5–10 m thick) would contain a mass of about 300–400 kg, or about the mass of four to five people. 5-19

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Figure 5.20. The Mimas 5:3 bending wave at the right propagates away from the moon, toward the left in this Cassini image. The larger context for these waves can be seen in figure 5.17. The wave at the left is the Prometheus 12:11 density wave, propagating in the opposite direction. The direction of propagation can be determined from the changing wavelength: the wavelength gets smaller for both types of wave as distance from resonance increases. Image Credit: NASA/JPL-Caltech/SSI.

If you were drifting among the particles in the A ring you would find yourself in a relatively crowded region filled with bits of slightly dirty snowballs, some as large as cars, but most the size of marbles or basketballs, likely worn down to a roughly spherical shape by countless gentle collisions. If you looked within the ring plane your view would be obscured by ring particles, but looking up or down you would see the emptiness of space a few meters away. The rings, for all their great expanse, are as thin as a sheet of tissue paper on a football field. Particles would drift slowly by you at a barely noticeable pace. Eventually you might find yourself bumping into a large stringlike assemblage of snowballs, moving together but not bound to each other. You might stay with this clump, a self-gravity wake, for several hours before drifting away (figure 5.21). In a density wave things would get quite crowded several times each orbit. Particles would press in on you from all sides, slowly but insistently, driven by the unrelenting distant arm of gravity. In some regions of the outer A ring you might find yourself next to a larger object, perhaps several hundred meters across. This propeller object might have formed from accretion of many ring particles, but the distribution of these objects in discrete

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Figure 5.21. With a resolution of only 340 m/pixel, this image from the final phase of Cassini’s orbital tour of Saturn shows a region of the outer A ring marked by a density wave (the dark, unevenly spaced bands at the left edge are the opaque peaks of the wave) and winding record-groove-like spiral wakes from the moon Pan orbiting in the nearby Encke gap. In the troughs between the density wave peaks the ring has a splotchy texture likely due to large, temporary agglomerates of ring particles. Image Credit: NASA/JPL-Caltech/SSI.

bands within the A ring suggests that they may instead be the leftover largest pieces of the fragmentation of a small moon by a cometary impact. If you came near this mini-moonlet your orbit would be strongly perturbed, and you might find yourself drifting away from it, pushed onto a different orbit by the combined interactions of the propeller object and Saturn (figure 5.11). Particles on nearly the same orbit as the object reverse course (relative to the propeller object, still moving in the same direction around Saturn), moving to a slightly larger or smaller orbit and then drifting further away from the object. Particles whose orbits are slightly different than the object’s receive a gravitational kick that increases the eccentricity of their orbits. This produces a wavy pattern on either side of the object. This two-lobed perturbation to the ring particles produces the propeller-shaped pattern observed in the A ring around small, unresolved moonlets at their centers (figure 5.22 and chapter 3 figure 3.6). Of Saturn’s large moons, Mimas is the closest to the rings, and its resonances produce strong perturbations. Janus, while smaller than Mimas, is closer to the rings, meaning that its orbit and orbital frequencies are closer to those of the ring

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Figure 5.22. This image of the Encke Gap, the empty diagonal band, shows two density waves on either side of the gap and a prominent propeller object nicknamed ‘Earhart’ after the aviator Amelia Earhart. The object producing the two-lobed disturbance is too small to be seen directly, but based on models it may be nearly 1 km across (Tiscareno et al 2008). Image Credit: NASA/JPL-Caltech/SSI.

particles. As a result it has more of the strong, first-order resonances in the rings. In fact, the only first-order resonance with Mimas that lies within the main rings is the 2:1 resonance5. For Janus, the 2:1 through 6:5 resonances all lie within the rings, and the 7:6 Janus resonance marks the boundary of the A ring. In stellar occultation data (figure 5.23) and images (figure 5.16 and 5.20), these waves show an almost mathematical precision. The Mimas 2:1 resonance marks the outer edge of the B ring. The perturbing effects of this resonance on the ring particles is likely what has created the outer edge of the B ring and opened up the gap outside the B ring known as the Huygens gap. This resonance is so strong that, rather than launch a wave, the perturbations to the particle orbits are large enough to clear a gap. The resonance with the moon almost acts like a moon itself, opening a gap the same way Pan and Daphnis maintain gaps in the outer A ring.

5.6 The B ring, ballistic transport, and spokes The B ring edge, like the A ring edge, is not circular, in this case due to the perturbations from the Mimas 2:1 resonance. These perturbations also force the particles in the outer region of the B ring to pile into each other. The resulting crush of particles forces them out of the ring plane, producing giant piles of particles more than a kilometer high as can be seen in the stunning images taken when the Sun was shining nearly edge-on to the rings (figure 5.24). These images dramatically illustrate how ring particles can be coerced into clumping together to form moon-sized objects by the gravitational forcing effects of a distant moon. The changing shape of the B ring edge due to Mimas can be seen in figure 5.25 where the pattern of motion is 5 The order of a resonance refers to the difference between the two numbers of the resonance. A 7:6 resonance is first order, while a 5:3 resonance is second order. There are no first-order vertical resonances.

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Figure 5.23. The brightness of the star Beta Centauri as it was occulted by Saturn’s rings in 2008. The occultation shows the changing density of the ring in two strong density waves, one created at the Janus 4:3 Lindblad resonance (left) and one created at the Pandora 6:5 Lindblad resonance. The wavelength of each wave decreases with increasing distance from the location of the resonance, and the rate of this change is determined by the local mass of the ring. In the Janus 4:3 wave there are four spiral arms wrapped around Saturn multiple times giving rise to the large number of oscillations seen in this occultation.

more complicated than a simple two-lobed ellipse as would be expected for the 2:1 resonance. In addition to the Mimas perturbation, the ring is ringing, for want of a better word, at its own natural frequencies, with waves traveling back and forth across its outer region. These waves are created not by an external moon such as Mimas, but by instabilities in the ring that lead to clumping of the particles into very dense regions separated by less dense regions. These so-called viscous overstabilities may be responsible for the complicated alternating structure of the transparency seen in much of the B ring (figure 5.3) (Salo & Schmidt 2010). The large clumps piled up at the outer edge of the B ring (figure 5.24) and their perturbed motions (due to Mimas) may act like pseudo-satellites with resonances in the nearby Cassini Division. The B ring edge, sculpted by Mimas, may in turn be shaping the structure and gaps in the Cassini Division, where we had previously expected to find more moonlets (Hedman et al 2010). While density waves are the dominant feature of the A ring, most of the structure in the B ring is not understood at all. There are regions where the ring is featureless for hundreds of kilometers (the so-called flat spot in the inner B ring), regions of quasi-periodic oscillations in transparency, and regions where there seems to be no pattern in the variations of ring transparency (figure 5.3). Some of this structure may be due to the so-called viscous overstability mentioned above. Particle traffic jams have also been suggested, in which the particles are so tightly packed that, rather than orbiting Saturn independently, they lock up into slabs, resulting in unusual density patterns (Tremaine 2003). Other aspects may be related to a process known 5-23

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Figure 5.24. This image of the outer edge of the B ring was taken near Saturn’s equinox when the Sun was shining nearly edge-on to the rings. In this geometry, things that stick out of the ring plane cast long shadows, like a building at sunset. By measuring the lengths of the shadows in this image we know that the pile-ups of ring particles at the ring edge are up to 2.5 km high, while the normal ring thickness is only a few meters. These particles are pushed inward by the resonance with Mimas, but have nowhere to go up but up (or down) and out of the ring plane. The narrow Huygens ringlet is the first thin bright band beyond the edge of the B ring. The non-circular shape of the B ring edge is evident from the varying distance between it and the Huygens ringlet from left to right across this image. Image Credit: NASA/JPL-Caltech/SSI.

as ballistic transport in which meteoroids striking the rings produce ejecta that reimpacts the rings at other locations. This transfers mass6 from one location to another, and if the initial distribution of material in the ring is not uniform, this process can either enhance or diminish these variations, depending on the initial conditions. This is believed to be responsible for the structure of the inner edges of the A and B rings, which share some striking similarities. As can be seen in stellar occultation data in figure 5.26, both edges have a notch feature with a transparency of about 0.6 and then a linear ramp feature that transitions to the C ring in the case of the B ring inner edge and the Cassini Division in the case of the A ring. At the 6

It also transfers angular momentum, a conserved, physical quantity that measures the rotational or orbital motion of matter. The reimpacting ejecta makes the particles it hits move onto new orbits, changing the local structure of the ring.

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Figure 5.25. The position of the outer edge of the B ring varies with time (or equivalently with longitudinal position around the ring). The green line indicates the location of the Mimas 2:1 Lindblad resonance, and the white line is the average location of the ring edge. The other colored lines indicate reflection boundaries for natural oscillations of the ring that also shape the edge. Image Credit: NASA/JPL-Caltech/SSI. Movie available at https://iopscience.iop.org/book/978-1-64327-714-1.

Figure 5.26. These plots show the transparency of the rings at the inner edge of the B ring and A ring measured during a stellar occultation observed by the Cassini UVIS.

bottom of the ramp there is a feature with transparency of about 0.7, followed by a narrow increase in transparency to a gap (transparency of 1), and then a narrow ringlet. All of these features are consistent with formation from the process of ballistic transport (Durisen et al 1989, 1992). The meteoroid impacts onto the rings preferentially launch ejecta in the orbital direction of the ring particles, enhancing a previously existing ring boundary. One of the most surprising discoveries of the Voyager encounter with Saturn were radial features in the rings dubbed spokes (figure 5.27). Spokes appeared spanning thousands of kilometers across the B ring in regions where as few as 5 min before images showed no sign of them. Particles moving only under the influence of Saturn’s gravity could not travel across such a great distance so quickly. This 5-25

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Figure 5.27. The bright spots on the right are ‘spokes’ that appear in the B ring and last for about one orbit before fading away. Image Credit: NASA/JPL-Caltech/SSI.

suggested that the particles making up the spokes were small enough to be affected by the Lorentz force, the force due to electric and magnetic fields on charged particles. Gravity knows how to bind particles together, and the shapes of their orbits are elliptical. While three-body interactions such as those due to perturbations from moons described above can produce some complicated deviations from perfect ellipses, these are just small perturbations, and the orbital speeds remain at their orderly pace dictated by the distance to Saturn. Radial features streaking across the rings, apparently fleeing Saturn, and then disappearing in a matter of hours, were an entirely unexpected phenomenon and require forces other than gravity to explain. Saturn, like the Earth, has a magnetic field generated in its interior. Magnetically, the planet can be approximated as a bar magnet spinning around its axis. As described in chapter 4, the motion of charged particles in a magnetic field can be quite complex. While magnetic fields deflect charged particles, creating spiral motion and bouncing north and south along the field lines, an electric field pushes a positively charged particle in the direction of the field (or in the opposite direction for a negatively charged particle). The rotation of Saturn’s magnetic field produces the effect of an electric field, called a corotation electric field, directed radially away from Saturn. This is precisely the geometry needed to explain Saturn’s peculiar radial spokes. We know from our experience of seeing houseflies safely stand on vertical walls while dogs, cats, and children who attempt the same feat invariably plummet to the floor that particles must be small for the electric force to win out over gravity. Another aspect of the spoke observations supported the idea that they are small particles subject to strong perturbations from the Lorentz force. In some geometries

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the spokes appear darker than the background rings, and in others they appear brighter. When looking back toward the Sun, the spokes are bright, but when the Sun is behind Cassini they appear dark, precisely what is expected from micron-sized (10−6 m) dust particles. Detailed modeling of the spoke brightness is complicated by the brightness of the background ring material which also depends strongly on viewing geometry, but has confirmed that the spoke particles are about one micron across. Intensive imaging campaigns designed to track the formation and evolution of spokes were planned for Cassini, but when it arrived at Saturn it found the rings to be spoke-free. A clue that this might be the case was provided by images obtained with the Hubble Space Telescope during the interval between the Voyagers and Cassini’s arrival at Saturn. Images showed the spokes gradually decrease in number as Saturn’s seasons changed. The Voyagers observed the spokes when the Sun was shining on the rings from a relatively low angle, about 8° out of the ring plane. Hubble observations made with a similar solar geometry also showed the spokes, but as Saturn’s seasons changed and the Sun moved further from the ring plane, the spokes faded away. At the time it was thought that this might be due to our changing vantage point relative to the ring plane and the illumination of the spokes by the Sun. In other words, the spokes might still be there, but were harder to see with the changing viewing and illumination geometry. When Cassini failed to find any spokes at all, regardless of viewing geometry, scientists realized that the changing seasons at Saturn weren’t affecting the visibility of the spokes but the conditions that allow them to form in the first place. When the Sun shines on a surface, high-energy photons from the Sun can kick electrons off the surface7. These photoelectrons can produce a tenuous electron atmosphere or ‘sheath’ above the surface which is left with a positive charge by the departure of the photoelectrons. When it is close to mid-summer or mid-winter, the Sun shines more directly on the rings and produces more photoelectrons, while close to the equinoxes, when the Sun is close to the ring plane, photoelectron production decreases. While the details of spoke formation remain an area of active research, the charge of the ring plane, which changes with the seasons at Saturn, controls the trajectories of the charged dust particles. One model for the spokes is that a meteoroid impact onto the rings produces a shower of micron-sized dust particles as well as a plasma of ions and electrons. The positive ions (or positively charged dust, depending on the particular model) move radially outward, away from Saturn, due to the electric force resulting from its rotating magnetic field. As they do so, they either charge dust on the surfaces of other ring particles, causing them to lift off the positively charged ring particles, or the initial dust particles knock off still more when they land on ring particles. Either way, an outward-moving population of dust or ions kicks dust particles off the ring particles along a radial band producing the 7 This doesn’t happen on the Earth because the atmosphere absorbs the most energetic photons before they reach the surface. Explaining this phenomenon, observed in laboratory experiments, is what won Einstein his Nobel prize, not his more widely known discoveries about the nature of space and time in the general theory of relativity.

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Figure 5.28. The spokes appear dark against the bright B ring in this Cassini image of the lit side of the rings. The dark diagonal line at lower right is the shadow of the planet across the rings, and the ring particle motion is clockwise in this view. The spokes are newly formed and emerging from the shadow. Image Credit: NASA/ JPL-Caltech/SSI.

characteristic spoke appearance (figure 5.28). But if the solar charging of the rings is too strong then these charged particles simply escape into space away from the rings entirely rather than cruise over the surface of the rings, kicking off more spoke particles. After this prediction was made, the spokes reappeared as Saturn neared its equinox in 2008 (the start of northern spring). Thus, spokes appear to be a phenomenon restricted to spring and fall at Saturn (figure 5.29). Although the details of how spokes form are still being studied, it seems that impacts play a role in triggering them. Most spoke formation models rely on impacts onto the rings to produce either plasma, an initial spray of dust particles, or both. Spokes are also seen to form preferentially near morning in the rings, just as the ring particles emerge from Saturn’s shadow. In the shadow, on the opposite side of Saturn from the Sun, the ring particles have the highest speed relative to the rest of the solar system because their orbital velocity around Saturn at that location is in the same direction as Saturn’s orbital velocity around the Sun. Impacts onto the rings on the nightside of Saturn, just before morning, are therefore at a higher speed and energy because the relative speed of the ring particles and an impactor is largest 5-28

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Figure 5.29. This movie made from several sequences of Cassini images shows spokes orbiting in Saturn’s B ring. Image Credit: NASA/JPL-Caltech/SSI. Movie available at https://iopscience.iop.org/book/978-1-64327-714-1.

there, on average. This results in more material being knocked off ring particles by an impact at that location in the rings.

5.7 The C ring and Cassini Division The innermost of the main rings, the C ring, shares many characteristics with the Cassini Division, the mostly transparent ring in between the A and B rings (figures 5.2 and 5.3). Based on a handful of weak waves present in each region we know that the surface mass density of these rings (the amount of mass per square meter of the rings) is at least 10 times smaller than that of the A and B rings. This has allowed the impacting meteoroids to make the particles there darker than in the A and B rings because there is less icy ring material to bury the incoming dirt in. While the C ring structure appears more orderly than that of the B ring, it is just as mysterious. Overall the C ring has a minimum in transparency of about 90%, roughly at its center, but the transparency fluctuates across the whole C ring (figures 5.3, 5.30). Punctuating these gradual fluctuations are a set of about a dozen embedded ringlets8 called plateaus with a transparency of only 60%–70%, ranging in width from about 10 km to about 240 km (figure 5.30). Some of this structure may also be related to ballistic transport, the process that sculpted the inner edges of the A and B rings (figure 5.26). The embedded ringlets immediately interior to the inner 8

More or less, depending on what criteria are used to identify a plateau.

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Figure 5.30. This image of the C ring shows a smoothly undulating pattern in brightness punctuated by brighter ‘plateaus’ in the outer half and narrow plateaus and gaps in the inner region of the ring (left). Image Credit: NASA/JPL-Caltech/SSI.

edges of the A and B rings can be explained by ballistic transport and resemble plateaus (figure 5.26) (Estrada et al 2015). The C ring is also home to other intriguing examples of impacts into the rings beyond the continuous hail of micrometeoroids that both darken the rings and gradually sculpt them through ballistic transport. When the Sun passed through the ring plane during Saturn’s equinox of 2009, features in the rings that stuck out of the ring plane cast long shadows, enabling us to observe small vertical perturbations in the rings. Matt Hedman, a Cassini scientist working with imaging data, noticed a spiral pattern in the C ring and the even more tenuous D ring interior to the C ring as the Sun approached the ring plane (Hedman et al 2011). The spiral is caused by particles in the rings moving on inclined orbits and casting a shadow on them. This spiral has a very different shape than the spiral bending waves produced by resonances with moons (figure 5.20). It extends from the D ring across more than 19 000 km to the outer edge of the C ring (figure 5.31). The wavelength of this spiral changes not due to the gravitational pull between ring particles like in the bending and density waves described above, but due to the different distances of the particles from Saturn itself and the passage of time. Here’s how that works. If Saturn were a perfect sphere, then if ring particles were put on an inclined orbit they would stay aligned with each other on that orbit. But because Saturn is highly flattened, the vertical and orbital motion of the particles are not the same, as described above in the context of bending waves. Also, those two rates of motion (vertical and orbital) have different dependencies on distance from Saturn. This causes inclined orbits to precess, or wobble, with a faster wobble for orbits closer to Saturn. The spiral seen in the C ring is what would be produced if somehow ring particles across the entire ring, at one longitude only (like a spoke, except not having anything to do with spokes!), were knocked vertically out of the ring plane. All those particles would be on wobbling, inclined orbits, and those closer to Saturn would wobble faster than those further from it. This causes the ring to twist up into a spiral (figure 5.32). As time goes on, the spiral gets more and more tightly wound. The rate of tightening of the spiral can be calculated from our knowledge of Saturn’s gravitational field and compared to the observed spiral pattern. Since the spiral tightens with time, the rate of tightening can be used to 5-30

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Figure 5.31. This mosaic of the D ring (dark, lower right) and C ring was taken shortly before the Sun crossed the ring plane, enhancing vertical structure in the ring. There is a periodic brightness variation that looks like the groove in a record that is only seen when the Sun is in this geometry, suggesting that it is due to particles on inclined orbits. This warping, or corrugation, extends across the entire C ring for a total width of more than 19 000 km. Image Credit: NASA/JPL-Caltech/SSI.

Figure 5.32. This computer animation shows how an initial perturbation to the vertical motion of ring particles across a ring evolves into a tightly wound spiral over time due to the different rates of orbital precession, or wobble, with distance from Saturn. NASA/Cornell. Movie available at https://iopscience.iop. org/book/978-1-64327-714-1.

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determine when the spiral winding began. The surprising answer is that something hit the rings and changed the orbits of particles across the entire C and D rings in 1983! If only we had been there to see it. The Voyagers flew by in 1980 and 1981, and Cassini arrived in 2004. The event was probably the impact of the debris from a comet, disrupted by tidal forces so that particles hit the ring over a large radial extent of the ring. Hints have been seen of the remnants of more ancient impacts in the rings, and a similar phenomenon has been seen in Jupiter’s dusty ring as well. Although the mass of the C ring and Cassini Division is too small to produce the clumping self-gravity wakes seen in the A and B rings, there are intriguing hints that particles may be accreting. Images taken during Cassini’s Grand Finale orbits between the C ring and the atmosphere show streaky features in the C ring plateaus that resemble clumps seen in the strong density waves of the A ring (figures 5.33, 5.34). Analyses of many stellar occultations of the C ring and Cassini Division have found small holes in the rings that may be empty regions similar to those near propeller objects (figure 5.22) (Baillié et al 2013). Tidal forces from Saturn would not

Figure 5.33. This image of the P1 plateau in the inner portion of Saturn’s C ring was taken during the proximal orbits of Cassini’s Grand Finale, on 4 June 2017, with a resolution of 325 m/pixel, among the highest resolution of any images of the rings. The space on either side of the plateau is not empty, but is more transparent. A faint streaky texture can be seen in the plateau as well as in the material to the left of the plateau. The two short diagonal lines are smeared stars. Cassini’s cameras scanned in the direction of the motion of the ring particles, so the streaks in the ring are real structure. Image Credit: NASA/JPL-Caltech/SSI.

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Figure 5.34. An enhanced view of the image of the C ring shown in figure 5.33 produced by Cassini imaging scientist Matt Tiscareno emphasizes the streaky texture. The streaks in the plateau are longer and more widely spaced than those in the surrounding material, while streaks are completely absent immediately next to the plateau. Image Credit: NASA/JPL-Caltech/SSI.

allow ring particles to stick to each other just from their gravity alone, so either the particles are physically adhering to each other, or they are clumping through some collective instability in spite of the low mass density of the ring. The answers to these questions, an active area of research, are relevant to the study of how planet formation got started in our own solar system and around other stars.

5.8 The F ring While the discovery of clumping and accretion in the C ring is surprising given the proximity to Saturn, the F ring, a narrow ring just beyond the edge of the A ring, lives just at the edge of Saturn’s Roche zone, where tidal forces give way to accretion and the formation of moons. The F ring is thus the perfect environment to see the coexistence of rings and moons at work. The F ring is straddled by the moons Prometheus (figure 5.5) and Pandora. When the F ring and these moons were first discovered it was thought that they acted as shepherd satellites, confining the ring particles between them via resonances in the same way that Janus confines the A ring edge, but in this case with a moon on either side confining an edge and thus 5-33

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Figure 5.35. This mosaic of 15 images of the F ring has been reprojected and shows about 1/6 of the circumference of the ring (147 000 km), and the radial dimension of the image is about 1500 km. The core of the F ring is narrow, eccentric and exhibits lots of bends and kinks, and it is straddled by bands of dust that were produced by collisions within the core. The dark diagonal gores in the ring indicate recent encounters with Prometheus (partially visible at lower right) whose eccentric orbit makes it approach the F ring core every orbit, clearing out channels of particles and perturbing the orbits of the particles throughout the region. Image Credit: NASA/JPL-Caltech/SSI.

maintaining a narrow ring between them. Instead it appears that the ring exists in a dynamically chaotic region where most orbits are unstable. The ring may have a very narrow (~ 1 km) core of large particles (meter-scale and larger) that lies between resonances (Cuzzi et al 2014), while the gravitational stirring due to the nearby moons causes these and other particles in the region to collide, releasing clouds of dust (figure 5.35). The ring shows a remarkably complicated structure that changes on a timescale of weeks and months (Murray et al 2008). Prometheus is more massive than Pandora and also closer to the F ring, so it has a larger effect on it (figure 5.35). Small objects or clouds of debris have been observed moving through the F ring region, and these objects cross the ring, colliding with other particles (figure 5.36). The whole area is in a state of constant accretion and disruption. Prometheus stirs the particle orbits, leading to collisions which allow some of the snowball-like particles to stick together. The largest of these agglomerations may get up to 1 or 2 km in size before being broken apart by a particularly energetic collision with another forming moonlet or by passing through the dense F ring core and colliding with a series of ring particles (figure 5.37). The F ring itself is non-circular, as is the orbit of Prometheus. The different radial and azimuthal periods of particles orbiting Saturn discussed above in the context of Lindblad resonances means that an eccentric orbit precesses. That is, the orientation of the orbit changes slowly with time due to the difference between the azimuthal orbital frequency and the radial motion or epicyclic frequency. If you drew the elliptical orbit of the moon or the ring, the orbit itself would precess or rotate. So the point in the orbit where the object is closest to Saturn, called perikrone, drifts around Saturn at a rate much slower than the time to complete one orbit. For Prometheus, the period for its perikrone direction to move through 360° is 130.6 days, while the time for Prometheus itself to orbit Saturn is only 14.7 h. The F ring itself also has a drift in its perikrone direction, but at a slightly slower rate than the drift in Prometheus’s orbit, taking 133.1 days to complete a full circuit. As a result of this slight difference in precession rate, every 19 years the two orbits are aligned, with Prometheus at perikrone at the same longitude as the perikrone of the F ring. This means that 9.5 years later, the apokrone

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Figure 5.36. These images show features produced by collisions between objects about 1 km across with particles in the F ring core. Some of these collisions release large amounts of dust that go on to form strands all the way around the planet (figure 5.35). Image Credit: NASA/JPL-Caltech/SSI.

of Prometheus (the point in its orbit where it is furthest from Saturn) will be in the same direction as the perikrone of the F ring. At that time Prometheus is at its greatest distance from Saturn at the same longitude that the F ring is closest to Saturn. At this time Prometheus makes its closest and most disruptive approach to the F ring and this configuration repeats every 19 years. This near-collision between the F ring and Prometheus occurred in 2009 and will next occur in 2028. During the close encounter between Prometheus and the F ring every 19 years, the ring gets stirred up even more than usual by gravitational perturbations. If the edges, wakes, and waves of the rings of Saturn and their relationships to its moons teaches us anything it is that gravitational perturbations can have a wide variety of outcomes. Sometimes they scatter particles away from a region, other times they may confine or concentrate particles. This happens on a large scale in the solar system’s asteroid belt, where Jupiter plays the role of the perturbing moon, and the Sun plays the role of the central planet. Some resonances with Jupiter stabilize and confine asteroid orbits, producing groups of asteroids with similar orbits, while other locations are devoid of asteroids due to the scattering influence of resonances. The dynamic F ring region, stirred by Prometheus and Pandora and lying at the edge of Saturn’s Roche zone, also experiences a full range of phenomena from collisional 5-35

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Figure 5.37. This series of montages of the F ring illustrate how quickly the appearance of the ring changes and the effects of collisions between moonlets and the ring. Each image shows the F ring reprojected as if looking down on it from above Saturn’s pole, while Saturn has been removed from the images and the radial extent of the ring has been stretched by a factor of 140 to highlight structure in the ring while still making it possible to see the whole ring. The object known as S/2004 s 6 crossed through the ring shortly before the picture at upper left was taken. Debris from this collision eventually spread out around the planet forming a loose spiral that gives the ring a multi-strand appearance by 2008. Image Credit: NASA/JPL-Caltech/SSI.

accretion of moonlets to violently disruptive collisions. Hundreds of features in the rings have been identified that are the remnants of a recent collision, usually from an unseen moonlet too small to be captured by Cassini’s cameras. Some moonlets or clumps of material in the region have been detected by stellar occultations when they serendipitously pass in front of the star being observed. Voyager scientists Jeff Cuzzi and Joe Burns first deduced the presence of a broad moonlet belt throughout the F ring region from measurements of dropouts in the population of charged particles in that region of Saturn’s magnetosphere (Cuzzi & Burns 1988). The F ring lives at the boundary between moons and rings. It is constantly changing form, both spawning moonlets and destroying them. It may itself be the remnants of a moon similar to Prometheus that was destroyed by an interplanetary interloper or a collision with a sibling moon. Those moons may themselves have been accreted from material at the outer edge of the A ring and evolved outward due to tidal interactions with Saturn. The close relationship between rings and moons is perhaps not surprising given that each ring particle is, essentially, a small moon trying to find its way around Saturn. Less massive than their larger companions, they make their way in a crowd, frequently jostling each other, sometimes with no

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consequence, sometimes joining together, and occasionally dispersing the crowd, like a police officer at a brawl.

5.9 The dusty rings An even clearer connection between ring and moon is provided by Enceladus. The water vapor geysers at Enceladus’s south pole provide a continual supply of micronsized9 ice grains into orbit around Saturn. Due to their small masses, these dust10 grains are easily pushed around by interactions with sunlight. Remarkably, a solid micron-sized dust particle orbiting Saturn can have the shape and size of its orbit dramatically altered due to the momentum it receives from sunlight. The primary effect of this radiation pressure is to put the dust particles on highly eccentric orbits so that the dust from Enceladus covers a broad region around Saturn with the highest concentration of dust at the source (figure 5.38). Other faint rings are visible in the right viewing geometries (figure 5.38), but the sources of dust in those rings are micrometeoroid impacts onto small moons knocking dust off their surfaces rather than the active geysers of Enceladus. While these rings are vast, like the Phoebe dust ring (chapter 4) they contain much less material than the main rings. The transparency of the E ring, the densest of Saturn’s dusty rings11, is greater than 99.99%. The efficiency of dust production from a moon depends on the

Figure 5.38. This mosaic of Saturn and its rings was taken by Cassini when the Sun was blocked by Saturn. In this geometry, looking back toward the Sun, dusty regions are particularly bright because they scatter light forward more than backward. The central B ring is dark because it is nearly opaque, while the F ring, the narrow bright ring just outside the A ring edge, shines brightly due to dust released in its collisionally active environment. Beyond the F ring are four fainter dusty rings: the very faint and narrow Janus ring, the bright G ring (unlabeled), the Pallene ring (a barely visible enhancement in the E ring), and the broad and diffuse E ring. Looking back toward the Sun, this remarkable mosaic also captured Earth, Mars, and Venus as well as several of Saturn’s moons. Although we are looking at the nightside of Saturn, its disk is not completely dark due to sunlight reflected on its nightside by the rings. Image Credit: NASA/JPL-Caltech/SSI.

9

The diameter of a strand of human hair is about 100 microns. ‘Dust’ is used in planetary science to denote small particles, regardless of what they are made of. 11 The F ring has a broad dusty component, but due to its core of larger objects is not usually considered in the same category as the other dusty rings. 10

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competition between its surface area (a larger moon has more area for micrometeoroids to hit and knock dust off) and its gravity (a larger moon has a stronger gravitational pull which prevents the ejected dust from leaving it). Gravitational focusing of the interplanetary impactors (chapter 4) enhances micrometeoroid impact rates on inner moons. The result of these competing processes is the system of faint dust rings associated with Janus, Anthe, and Pallene seen in figure 5.38. The E ring particles, launched from the geysers of Enceladus, are nearly pure water ice, and their orbits evolve to span the entire system from the outer edge of the main rings out to Titan. While the origins of the dusty rings are relatively straightforward, the origin of the main rings (A, B, and C) has been a particularly vexing problem for planetary scientists, especially since the Voyager spacecraft provided the first close look at the rings in the early 1980s. One of the main objectives of the Cassini mission was to provide the data to enable us to resolve that problem, and some of the key measurements were planned for Cassini’s Grand Finale. These observations and our current understanding of the history of the rings is the subject of the next chapter.

Further reading Planetary Rings by L W Esposito (2014) provides an overview of planetary ring physics. The book Planetary Ring Systems (Tiscareno & Murray 2018) contains review chapters on all aspects of planetary rings following the Cassini mission. Tiscareno et al (2019) provides a detailed look at what was learned about the rings from Cassini’s Grand Finale orbits.

References Baillié K, Colwell J E, Esposito L W and Lewis M C 2013 Astron. J. 145 171–80 Cuzzi J N and Burns J A 1988 Icarus 74 284–324 Cuzzi J N et al 2014 Icarus 232 157–75 Dougherty M, Esposito L W and Krimigis S (ed) 2009 Saturn from Cassini-Huygens (Berlin: Springer) Durisen R H et al 1989 Icarus 80 136–66 Durisen R H et al 1992 Icarus 100 364–93 Esposito L W (ed) 2014 Planetary Rings 2nd edn (Cambridge: Cambridge University Press) Estrada P R et al 2015 Icarus 252 415–39 Hedman M M et al 2010 Astron. J. 139 228–51 Hedman M M et al 2011 Science 332 708 Lewis M C and Stewart G R 2009 Icarus 199 387–412 Murray C D and Dermott S F 1999 Solar System Dynamics (Cambridge: Cambridge University Press) Murray C D et al 2008 Nature 453 739–44 Salo H 1995 Icarus 117 287–312 Salo H and Schmidt J 2010 Icarus 206 390–409 Tiscareno M S and Murray C D 2018 Planetary Ring Systems (Cambridge: Cambridge University Press) Tiscareno M S et al 2019 Science 364 eaau1017 Tiscareno M S et al 2008 Astron. J. 135 1083–91 Tremaine S 2003 Astron. J. 125 894–901

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The Ringed Planet (Second Edition) Cassini’s voyage of discovery at Saturn Joshua Colwell

Chapter 6 Divining the age of the rings

6.1 The problem If anything is clear from the observations of the rings made by Cassini, not to mention the wealth of data collected from other observatories, both ground-based and space-based, it is that the rings are a dynamic, changing system. Any time something changes or evolves it raises the questions of how rapidly those changes occur, what that something looked like long ago, and what it will look like in the future. Consider the discovery of plate tectonics on the Earth. What had long been considered an immutable and static configuration of continents for billions of years was found to be a shifting, sliding system of puzzle pieces. This led to the realization that continents merged, separated, and reoriented themselves over the globe. While this dramatically changed our understanding of the age of the continents, it did not change our understanding of the age of the planet. A similar transformation occurred with our understanding of the history of Saturn’s rings since the Voyager flybys of 1980 and 1981. When Saturn and its moons formed there would naturally be particles over a range of distances from Saturn. Those outside the Roche limit would accrete to form moons, while those closer to Saturn would be prevented from accreting by its strong tidal force. At first glance, therefore, there would seem to be no reason that the rings could not have remained in orbit around Saturn since the planet formed 4.5 billion years ago. But the discoveries of small moons in gaps, the complicated F ring with its intricate interactions with Pandora and Prometheus, detailed studies of the rate that the rings would spread due to collisions between the particles, the surprisingly high reflectivity of the rings, and many other observations and calculations, showed that if the rings have been around for billions of years they certainly haven’t looked the same all that time. Of course that doesn’t preclude the rings being ancient, just as continental drift on the Earth on the timescale of 100 million years doesn’t mean that the planet is only 100 million years old. But it does require an explanation for how they have evolved over time. In other words, the rate that features in the rings

doi:10.1088/2053-2571/ab3783ch6

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change doesn’t necessarily tell us the age of the rings any more than the time it takes for someone to grow a beard tells us the age or lifetime of that person. In this chapter we will discover the various ways that rings evolve over time and the implications of Cassini’s measurements to weave those processes together into a story of the origin and evolution of the rings to their present appearance and ultimate fate. While there was plenty of material available to form rings at the origin of the solar system, creating rings more recently is challenging: some significant amount of material would have to be delivered to Saturn’s Roche zone, either from interplanetary space or from a moon within the system. Our calculations show that both scenarios are unlikely (about 1% chance), at least given the current abundance of things flying around in the outer solar system. So, either the rings formed with Saturn and have been rapidly evolving for 4.5 billion years without disappearing altogether, all the while maintaining the appearance of a recently created system, or they formed relatively recently from what seems to be a very rare event.

6.2 The spreading of the rings Nature abhors a vacuum, as the saying goes. If there’s stuff over here, and nothing over there, some of the stuff is going to move from here to there. This basic phenomenon can be thought of as a consequence of pressure, which comes from collisions between the particles in the stuff. The Earth’s weather is driven by areas of high pressure, where atmospheric molecules collide more frequently than in areas of low pressure, causing air to move from the high pressure region to the low pressure region. Within a planetary ring, the particles collide, even though the collisions are gentle and infrequent. At the edge of a ring, a particle may collide with a neighbor on the ring side, but there is no particle on the opposite side to knock it back into the ring. This allows the ring to gradually spread. This is called viscous spreading because the rate of spreading is governed by the viscosity, which is a measure of how frequently particles collide with each other and how far apart they are from each other. Orbital resonances between ring particles and moons can slow viscous spreading. The gravitational interaction between the moon and ring particles, as described in chapter 5, lead to the moon repelling the ring particle just as a physical collision would. The more massive the moon, the more effective it is at confining the ring. The moon acts kind of like a bookend on the rings. Janus and Epimetheus have resonances near the outer edge of the A ring, while Mimas has a 2:1 resonance at the outer edge of the B ring. This means that particles at the outer edge of the B ring orbit Saturn exactly twice as fast as Mimas does. This resonant interaction may have created the edge of the B ring in the first place. Similarly, the more massive the rings are, the more rapid is the viscous spreading. We can estimate the viscosity of the rings from the study of spiral density waves (chapter 5). The rate that those waves damp is governed by the ring viscosity (figure 6.1). With measurements of the viscosity from waves as well as measurements of the masses of moons we can calculate the rate of viscous spreading. Another way to look at the spreading of the rings is to realize that some of the mechanical energy (energy of motion) of ring particles prior to a collision is converted

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Figure 6.1. This region in the A ring is rife with density waves. This region of the rings also shows grooves due to perturbations from the moon Pan that orbits within the Encke gap in the outer part of the A ring. The spirals are so tightly wound up around Saturn that the spiral arms look like parallel ringlets. The wavelength (distance from one wave crest to the next) gets smaller as the wave propagates toward the moon, and the amplitude of the wave damps out until the wave fades into the background. The arrows indicate the direction of the wave propagation and the distance of propagation which is determined by the resonance and the ring viscosity. The largest wave in the center is due to a resonance with the moon Janus. The image scale is 669 m/pixel. Background image: NASA/JPL-Caltech/SSI.

to minuscule quantities of heat at the contacts between the particles. This means some of the energy of orbital motion of the ring particles is lost in the collision which should put them on a lower orbit, closer to Saturn. However, at the same time, the angular momentum of the system of particles is conserved (chapter 5) even though mechanical energy is lost. The only way for the combined orbital energy of the two particles to be decreased while conserving angular momentum is for one particle to move to a lower orbit and the other to move to a higher orbit. Extended to a full system of ring particles, this again means that collisions lead to the ring spreading. While the rings themselves are likely the debris of some progenitor object, whether a moon or a large captured comet, it is possible that the spreading of the ring itself created some of the moons that now in turn sculpt the rings. As collisions between ring particles cause the ring to spread, the outer edge approaches the edge of Saturn’s Roche zone, where tidal forces cease to be effective at inhibiting accretion.

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Small clumps of ring particles may accrete with each other there and drift off the edge of the ring. These nascent moons then tidally evolve further outward, collecting more material that evolves outward from the ring edge. We see in the images of the small ring moons that they have accreted small particles from the rings (chapter 4). Perhaps some of these moons are made entirely of ring particles in the first place. Professor Sébastien Charnoz of the Université de Paris 7 and his colleagues have proposed that the rings are ancient and started out very massive, perhaps several times more massive than the moon Mimas. The ring would have viscously spread quite rapidly for the first billion years or so, leading to most of the ring material drifting beyond the Rochel limit where it then could accrete to form many of Saturn’s small inner moons, such as Janus and Epimetheus. In this model, the rings gave birth to many moons, and due to the natural viscous spreading the current ring system is a pale shadow of its original massive self.

6.3 The mass of the rings The first edition of this book, completed prior to the Grand Finale orbits, predicted that ‘knowing the mass of the rings will help settle the nettlesome open question of just how long Saturn has had its spectacular rings’. The model of Charnoz showed that massive rings would spread, and that the rate of spreading slows as the rings lose mass beyond the Roche limit. Their models indicate that if you start with rings when Saturn formed, as long as they were at least about as massive as Mimas1 originally, they would end up having a mass of about half a Mimas mass now, almost regardless of their initial mass. The mass is important in determining how fast the rings are evolving now, but may not be telling us much about how long they have been around to evolve in the first place. Even getting a measurement of the mass, predicted to be readily measured by the Cassini Radio Science Subsystem (RSS) during the Grand Finale orbits, turned out to be unexpectedly complicated. The technique for measuring the ring mass involves tracking a radio signal from Cassini to the Earth as it flies by Saturn. The mass of Saturn, its moons, and the rings exert a gravitational pull on Cassini causing its speed to vary. These changes in speed produce a change in the frequency of the radio signal from Cassini received by the antennas of the Deep Space Network on Earth (figure 6.2) due to the Doppler effect. Only motions toward or away from the Earth produce a Doppler effect, rendering the determination of the spacecraft’s actual speed that much more complicated. The acceleration of the spacecraft was measured with astounding precision: better than one part in a billion. That means that the change in the frequency of the radio signal from Cassini was measured to that level of precision. The rotation of the Earth, its orbital motion, and the orbital motion of Saturn all have to be taken into account to make these measurements. Even errors in 1

Mimas orbits close to the rings, and has strong resonances in them that sculpt it in various ways, including controlling the outer edge of the largest ring, the B ring. As it turns out, the mass of the rings is in roughly the same ballpark as that of Mimas, so it is more convenient to refer to the mass of the rings in terms of that of Mimas than in kg. The mass of Mimas is 3.75 × 1019 kg. It is slightly more dense than water, and its diameter is 396 km.

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Figure 6.2. The 70 m antenna at the Goldstone, California array of antennas of the Deep Space Network. The Deep Space Network antennas are located in Goldstone, Madrid, and Canberra, and were used to send commands to Cassini and receive data from it. Image Credit: NASA.

the position of the giant Deep Space Network antennas can ruin the measurement because the speed of the antenna due to the rotation of the Earth depends on precisely where it is. An early error in the location of an antenna of only 10 cm due to plate tectonics had to be corrected in order to get the right result2. Based on similar measurements made by the Juno spacecraft orbiting Jupiter as well as previous measurements made with Cassini, the RSS team modeled the Saturn system and predicted that the ring mass would be measured to a precision of about 0.03 times the mass of Mimas. Saturn had some surprises in store. The Grand Finale orbits were critical to getting the mass of the rings. Prior to these orbits, Cassini always crossed Saturn’s equatorial plane exterior to the rings. From this location, the gravitational pull of the rings has the same effect on Cassini as does a component of the gravitational pull of Saturn itself. Saturn is not spherical but is more like a squashed ball in shape; it has more mass around the equator than it does around the poles. The rings, orbiting above the equator, pull on Cassini in the same way as the material within Saturn at its equatorial region. But in the Grand 2

Richard G French, Cassini Radio Science Subsystem Team Leader 2017, personal communication.

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Figure 6.3. This series of photos was taken by Cassini as it passed between the rings and the atmosphere of Saturn during its Grand Finale proximal orbits. The images are show stitched together in figure 6.4. Image Credit: NASA/JPL-Caltech/SSI. Movie available at https://iopscience.iop.org/book/978-1-64327-714-1.

Figure 6.4. The ‘inside-out’ image sequence of the rings made by Cassini as it passed between the planet and the rings presents Saturn’s rings from a perspective that will never be seen again until another spacecraft ventures into that space. The central, massive B ring is dark in this view because it is the unilluminated face of the rings. Image Credit: NASA/JPL-Caltech/SSI.

Finale orbits, Cassini plunged between the rings and the planet (figure 6.3). Figures 6.3 and 6.4 show the rings from this unusual vantage. By combining RSS measurements from the Grand Finale orbits with those from prior orbits, Cassini scientists planned to separate the gravitational pulls due to the rings from those due to the planet and finally measure the mass of the rings definitively. The density waves discussed in chapter 5 provided measurements of the mass of the regions of the rings where the waves are located, but large swaths of the dense B ring have no waves, leaving open the question of the total mass of the ring system. In chapter 8 we will take a look at the implications for the RSS measurements made during the Grand Finale for the structure of Saturn itself. It turns out that the interior of Saturn is more complicated than predicted. This means that the RSS scientists had to make some assumptions about the interior of the planet in order to get a measurement of the rings mass. As a result, the uncertainties in the measurement are larger than anticipated. Nevertheless, the team was able to determine that the mass of Saturn’s rings is about 0.41 times that of Mimas3, with an uncertainty of about 0.13 times the mass of Mimas (Iess et al 2019). If the rings had a larger mass, it 3

A way to visualize this value is that if all the material in Saturn’s rings were collected into a single ball of water, that ball would be about 300 km across. This is almost as big as Mimas, even though the mass is much less, because Mimas is more dense than water.

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would require some extreme and unlikely configurations of material in the interior of Saturn. Remarkably, the measured mass lies right in the range predicted by Charnoz for the final mass of an ancient, massive ring system following 4.5 billion years of viscous spreading. This does not mean the rings are ancient, and started off massive. But it does mean that, based on mass alone, they are consistent with that history. If Cassini had found the rings to be, say, only 0.1 times the mass of Mimas, that would have been inconsistent with the Charnoz model. Similarly, if Cassini had found the mass to be comparable to, or greater than, the mass of Mimas, it would have been difficult to explain how it could still be that massive if it had been viscously spreading for the age of the solar system. While very massive rings might make it easier to explain the ring brightness for an ancient ring system, as explained below, the prediction of viscous spreading suggests that if Cassini had measured a very massive ring system that would have suggested the opposite: that the rings must be quite young. If the rings formed recently (and in astronomical terms, 100 million years ago is recent), it’s easier to form a less-massive set of rings from disruption of a moon or a comet than it is to form a very massive ring system. So it’s possible that the mass of the rings is just a reflection of the mass of the progenitor moon or comet that was disrupted recently to produce the rings, and that that mass is coincidentally about the same as predicted by the Charnoz model for an evolved, ancient, massive ring. The bottom line is that Cassini measured the mass of the rings, but that measurement alone does not definitively tell us their age. It could, in principle, have ruled out some scenarios. But the only ones it did rule out were not considered likely in the first place (such as a very young and very massive ring system). However, the mass is an important piece of the puzzle that, combined with the other observations outlined below, provides strong constraints on the origin and history of the rings.

6.4 The brightness of the rings The rings may be only about half as massive as the moon Mimas, but their surface area is about 60 000 times that of Mimas. So they are a huge target in the sky, and should thus be particular sensitive to the effects of impacts. They have a large area to hit, but not a lot of mass to absorb the effects of those impacts. In addition to the sculpting of the edges of the A and B rings by ballistic transport and the creation of spokes, impactors from interplanetary space contribute non-icy material (anything from carbon-rich compounds to silicates or rocky material to metals like iron and nickel) to the rings that should gradually darken them. If we assume that the rings began as pure water ice (an extreme assumption to provide a limiting case on the age of the rings), then in principle we can calculate their age from knowledge of the rate of impacts onto the rings. Similarly, if we assume an initial sharp edge for the inner edges of the A and B rings, the impact rate onto the rings combined with a model of ballistic transport (chapter 5) could tell us how long it would take the edges to evolve to their current shapes. The key to both of these approaches is to measure the rate of impacts onto the rings: easier said than done.

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One of Cassini’s dozen scientific instruments was the Cosmic Dust Analyzer, or CDA. The CDA has a metal target the size and shape of a large salad bowl beneath a set of wire grids (figure 6.5). When a dust particle strikes the target, its speed is so great that it creates a tiny puff of plasma, and those charged particles are collected by the wire grids, producing a current that the instrument measures. The magnitude of the electrical signal is related to the mass and speed of the impacting particle. Also, if the incoming particle is electrically charged (and most are, due to solar photons knocking electrons off of them), it produces a small signal in the grids as it passes through them, providing additional information on the incoming velocity of the particle. There are several complicating factors, however, in taking these measurements and trying to figure out where the particles originally came from. Cassini measured these impacts as it was orbiting Saturn, so the dust particles that hit the CDA were also affected by Saturn’s gravity, causing their orbits to bend and their speeds to change. Most of the particles detected by the CDA are those in Saturn’s E ring that ultimately came from the moon Enceladus. Some were eroded off the main rings or other moons in the system. Through painstaking analyses of years of data, the CDA team has been able to separate out those signals due to interplanetary particles from the far more abundant impacts from dust particles orbiting Saturn. The key result from these measurements is the flux of micrometeoroids in interplanetary space at Saturn’s location in the solar system. The flux is the number of micrometeoroids of a particular size flying through a given area, say one square centimeter, each second, before their orbits were affected by Saturn’s gravitational pull. By adding this flux up for all different particle sizes a total mass flux of

Figure 6.5. Cassini’s Cosmic Dust Analyzer. The large opening is covered with grids of thin wires that sense the charge on dust grains passing through them to the impact detection plate at the bottom. The two gray circles outside the main opening are able to measure impacts at high rates from small grains when Cassini passed through particularly dusty areas in the Saturn system. Image Credit: NASA/JPL-Caltech.

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interplanetary dust can be calculated. Then the rate of impacts of these particles on the rings can be calculated. It depends not only on the mass flux of the particles but also what their orbits are and how fast they are moving. Slower moving dust particles spend more time in Saturn’s vicinity, allowing them to be accelerated or focused by Saturn’s gravity in towards the planet and the rings. CDA found that most of the dust in Saturn’s vicinity are particles on relatively circular orbits, similar to Saturn’s, indicating that the source of those dust particles are comets in the Kuiper belt. The Kuiper belt is a vast but sparsely populated region of icy objects beyond the orbit of Neptune. Pluto is its most famous resident. Prior to Cassini it was suspected that much of the dust in the outer solar system originated on comets with highly elongated orbits, like that of Halley’s comet. With the more circular dust orbits found by Cassini, Saturn’s gravity is able to increase the flux of those particles hitting the rings more than if the particles were coming in on Halley-like orbits. The net result of these measurements is that the meteoroid flux would be expected to darken an initially pure ice ring to its current reflectivity in only about 100 million years. But this conclusion comes with a number of caveats and assumptions about how much ejecta is produced by a meteoroid impact and the composition of the impacting material. The measurement of the impacting flux also has fairly large uncertainties. Furthermore, it is a measurement of the impacting flux now. There is the possibility that the present epoch has a significantly higher (or lower) flux of interplanetary dust out at Saturn than the average value for the history of the solar system. Nevertheless, taken at face value, the high reflectivity of Saturn’s rings combined with our knowledge of the impacting flux suggests that the rings are much younger than the solar system, perhaps only a few tens of millions of years.

6.5 The erosion of the rings Darkening of the rings due to impacts and spreading of the rings due to their viscosity (collisions between the ring particles) are not the only evolutionary processes relevant to the question of the age of the rings. Some ring material is lost inward to Saturn’s atmosphere. During the Grand Finale orbits, several instruments directly measured particles in between Saturn and the inner edge of the rings as the spacecraft dove through this gap. The CDA measures particles that manage to enter its bowl-like aperture (figure 6.5). But it has also been known since the days of the Voyager spacecraft that the entire spacecraft body also acts as a dust detector in conjunction with the Radio and Plasma Wave Science (RPWS) investigation. Another instrument, the Ion and Neutral Mass Spectrometer (INMS), measured the abundance and composition of molecules near the rings. The RPWS consisted of three long antennas sticking out of the body of Cassini. Its primary scientific purpose was to measure radio waves emanating from Saturn, such as from lightning in the atmosphere, and the ionized gas in Saturn’s magnetosphere by collecting charged particles in the space around the planet. But when a small dust particle smacked into the spacecraft it would vaporize and ionize a tiny piece of the spacecraft body, and those ions were collected by the RPWS antennas. It could measure the rate of tiny dust particle impacts onto the spacecraft at a higher

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rate than the CDA, but without any information on the direction of the dust particles or their composition (figure 6.6). Much to everyone’s surprise, when Cassini flew between the planet and the rings during its Grand Finale orbits there weren’t many impacts onto the spacecraft by dust particles at all. On the first Grand Finale orbit, Cassini mission planners oriented the spacecraft so that it used its large radio dish antenna as a shield to protect the sensitive electronics and instruments from particle impacts. There were far fewer dust impacts than anticipated and far fewer than had been seen when Cassini crossed the plane of the rings beyond the outer edge of the rings. Compare the animations in figures 6.6 and 6.7. The lack of impacts led to the region between the rings and the planet being dubbed The Big Empty. However, while it was empty of relatively large dust particles, other instruments measured molecular and nanograin particles in this region, showing a surprisingly large erosion rate of the rings. The CDA measured the composition and sizes of dust particles throughout the Grand Finale ring-plane crossing. Based on their measurements, if the flux of dust from the rings has been constant, the rings would erode in a few hundred million years (Hsu et al 2018). The INMS instrument measured molecules falling into the atmosphere from the rings and found a rate of about 5000–50 000 kg s−1. If we combine these rates with the measured mass of the rings, we find a timescale for them to erode of only 10–100 million years. Again, it’s important to remember that

Figure 6.6. RPWS data show the intensity of radio waves as a function of the frequency of the radio wave in a spectrogram, shown here. The color indicates the intensity of the wave. Because the frequencies of the radio waves are the same frequencies as the sound waves that humans hear, so this movie includes a soundtrack that plays the radio waves as though they are sound waves. Note, though, that they are not sound waves; sound is just another way of representing the data in the spectrogram. These data were collected during a ring-grazing orbit when Cassini flew just outside Saturn’s F ring. As it crossed the ring plane there was a marked increase in radio waves produced by plasma generated by impacts onto the spacecraft. Animation credit: NASA/ JPL-Caltech/University of Iowa. Movie available at https://iopscience.iop.org/book/978-1-64327-714-1.

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Figure 6.7. This RPWS spectrogram and animation shows data collected during the passage of Cassini between Saturn’s rings and its atmosphere on the first Grand Finale orbit. The time of the ring-plane crossing shows no significant increase in dust particle impacts. The pattern of emissions includes whistles that are due to plasma waves from Saturn’s magnetosphere and atmosphere that are not related to dust impacts. Animation credit: NASA/JPL-Caltech/University of Iowa. Movie available at https://iopscience.iop.org/book/978-1-64327-714-1.

we are measuring a snapshot of the erosion rate with all these instruments, and it is speculative to assume that the rates have been constant over the age of the solar system. Indeed, the faint, innermost D ring, which is the source of much or all of this material, changed just during the 13 years of Cassini’s mission at Saturn. Bright clumps appeared in a ringlet within the D ring in 2014 or 2015 and may be due to collisions among some larger objects within the D ring (Hedman 2019). This just emphasizes how challenging it is to reconstruct the history of the rings over timescales of millions to billions of years when the rings have significant changes on timescales of a decade. Both CDA and INMS measured the composition of the particles falling into Saturn and found them to be quite different than the measured composition of the rings as a whole. As we discuss below, this provides a tantalizing clue to the history of the rings.

6.6 The composition of the rings Astronomers’ first tool to determine the composition of a distant object is spectroscopy. Different elements and molecules interact with light in different ways. This gives us a colorful Universe to live in and a remarkably powerful tool to learn about the distant Universe. From measurements of the amount of light reflected by Saturn’s rings at different wavelengths (a technique called reflectance spectroscopy), we know that Saturn’s rings are very nearly pure water ice. Or, rather, 6-11

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we know that the surfaces of the ring particles are water ice. Just from spectroscopy in the visible and infrared parts of the spectrum, for all we know the ring particles could be solid iron with a thin coating of ice. Of course, collisions between the particles and impactors from afar would smash through a thin veneer and expose the underlying material. Thus, while we only see the surfaces of the particles, it is challenging to explain how those surfaces could show a nearly pure water ice composition if the bulk composition of the ring particles were not also mostly water ice. Since the impacting micrometeoroids are not pure water, over time they change the composition of the rings as they bombard them, year after year, millennium after millennium. Nevertheless, it is possible that by looking only at the surfaces of the ring particles we are missing something about the bulk composition that would alter the narrative of young rings suggested by their high reflectivity and the impactor flux. Cassini made measurements that helped resolve this problem. The Cassini Radar (radio detection and ranging) experiment uses radio waves emitted from Cassini’s highgain antenna to illuminate the rings and then look at the radio waves reflected back from the rings. Because radio waves have a longer wavelength than visible light waves, they penetrate below the surfaces of the ring particles (or whatever they hit), providing us with a way to peer inside the particles. Based on these observations, Z. Zhang and her colleagues were able to show that the bulk composition of the rings really is almost pure water ice, with less than 1% non-icy material in the massive A and B rings, and a few per cent of non-icy material in the C ring and Cassini Division (Zhang et al 2017).

6.7 The age and origin of the rings If the rings are not leftover planet-forming material from the days of the formation of the solar system, something, either a moon or a comet, must have been broken apart by an impact or by tidal forces from Saturn. All of these scenarios are problematic in one way or another. So where do we stand at the conclusion of the Cassini mission? We now have a good measurement of the mass and composition of the rings, indicating that they are mostly water ice and have a total mass that is about 0.4 times that of the moon Mimas, about half the mass that was inferred in the 1980s from Voyager measurements. We also have a measurement of the current rate of interplanetary micrometeoroids onto the rings that suggests that a pure icy ring would reach its current level of non-icy contamination in less than 100 million years, and we have measurements that show material falling from the rings into the planet that, if the rate remained constant, would lead to the rings eroding away in less than 100 million years. But while those measurements seem to make a compelling case for the rings being young, there are some significant questions that must be addressed to have confidence in that conclusion. The current mass of the rings agrees very well with that predicted by spreading models of a ring system that started off 4.5 billion years ago with a much greater mass. Is this just a cosmic coincidence? The composition of

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the material falling from the rings into the planet does not match the composition of the rings themselves. Perhaps the rings maintain a nearly pure water ice composition not because they are young, but because some process, yet to be clearly defined, preferentially removes the non-icy material from the rings. In other words, are the rings being gradually cleaned by Saturn somehow? Finally, if the rings are less than 100 million years old, what could have happened at that time to create a nearly pure ice ring system? Even with the relatively low mass measured by Cassini, our understanding of the population of objects in the outer solar system suggests that it is very unlikely (about one in a hundred probability) that an object large enough to form Saturn’s rings would be disrupted within the last 100 million years. We are thus left, as is frequently the case in science, with as many new questions as we have new answers from this investigation. Research continues on the data returned by Cassini, as well as on the processes active in the rings and the history of the outer solar system that will help us answer these questions, and undoubtedly raise new ones. For now, the weight of evidence points to the rings being young, but the questions above are important and are an area of ongoing research. Their resolution may yet point to an older origin for Saturn’s rings.

Further reading See Charnoz et al (2010), Hedman (2019) and Zhang et al (2017). A review of the principle findings of Cassini for Saturn’s rings with an emphasis on the final stages of the mission is in Tiscareno et al (2019). INMS measurements of ring erosion are in Waite et al (2018). The mass measurement is described in Iess et al (2019). CDA measurements of the ring erosion are in Hsu et al (2018).

References Charnoz S, Salmon J and Crida A 2010 Nature 465 752–4 Hedman M M 2019 Icarus 323 62–75 Hsu H-W et al 2018 Science 362 eaat3185 Iess L et al 2019 Science 364 eaat2965 Tiscareno M S et al 2019 Science 364 eaau1017 Waite J H et al 2018 Science 362 eaat2382 Zhang Z et al 2017 Icarus 294 14–42

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The Ringed Planet (Second Edition) Cassini’s voyage of discovery at Saturn Joshua Colwell

Chapter 7 Titan: the planet moon

Worlds can be categorized many different ways. This is why we have so many confusing and overlapping names for them, such as planets, minor planets, moons, dwarf planets, asteroids, and comets, to list just the most common categories, and this is why there is a ridiculous debate about how many planets there are in the solar system. One way of categorizing them is by the presence or absence of an atmosphere. Almost every world in the solar system is airless: a solid mass, either icy, rocky, or metallic in bulk, that has no atmosphere and whose surface is exposed directly to space. Four notable exceptions are the four largest planets, and they are essentially entirely atmosphere. While the state of the material deep in their interiors changes from gaseous to solid, there is never really a surface to speak of. And then there is a handful of worlds that have a surface that is protected by the comforting blanket of an atmosphere. Our world and its nearest neighbors Mars and Venus fall in this category. The only other world with a surface we could stand on and an atmosphere substantial enough to hide stars in the daytime is Saturn’s moon Titan. There are only two worlds in the solar system where a human can survive on the surface without a pressure suit: Earth and Titan1. Titan has an atmosphere with many similarities (and some crucial differences) to that of Earth. It is mostly composed of molecular nitrogen (N2), and the atmospheric pressure at the surface of Titan is about 1.5 times that at sea level on the Earth. Like water on the Earth, there are molecules in Titan’s atmosphere that can form clouds and rain that contribute to

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A pressure suit, or more commonly a spacesuit, has an airtight seal maintaining enough air pressure on the inside to permit a human to survive. On Titan the temperatures are so low that contact with its air would cause instant tissue death, but as long as one is covered and heated and has a supply of oxygen, the suit would not need to be pressurized. On the surface of Venus the atmospheric pressure is the same as being 900 m below the surface of the oceans on Earth. However, at an altitude of 50 km in the atmosphere of Venus the pressure would be Earthlike and the temperature like a cold winter day, but the air would still be lethally devoid of oxygen.

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lakes in its polar regions. Vast fields of dunes are formed by its weak winds. To understand Titan, we must understand the basics of how atmospheres work.

7.1 The atmosphere Atmospheres are hit from above by sunlight and by the solar wind, the diffuse flow of mostly electrons and protons outward from the Sun. If the moon is within a planet’s magnetosphere, like Titan is, the atmosphere is also hit by charged particles that are trapped in that magnetosphere. On the other side, the bottom of the atmosphere is bounded by the surface of the world, Titan in this case. The story of atmospheres is the story of the transport and exchange of energy between these two boundaries, one at the top and one at the bottom of the atmosphere. One thing the Universe hates is sharp gradients in energy. Put a hot thing next to a cold thing and energy will move from the former to the latter until the temperatures even out2. The greater the difference (the gradient), the faster energy will flow. Atmospheres absorb energy at the top and the bottom and what goes on in between is a complicated mediation between those two energy sources involving chemistry, fluid dynamics, and radiative energy transport. Molecules, like the individual atoms they are composed of, can absorb or emit energy only in discrete quantized amounts, and those amounts depend on the type of molecule. There are three main types of energy levels in a molecule: electronic, vibrational, and rotational. Electrons within an atom can be excited to a set of discrete energy levels by absorbing exactly the right amount of energy from a photon of light. Because the energy of a photon is uniquely determined by the color, or wavelength, of the light, that means only certain colors of light can be absorbed or emitted by any particular atomic element. In addition to these electronic transitions, molecules can also exchange energy by changing their rotational state or their vibrational state. Our normal intuition for macroscopic objects is not correct for molecules. They cannot spin at arbitrary rates like an ice skater can. They can only spin at a finite set of discrete spin rates, each one corresponding to a specific rotational energy, and in turn that energy corresponds to a specific wavelength of light. The same is true for the vibrational states. Each type of oscillation of the atoms in a molecule, like masses on the ends of a spring, are governed by quantum rules as well. It is this remarkable relationship between energy, the wavelength of light, and the behavior of atoms and molecules that enables us to learn so much about the Universe simply by studying the patterns of light from distant objects. Every pattern of light emitted or absorbed by something is related to the molecular composition of that thing, and it is altered by the motion, temperature, and physical state (solid, gaseous, ionized) of the material as well. Spectra, the maps of light from an object, are packed with information. 2 It’s worth noting here that energy will never flow from the cold thing to the hot thing. This is one way of stating the Second Law of Thermodynamics. It can also be expressed as: everything ultimately runs down and gets worse, which is why it’s my least favorite law of nature. Although, to be fair to the Second Law, it would be a mighty confusing Universe if this were not the case.

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The energy levels associated with electronic transitions usually correspond to ultraviolet (UV) wavelengths of light, while those due to vibrational states are generally in the infrared (IR) part of the spectrum3. Rotational states are important in the IR part of the spectrum for molecules that have a complicated shape like methane (CH4) and less so for simple molecules like N2. The top of the atmosphere is hit by the full solar spectrum, from short-wavelength x-ray and UV radiation to long-wavelength IR radiation. The Sun is brightest in the visible portion of the spectrum, but the shorter x-ray and UV wavelengths carry more energy per photon. Those energetic, short-wavelength photons are absorbed by atoms and molecules at the top of the atmosphere. The upper atmosphere is heated by this radiation to high temperatures. This is true of all planetary atmospheres because all gases can absorb these short-wavelength photons, usually by being ionized or by molecules being broken apart. In addition, solar wind particles (electrons and protons streaming outward from the Sun) and electrons and ions within Saturn’s magnetosphere can impact atoms and molecules in the upper atmosphere, causing them to break apart into different molecules or their constituent atoms or to glow when they absorb energy from the particles and then release that energy in the form of photons. This is the process that gives rise to aurorae on Earth and on other planets with magnetic fields. The magnetic fields concentrate the flux of charged particles onto certain areas of the atmosphere (see chapter 8 for Saturn’s aurora). Without this concentrating effect there can be a more diffuse glow high in the atmosphere called airglow or diffuse aurora (figure 7.1). At the bottom of the atmosphere, the solid surface of Titan radiates its own spectrum of light upward. That spectrum is determined by the temperature of the surface. The hotter the surface, the shorter the average wavelength of the radiation. The surface of the Earth is about 290 K and the peak wavelength of the Earth’s radiation to space is about 10 microns4. The surface temperature of Titan is about 100 K, and the peak wavelength of its radiation is about 30 microns. Diatomic molecules, such as molecular nitrogen (N2) which makes up the bulk of Titan’s (and Earth’s) atmosphere, do not have rotational energy transitions in the near-IR part of the spectrum, but molecules such as H2O (water), CO2 (carbon dioxide), and CH4 (methane), which have complex, asymmetric structures, have multiple rotational energy transitions in the near-IR. They are known as greenhouse gases because planetary surfaces cool off by radiating energy in the IR part of the spectrum (wavelengths longer than 1 micron), and these gases absorb those wavelengths of light. This traps heat in the atmosphere and prevents the radiation from escaping to space. A greenhouse gas is like a blanket: it prevents heat from leaving a world. Greenhouse gases are transparent in the visible part of the spectrum, however, where the incoming energy from the Sun is concentrated. Thus, heat in the form of solar radiation gets through the atmosphere to the planet, but it has a hard time getting

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Short wavelengths correspond to higher-energy photons, and lower-energy photons have longer wavelengths. The visible part of the spectrum (where humans can see) is about 0.4–0.7 microns coinciding with the peak of the Sun’s spectrum at about 0.5 microns. 4

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Figure 7.1. Ultraviolet (UV) images of Titan taken by the Cassini Ultraviolet Imaging Spectrograph. The white circle shows the size of the solid body of Titan. The lower right image is a composite of the other three which use different parts of the UV spectrum. The Lyman alpha component (upper left) shows the shadow of Titan and its extended atmosphere against the diffuse glow of interplanetary hydrogen. The LBH component (refers to spectral features of molecular nitrogen named after Lyman, Birge, and Hopfield) shows airglow emission in the UV from the upper atmosphere, while the solar component shows reflection from haze layers in the upper atmosphere. Image Credit: NASA/JPL-Caltech/Univ. Colorado/Kris Larsen.

out in the form of IR radiation that greenhouse gases absorb. This is how the greenhouse effect in an atmosphere works5. Although N2 is not a greenhouse gas, Titan’s atmosphere is a few per cent methane. The greenhouse warming provided by methane is essential to maintaining temperatures warm enough for N2 to remain a gas in the atmosphere. Without this greenhouse warming, Titan’s atmosphere would be far less dense than our own. As we journey up from the surface of Titan there is less and less methane to trap radiation from the surface, and so the temperature drops until it reaches a minimum at what is called the 5

A botanical greenhouse also lets sunlight enter (via its transparent glass roof), but the heat is trapped primarily by the physical barrier of the roof preventing convective cooling (the expansion and rising of warm air) rather than the prevention of IR radiative cooling.

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Figure 7.2. Temperature and structure of Titan’s atmosphere. NASA/JPL-Caltech.

tropopause (figure 7.2). The layer of the atmosphere between the surface and the tropopause, the troposphere, is where most weather occurs because the decreasing temperature with altitude allows convection to carry warm, wet air upward, like the rising bubbles in a pot of boiling water. As we approach the tropopause the temperatures are lower, and the wet stuff may condense into clouds. On Earth the wet stuff is H2O. On Titan it is methane (CH4) and ethane (C2H6). Convection is much weaker in Titan’s atmosphere than in the Earth’s, in part due to the much lower amount of solar heating reaching the surface. Saturn (and Titan) is 10 times further from the Sun than the Earth so the flux of energy, the amount of energy hitting a surface each second is 100 times less. Titan’s global haze further reduces the amount of sunlight reaching the surface. The difference between daytime and nighttime temperatures on Titan is only 1.5 K6. Seasonal temperature variations and temperature variations from equator to pole are only slightly larger at the surface (Cottini et al 2012). However, at higher altitudes the seasonal temperature variations are larger. Cassini found a seasonal temperature change of 19 K at the south pole (Sylvestre et al 2019). As we continue to move up in the atmosphere, above the tropopause, the atmosphere begins getting warmer again, even as the atmospheric pressure and density continue to decrease. This is due to warming from above: the upper atmosphere is heated by energetic solar photons that are absorbed before making it deep into the atmosphere or to the surface. Here, in the upper atmosphere, there is a different kind of

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A temperature difference of 1 K is the same as a temperature difference of 1 °C.

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Figure 7.3. This schematic shows the formation of large organic molecules called polycyclic aromatic hydrocarbons, or PAHs, as well as particulate aerosols that make up the haze on Titan, obscuring its surface from view. Note the difference in altitudes compared to figure 7.2. Image Credit: ESA/ATG medialab.

rain. Hydrocarbon gunk produced with the help of those energetic solar photons as well as energetic electrons from the Sun and from Saturn’s magnetosphere is formed, producing a global haze layer. There is another striking difference between Titan’s atmosphere and Earth’s. Titan’s weaker gravity (about one-seventh of the Earth’s, slightly less than that on the Earth’s Moon) means the atmosphere extends to much greater heights than the Earth’s. At 140 km altitude, for example, Titan’s atmospheric pressure is about 0.2% of the surface pressure. That same pressure is reached at the Earth at an altitude of only about 42 km, while at 140 km above the surface of the Earth the pressure is down to five billionths of the surface pressure. The chemistry of hydrocarbons can be quite complex due to carbon’s ability to form very large molecules. In the upper atmosphere, methane is broken apart by solar photons and electrons, and the constituent pieces recombine to form larger molecules with a greater ratio of carbon to hydrogen than the 1:4 ratio in methane (figure 7.3). Some of the hydrogen atoms, being low mass and therefore moving 7-6

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faster than carbon and the heavier molecules at a given temperature, escape into space. The large hydrocarbon molecules that form from the byproducts of methane can form microscopic particles called aerosols that make up Titan’s haze layers (figure 7.4). The haze is what makes it particularly difficult to see the surface of Titan from space at visible wavelengths. Clouds are actually much rarer on Titan than on Earth (figure 7.5).

7.2 Lakes, dunes and mountains Titan and Earth are the only bodies in the solar system with bodies of liquid on their surfaces, though the amounts are vastly different. The evidence for liquids on the surface of Titan was tantalizing and contradictory for many years prior to Cassini’s arrival. The Huygens atmospheric probe that Cassini carried to Titan was designed to float in a hydrocarbon sea, but that capability turned out not to be necessary. The equatorial region of Titan where Huygens landed does not appear to have liquids on the

Figure 7.4. This view of the south polar region of Titan shows multiple high-altitude haze layers that appear bluish, and the main atmospheric haze layer which gives Titan its distinctive orange coloration. There is a kink or depression in the haze layers near the south pole that may be related to seasonal changes. This natural color image was taken in September 2011, shortly after equinox. Southern winter is beginning, and a polar vortex may be forming in the south, resulting in the depression in the haze altitude. Image Credit: NASA/JPL-Caltech/SSI.

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Figure 7.5. Thin clouds can be seen circulating around the north polar region of Titan in this series of images. Dark hydrocarbon lakes dot the polar region. Image Credit: NASA/JPL-Caltech/SSI/Univ. Arizona. Movie available at https://iopscience.iop.org/book/978-1-64327-714-1.

surface now, though there are features reminiscent of rivers and shorelines, suggesting enough liquid flowed in the not-too-distant past to carve these channels (figure 7.6). The difficulty in determining whether there is liquid on the surface stems in part from the global haze layers that obscure the surface from view in visible light (figure 7.7). In certain wavelength ‘windows’ in the IR, the atmosphere is much clearer, and surface brightness variations can be seen, but it is not clear just from these variations whether the surface is liquid or not (figure 7.5). The landscape near where the Huygens probe landed looks like it is crossed by fluvial features, yet Huygens came to rest on dry ground. Accelerometer data from Huygens indicate that it may have punched through a thin crust on the surface, leading the penetrometer team to famously compare the surface texture to crème brulée (Lorenz & Mitton 2010). One telltale indicator of a liquid surface is the bright specular reflection of sunlight, or ‘sunglint’, off the smooth liquid surface. This is the kind of reflection one sees on a body of water on the Earth or a bright light reflecting off a mirror or other smooth surface. These reflections proved elusive on Titan because, as it happens, the liquid hydrocarbons are restricted to a handful of lakes near the poles, primarily

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Figure 7.6. As the Huygens probe descended through Titan’s atmosphere, cameras captured views in all directions. This mercator projection from an altitude of about 10 km reveals a complex landscape absent impact craters but showing channels reminiscent of rivers and a shoreline towards the northwest. Image: ESA/ NASA/JPL-Caltech/Univ. Arizona.

the north pole, and the relative geometry of the Sun and lake must be the same as the geometry between the lake and Cassini for the glint to be observed. Since Cassini orbited Saturn rather than Titan, the opportunities to observe them were few and far between. An example of a glint, observed in the IR by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) instrument at a wavelength where the atmosphere is more transparent, is shown in figure 7.8. Following the conclusion of the mission, the Cassini VIMS team assembled images of Titan made over the course of the 13 years at Saturn at different wavelengths and from different viewing geometries and produced the false-color views shown in figure 7.9. Titan has the look of a planet, with a complex variety of landforms and geology. It’s a world that demands the kind of exploration so far reserved for the Moon and Mars7. In June, 2019, NASA announced that Dragonfly, a New Frontiers class mission, will launch to Titan in 2026 with arrival in 2034 for the next step of this exploration (chapter 9). Cassini’s Radar instrument looked at the reflection of long-wavelength radio waves off the surface. These centimeter-scale photons pass freely through the hazy atmosphere. Each time Cassini flew by Titan it could point its radio dish and create an image strip of the surface underneath the spacecraft at radio wavelengths. These images can be deceptive, though. Things that appear bright in a radar image are 7

And Earth, of course!

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Figure 7.7. Yes, this is a real picture. Titan’s surface is concealed by a global haze layer. The rings cast an intricate shadow on Saturn in the background. Image Credit: NASA/JPL-Caltech/SSI.

reflective at radio wavelengths of a few centimeters, but would not necessarily appear bright to the human eye. To be reflective of radio waves, the surface must be rough at the scale of the wavelength of the radio waves. Thus, rocky structures like craters, mountains, and boulders appear bright, while smooth areas such as plains, deserts and, yes, lakes, appear dark (figure 7.10). Once Cassini started getting radar measurements near the polar regions, well-delineated dark regions with dendritic channels leading into them appeared. While these look for all the world like lakes, additional measurements confirmed the case. One particularly dramatic radar observation looked straight down as Cassini flew over one sea on Titan, Ligeia Mare, and measured reflection off the surface of the lake and a separate echo of the radar beam from the lake bottom. This radar observation showed the depth to be up to 160 m (Mastrogiuseppe et al 2014). The radar passed through the liquid methane and ethane of the lake and bounced off the bottom. Ligeia Mare (figure 7.10), and presumably the other lakes, are primarily liquid methane based on their radar reflection properties (Le Gall et al 2016). The dark appearance of the lakes at radar wavelengths indicates that the lakes are very smooth, with no waves to speak of. At more equatorial regions, radar swaths revealed sets of nearly parallel linear structures (figures 7.11 and 7.12). These dunes reveal the presence of winds near the

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Figure 7.8. This false-color image was taken in the infrared (IR) by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) in 2012. The bright point is a reflection of the lake Kivu Lacus near the north pole of Titan. The pink is light from the surface scattering off atmospheric haze layers. The first such specular reflection was observed in 2009 with the beginning of northern spring on Titan so that the Sun could shine on the polar lakes. Image Credit: NASA/JPL-Caltech/Univ. Arizona.

surface. Such winds might be expected to produce waves on the lakes, so they may be absent or much weaker at higher latitudes. The largest and most prominent equatorial feature is named Xanadu (figure 7.13). The hydrocarbon sand dunes lay in the lowlands in the equatorial region. The Dragonfly mission will land a dual quadcopter on these dunes in 2034. Surface winds deflect around higher regions such as Xanadu leaving them rough and bright in radar images. Mountains on Titan are mostly located at equatorial latitudes. The highest mountains are located in Xanadu (figure 7.15). An example of a complex landscape dubbed labyrinth terrain is shown in figure 7.14 in a radar image. These regions may have been eroded by methane rivers. Traces of valleys that were carved by these rivers, not currently present, drained liquid from the lower right to the upper left on the image. Even the dry regions of Titan may be playing an essential role in maintaining the lakes and Titan’s methane cycle. The processes that break down methane in the upper atmosphere producing ethane, which drifts downward, and hydrogen, which

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Figure 7.9. These six images show the complete surface of Titan in false color from IR data obtained by the VIMS instrument over the full course of the mission. The dune fields are brown in this representation. Image credit: NASA/JPL-Caltech/Université de Nantes, University of Arizona.

Figure 7.10. This is a colorized segment of a radar image of Titan taken in 2007 by Cassini showing the secondlargest lake or sea on the moon, Ligeia Mare, in the north polar region. Dark areas are smooth, transparent, or absorbing at the 2 cm wavelength of the radar beam from Cassini. NASA/JPL-Caltech/ASI/Cornell.

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Figure 7.11. This radar image-strip of Titan was taken in 2016 during Cassini’s 122nd flyby of Titan and shows sets of parallel features that are sand dunes produced by surface winds traveling from west to east (left to right in the image). The dunes are interrupted by mountains and other topogrophaic features that show up as bright regions in radar. Image Credit: NASA/JPL-Caltech/ASI/Université Paris-Diderot.

escapes, would empty the atmosphere of its current supply of methane in 10 million years (Lunine & Atreya 2008). At the same time, Cassini observations don’t show much activity in the way of active rainfall or cryovolcanism. The lakes also appear to be relatively stable, so the lakes themselves are not rapidly evaporating to replenish the atmospheric methane (Le Gall et al 2016). There must be a source of methane for the atmosphere, or we are living in a relatively unusual epoch and Titan’s current supply of atmospheric methane is due to a recent event that released it from the subsurface. The Huygens probe heated the surface after landing to help

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Figure 7.12. Close-up of the dune fields shown in figure 7.11. Image Credit: NASA/JPL-Caltech/ASI/ Université Paris-Diderot.

boil off volatiles such as methane that could be collected and analyzed by the probe’s gas analyzer. The amount of methane measured increased by 40% following this heating (Niemann et al 2011).

7.3 Titan’s interior We have seen the dramatic results of tidal flexing in powering the geysers of Enceladus (chapter 4). Titan’s orbit is, like Enceladus’s, slightly eccentric, meaning it should have a varying amount of tidal deformation as it orbits Saturn. If Titan were solid through and through, the size of its tidal bulge—the amount of deformation from a mean spherical shape due to the tidal force from Saturn—should be about 1 m. The average ocean tide on the Earth due to the Moon is not much larger than this, and is visible as the sloshing of the oceans up and down our beaches every 12 and a half hours or so. Titan provides no oceans for such measurements, and Cassini lacks the imaging resolution or temporal coverage to see such small changes

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Figure 7.13. This map of Titan is based on a combination of Cassini imaging and radar measurements. The dark equatorial regions are covered in dunes of hydrocarbon sands. The large bright area named Xanadu deflects the winds around it and is sand free. Lakes are found only in the polar regions. Image: NASA/JPLCaltech/SSI/USGS.

Figure 7.14. A synthetic-aperture radar image obtained on 7 June 2016, showing labyrinth terrain that appears to have been caused by erosion of a fluid, likely liquid methane. The direction and form of the channels shows a flow from the lower right to the upper left. On the right is a similar morphology on Earth in southern Java, but note the much smaller scale of the image of Earth. Image Credit: NASA/JPL-Caltech/ASI.

anyway. Instead, the Cassini Radio Science team was able to measure the amount of tidal flexing of Titan by analyzing the Doppler shift of a radio signal from Cassini to the Earth (Mitri 2014; Zebker et al 2009). Just as the siren of a passing ambulance changes tone depending on whether it is approaching or receding from you, the frequency (or equivalently wavelength) of light received from a moving source depends on that source’s velocity towards or away from the receiver. The large radio antennas of the Deep Space Network can

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Figure 7.15. These ridges known as Mithrim Montes include the highest mountain peaks on Titan. This is a radar image which shows rough surfaces as bright or reflective, and dark surfaces are smooth or have material properties that allow the radar waves to penetrate rather than reflect off the surface. Image Credit: NASA/JPL-Caltech.

measure minuscule changes in the velocity of Cassini by monitoring the small changes in its broadcast frequency as its speed is altered by the gravity of nearby objects. This technique is so powerful that, by measuring the rate of change of the radio frequency during flybys of Titan at different locations in its elliptical orbit, the Radio Science team could measure the amplitude of the tidal deformation and found it to be 10 m, 10 times larger than would be expected for a completely solid moon. The only way for such a large deformation to be produced is if Titan’s crust is a solid shell disconnected from the bulk of the moon by a liquid layer. Based on the abundance of water ice throughout the Saturnian system, as well as the density profile of the interior of Titan determined from radio Doppler measurements, this global subsurface ocean is almost certainly water. Just as methane plays the role of water on Titan, water ice plays the role of terrestrial rock. At the low temperatures on Titan, water ice is nearly, but not quite, as hard as rock. The rough terrain on Titan is primarily water ice. Over geological timescales, these features can relax and deform under their own weight. The tallest mountains on Titan are a little over 3 km in height (figure 7.15), significantly less than the tallest mountains or volcanoes on Earth. Just as rock is partially molten beneath the Earth’s crust, allowing convection that drives plate tectonics and volcanism, water ice is molten in Titan’s interior. The presence of large topographic relief, such as the mountains in figure 7.15, in spite of a global subsurface ocean and the generally softer nature of Titan’s ice ‘rocks’ suggests that Titan is geologically

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active, though the forces driving the mountain building on Titan are probably related to tidal flexing and cooling of the crust. This buried ocean may hold the key to the persistence of methane in the atmosphere. The destruction of methane in the upper atmosphere by solar photons and energetic electrons requires a source at the surface to maintain the current abundance for more than a few million years. Minerals in Titan’s interior react with water through a series of reactions called serpentinization that produces hydrogen and ultimately methane, where water is the source of the hydrogen and carbon dioxide (CO2) is the source of the carbon in the methane (Glein 2015). The methane percolates through the crust and into the atmosphere with some eventually finding its way into the polar lakes. What it must look like to float on a sea of liquid methane, glossy flat, with the faint orange-hued sunlight filtering through the haze high above and a shoreline of ice mixed with hydrocarbons, hard as rock. We must return to Titan with a mobile explorer to sample the wide array of vistas and understand more about how this world works.

Further reading Following the nominal mission of Cassini and the completion of the Huygens probe mission, a compendium book on results was published that is a companion to the Saturn from Cassini-Huygens book referenced in other chapters (Brown et al 2009). An early look at results from Cassini and Huygens with lots of behind-the-scenes information on the mission is in Lorentz & Mitton (2010). Harland (2007) provides an overview of the mission and results form Huygens. An overview of all Cassini Visual and Infrared Mapping Spectrometer (VIMS) observations of Titan is presented in Le Mouélic et al (2019).

References Brown R H, Lebreton J-P and Waite J H (ed) 2009 Titan from Cassini-Huygens (Berlin: Springer) Cottini V et al 2012 Planet. Space Sci. 60 62–71 Glein C R 2015 Icarus 250 57–586 Harland D M 2007 Cassini at Saturn—Huygens Results (Berlin: Springer) Le Gall A et al 2016 J. Geophys. Res. 121 233–51 Le Mouélic S et al 2019 Icarus 319 121–32 Lorenz R and Mitton J 2010 Titan Unveiled: Saturn's Mysterious Moon Explored (Princeton, NJ: Princeton University Press) Lunine J I and Atreya S K 2008 Nat. Geosci. 1 159–64 for a description of the methane cycle on Titan Mastrogiuseppe M et al 2014 Geophys. Res. Lett. 41 1432–7 Mitri G 2014 Icarus 236 169–77 Niemann H B et al 2011 J. Geophys. Res. 115 E12006 Sylvestre M et al 2019 Icarus doi:10.1016/j.icarus.2019.02.003 Zebker H A et al 2009 Science 324 921–3

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The Ringed Planet (Second Edition) Cassini’s voyage of discovery at Saturn Joshua Colwell

Chapter 8 Saturn’s hidden mysteries

Our journey through the Saturn system brings us now to the master of the realm, the planet itself. Here we’ll see what Saturn is made of and why and how it differs from the other planets in composition and structure. We’ll explore the stormy atmosphere, its magnetic field, and how it interacts with its family of moons and rings. The Grand Finale orbits enabled detailed gravity measurements of the interior, revealing surprising complexity compared to Saturn’s planetary sibling, Jupiter.

8.1 Origin and interior Saturn, like Jupiter, is what is known as a gas giant planet (figure 8.1). The bulk of its mass, which is 95 times the mass of the Earth, is made up of the light gases hydrogen and helium. This is not terribly surprising given that those are the most abundant elements in the Universe and also make up the bulk of the Sun. This bulk composition gives Saturn the lowest density of the planets, just 0.69 g cm−3. The density of water is 1 g cm−3. The low density and compressible nature of its thick atmosphere, combined with a rapid rotation period of just 10.6 h, result in Saturn being the most oblate, or flattened, planet in the solar system. Its polar diameter is just 90% of its equatorial diameter, giving it a noticeably flattened appearance (figure 8.2). The protostellar nebula was 98% hydrogen and helium, with the remaining 2% mostly comprised of volatile molecules such as water and carbon dioxide, with a small amount of heavier elements and molecules such as metals, SiO2 (silica), and other minerals. The inner planets of the solar system are composed almost entirely of these less-abundant heavier ingredients of the protoplanetary nebula because close to the Sun the temperatures were too high for anything else to condense into solids. Beyond a few AU from the Sun, however, temperatures were low enough for water to condense from the nebula into solid grains. This greatly increased the amount of

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Figure 8.1. This view of Saturn over the south pole, taken by Cassini in northern summer, reveals subtle atmospheric banding and the ever-present shadows of the rings on the planet. Image Credit: NASA/JPLCaltech/SSI.

solid material available to form planets, leading to larger protoplanetary objects beyond the so-called ice line1. These larger, icy protoplanets could grow to masses several times that of the Earth, but still much less than the current masses of the gas giants. However, the additional mass provided by ice increased the gravitational pull of these protoplanets enough so that they could capture the much more abundant hydrogen and helium gases (which don’t condense at any temperature) from the nebula, dramatically 1

The precise location of the ice line (also sometimes referred to as the snow line or frost line) where water could condense from the nebula varied with time as temperatures and pressures in the nebula evolved, but typical numbers would be 2–3 AU.

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Figure 8.2. This image, in near-true color, shows a large storm in the northern hemisphere. This storm appeared in late 2010 and is seen here on 24 December 2010, about three weeks after the first sign of it appeared. Over the course of the next few months the storm spread to completely circle the planet. Saturn is noticeably oblate, with an equatorial radius of 60 268 km and a polar radius of 54 364 km. Image Credit: NASA/JPL-Caltech/SSI.

increasing the masses of the planets. The gravitational capture of large amounts of material from the protoplanetary nebula also explains why the outer planets have moons and rings, while the four inner, or terrestrial planets, have a combined total of just three moons and no rings. The captured material formed a miniature solar system, of sorts, with the forming planet rather than the Sun at its center. This gravitational capture was also aided by the lower temperatures in the outer solar system which meant that the hydrogen and helium atoms did not have as much thermal energy. Jupiter, the first planet beyond the ice line, is larger than Saturn (and Uranus and Neptune) because the overall abundance of material available to build planets was largest close to the Sun. Orbital speeds also decrease with increasing distance from the Sun, so the pace of collisions necessary to build planetary cores is 8-3

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also slower further from the Sun. This scenario explains the general architecture of the outer solar system, with Jupiter weighing in at 300 times the mass of the Earth, followed by Saturn at roughly one-third that mass, and Uranus and Neptune further out and another factor of four less massive. It is worth noting, however, that hundreds of planetary systems discovered around other stars do not follow this pattern. Migration of planets after they form seems to have played a greater role in shaping the final planetary configuration than it may have in our own. Even for our own planetary system there are suggestions that it may have gone through a significant reshuffling of the planets after they formed. The Nice model (discussed in chapter 3) suggests that Neptune and Uranus may have swapped orbits, and the giant planets may have formed much closer to the Sun than they currently are. The architecture of our solar system may not be typical, if there even is such a thing as a typical planetary system. In the model of planet formation described above, the gas giants started off without so much gas, as large icy and rocky planets. These planetary embryos, as they are sometimes called, were much more massive than the Earth. Being in a colder part of the solar system and with a greater gravitational pull than the Earth, these embryos could gravitationally pull in the abundant hydrogen and helium. In the process their mass increased even more, leading to more gravitational accretion of light gases as well as any solid icy and rocky objects. This model predicts that Saturn and Jupiter have a core that is made of rock and ice at their center. It is the original seed of the planet plus additional material that was captured along with the gas that has long since settled to the center. Saturn, like all the planets, is differentiated: the densest material has settled to the center or core of the planet, with the least dense elements in the outer layers. In the case of Saturn and the other outer planets, the core is dominated by the most abundant compound that could condense beyond the ice line (namely, water ice itself) with significant fractions of rocky compounds mixed in. Differentiation is still occurring deep in the interior of Saturn as helium rains out of the primordial mixture of hydrogen and helium gases. This helium rain settles into a layer of so-called metallic hydrogen and helium that surrounds the core of rock and ice. Metallic hydrogen can form from hydrogen when the pressures are millions of times the atmospheric pressure at the surface of the Earth (Dias & Silvera 2017). The enormous mass of Saturn is able to produce such pressures deep in its interior. When the helium atoms fall toward the center, they produce heat as their gravitational potential energy is converted to thermal energy while they fall2. This ongoing process of differentiation explains why Saturn radiates roughly twice as much energy as it receives from the Sun. Above the metallic hydrogen layer there is a thicker layer of liquid hydrogen which can be produced on the Earth at very low temperatures and is used as a rocket propellant3. Most of the mass of Saturn, some 99.9%, is thus in a solid or liquid state The term ‘fall’ here is colloquial. This takes place deep in the interior of the planet where the pressures are crushingly high. The helium ‘rain’ is drifting slowly through dense liquid hydrogen until it meets the even denser layer of metallic hydrogen. Sediment falling slowly through water is a closer analogy. 3 The Centaur upper stage of the rocket that launched Cassini burned liquid hydrogen with liquid oxygen, producing water as exhaust. 2

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even though we refer to it as a gas giant. That is because the bulk of the material in the planet is hydrogen and helium, and these are gaseous under normal (terrestrial) pressures and temperatures. Above the bulk of the mass, however, there is a deep gaseous atmosphere composed, again, primarily of hydrogen and helium, but with a mixture of water and other molecules to provide some weather, as discussed below. One of the goals of the Cassini Grand Finale was to determine the interior structure of the planet by carefully measuring its gravitational field. The mass (and size) of the core is one of the results of these measurements. These measurements, described in chapter 6, were made during six of the 22 Grand Finale orbits, though only five of these provided useful data. The Cassini Radio Science team modeled the distribution of matter in the planet, rings, and moons to try to come up with a distribution that reproduced the exact trajectory followed by Cassini on these five orbits. The data are the changing frequencies of the radio signal sent from Cassini to the Deep Space Network antennas due to the changing speed of the spacecraft (the Doppler effect, chapter 6). The uncertainties in the speed of the spacecraft based on these measurements were less than a tenth of a millimeter per second. The tiny change in the speed of light (the radio wave) due to passage through the Earth’s atmosphere is one of many factors that had to be carefully modeled. The distribution of matter in the interior of Saturn was modeled in the customary way by using something called an expansion of spherical harmonics (Iess et al 2019). This involves adding up a series of increasingly lumpy objects, and weighting them so that the result matches the observations. The simplest model of Saturn would be a perfect sphere (not lumpy at all). I have previously mentioned that Saturn is flattened, or oblate. An infinitely oblate planet would have the shape of a disk. So one could imagine representing Saturn’s gravity as that due to a sphere plus that due to a disk or plate (a somewhat lumpy object, perhaps, at least compared to a sphere). The amount of oblateness would be represented by the fraction of the total mass that is in the disk part rather than the spherical part. This fraction is represented by a numerical factor. For the oblateness that factor is called J2. A large J2 means a more oblate planet, while a small J2 is a less oblate planet. J2 of Earth is about 10−3, while that of Saturn is 15 times larger. The 2 in J2 tells us that this is a symmetric distribution of matter, and that it is a simple, global-scale distribution. The even-numbered Js all describe symmetric distributions and, as the number of the coefficient increases, the size scale of the lumpiness they describe decreases. There can be, in principle, an infinite number of J coefficients to define the gravity field of a planet if we want to get it infinitely precise. Some models for the Earth have over 100 000 coefficients! The higher-order ones are capturing the small gravitational lumps due to things like mountains and valleys. The lumpier the planet, the larger the J values will be at higher order. For gas blobs like Saturn and Jupiter we expect fluid physics to explain the structure, and fluids don’t generally make lumps. When they do, they are nice and periodic and regular, like the ripples on a pond. So the standard model for the gravity includes primarily the evennumbered Js with rapidly decreasing values as we go to higher-order Js. Beyond the simple sphere and the disk represented by J2, there are terms that can be represented by imagining the planet to have a number of sections, like an orange, and terms that 8-5

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represent it is a bunch of stacked doughnuts. And there are some that involve a more checkerboard pattern of gravity that is a combination of doughnuts and oranges. Cassini’s Grand Finale Doppler-tracking orbits measured Saturn’s J values out to J12. Those measurements showed surprisingly large values of J6 and higher for Saturn. The data show that Saturn’s interior does not rotate like a solid body, but has different rates of rotation depending on how deep one goes into the interior. The equatorial region of Saturn rotates 4% faster than the deep interior. The results also show that these differential flows go many thousands of kilometers deep into the planet, about 15% of its radius. Curiously, while the Radio Science team could fit any one of the Cassini Grand Finale gravity orbits with a reasonable model of the planet and the rings, they could not find a single solution that simultaneously fit all five of the gravity-pass orbits (Iess et al 2019)! This means that there is something going on inside Saturn or in the Saturn system that is more complicated than the already complicated models used by the team. These unexplained accelerations of the spacecraft contribute to the uncertainty in the mass of the rings described in chapter 6. Nevertheless, the first 11J values for Saturn (J2–J12) and much else about the interior have been accurately measured. The core of Saturn was found to have a mass of about 15–18 times the mass of the Earth and about 20% the radius of Saturn, perhaps more massive than Jupiter’s core. The different orbital periods (azimuthal, vertical, and radial) that give rise to the waves in the rings discussed in chapter 5 are a result of a non-spherical distribution of matter in the planet. Some waves in Saturn’s C ring are created by resonances not with moons but with the rotation of the planet itself. If the planet were a perfectly homogeneous sphere, this would not be possible because there would be no change in the gravitational pull on the ring particles as the planet rotates. But because the planet apparently is somewhat lumpy, there is a kind of a drumbeat of changing gravitational pulls on ring particles as the planet rotates, and at certain locations in the rings these drumbeats resonate with the ring particle orbits, resulting in waves. Thus, the rings act as a kind of seismometer for the interior of Saturn, and research continues to piece this together with the data returned from the Grand Finale orbits to understand the complex interior structure of Saturn. Research and modeling of Saturn’s interior will continue. We know now that the planet has differential rotation to a much deeper level in the interior than does Jupiter, that there are additional aspects of the interior that do not fit a simple model that produced accelerations of Cassini during its Grand Finale, and that it has a fairly massive core. This provides an important constraint on models of formation of the giant planets.

8.2 Atmosphere Saturn’s atmosphere at altitudes above the pressure at the surface of the Earth (1 bar) behaves in general like that of Titan (chapter 7): the top is heated by the Sun and impacting charged particles, and the bottom is heated by the interior of Saturn. In the case of Titan the heating from below comes from its surface. Saturn does not have a surface per se but, as discussed above, heat is radiating outward from its

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interior, likely due to the ongoing process of differentiation discussed above (rain out of helium deep in its interior4). Thus, like Titan (and the Earth), Saturn has a troposphere where convection produces clouds, and a warm thermosphere above the troposphere, heated by energetic solar photons and charged particles, where the atmosphere thins into the emptiness of space. The rapid rotation of such a large planet means that the clouds generally stretch around it in latitudinal zones. In general, there is more heating over the equator causing air to expand and rise. At the top of the troposphere it is pushed aside by more rising warm air beneath, so it moves north and south where it cools and sinks, displacing air at the surface. Thus there is a circulation pattern of rising and sinking air, with air rising at the equator, and sinking at a more northern (or southern) latitude. As the air rises and cools, any molecules that can condense into droplets form clouds, which can produce rain. The sinking air is depleted of this moisture and is thus dry. This circulation pattern produces zones that circle the planet but in a north–south slice are simple convection cells. The number of these cells that fit between the equator and the pole is determined by the size of the planet and the speed of rotation. On Earth, that number is three and explains why it is rainy over the equator and at mid-latitudes while deserts occur where the first cell sinks (at the latitude of the Sahara, for example). These zones of rising and sinking air give Jupiter its prominent striped appearance, where the regions of rising air produce bright, reflective clouds at the top of the troposphere, and the neighboring regions are clear and allow us to see to deeper levels in the atmosphere where there are reddish and yellow sulfur-bearing clouds. Saturn has less dramatic color contrasts than Jupiter (figure 8.3). Saturn’s atmosphere is more vertically extended than Jupiter’s due to its smaller mass and therefore weaker gravitational compression of the atmosphere under its own weight5. With a thicker atmosphere it’s harder to see down to the levels where the darker, more colorful clouds reside. Saturn has three main cloud levels: ammonia clouds at an atmospheric pressure of 1 bar, ammonium hydrosulfide at a few bar, and water clouds at about 10 bar (Genio 2009 and references therein for details on Saturn’s atmosphere discussed in this chapter). The water clouds are too deep to be visible, normally. Occasionally, however, Saturn’s relatively uniform atmospheric appearance is interrupted in dramatic fashion by the eruption of a storm that gradually circles the planet. One such storm appeared during Cassini’s mission, in 2010 (figure 8.2), but they have been observed from Earth-based telescopes for over a century (Li & Ingersoll 2015). It is a coincidence that the appearance of these global storms roughly coincides with the length of Saturn’s year, or about 30 years. With a 13 year mission at Saturn Cassini had a roughly one in two chance of catching one of these big storms, and luckily we won the coin flip. On Earth, storms are launched by contrasts in heating of the atmosphere at the surface due to different surface temperatures (warm land next to cool bodies of 4

This process has been going on for 4.5 billion years! For the same reason we saw in chapter 7 that Titan’s atmosphere is much more vertically extended than the Earth’s. 5

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Figure 8.3. This true-color view of Saturn from high above the north pole is a mosaic of 36 Cassini images from 2013. A thin band of white clouds at 42° north latitude is left over from a major storm that peaked in 2011 (see figure 8.2). The north pole is sunlit at this time and is circled by a hexagonal feature. Image credit: NASA/JPL-Caltech/SSI.

water, for example, or even warm asphalt and concrete next to cooler vegetation) or the flow of the atmosphere over topography, such as hills and valleys. Saturn has no such obvious sources of uneven heating to drive the generation of storms, so it is not surprising that in general Saturn’s cloud cover is much more constant and uniform than that of the Earth (figure 8.3). And yet, storms do sometimes crop up on Saturn, and grow to circle the planet. The energy source for Saturn’s storm is heat liberated from the interior through differentiation discussed above. Why these storms appear episodically, and only every few decades, is not understood. The same processes that give rise to hurricanes on Earth act on Saturn. As rising air moves northward or southward, it is deflected east or west by the Coriolis force. Here’s why. Imagine tracing a large chunk of Saturn’s atmosphere. Such a parcel of air at Saturn’s equator is moving around its center at about 36 000 km h−1 because it has to go around the full circumference of the planet in a Saturn day (10.55 h). A parcel of air near the north pole, on the other hand, has a much smaller journey to make in a Saturn day because it is close to the rotation axis, and it is therefore traveling much slower (about 25 000 km h−1 at 45° latitude). So an air parcel moving northward from the equator is like a car moving from the fast lane into the slow lane: suddenly it is moving much more quickly than its neighbors, while an air parcel moving toward the equator from the north is like a slow car moving into the fast lane. This results in a circulation of the air around local highs or lows in the atmospheric pressure (figure 8.4).

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Figure 8.4. Clouds are emphasized in this view taken by Cassini in 2014 through a red filter illustrating the complicated fluid dynamics present in a rapidly rotating atmosphere. The resolution is about 100 km/pixel of this view of northern mid-latitudes. Image Credit: NASA/JPL-Caltech/SSI.

Saturn has a yellowish hue due to absorption of red light by methane and other trace molecules in the atmosphere and an upper-atmospheric haze of suspended particles that obscure our view of clouds below. The longer a path that sunlight takes through the atmosphere, the more red light is absorbed and the bluer that atmosphere appears (figure 8.5). This gives the northern hemisphere a bluish tint compared to the southern hemisphere. Another seasonal change in the atmosphere that is unique to Saturn is the large shadow that the rings cast on the planet (figure 8.6). However, its effect on Saturn’s weather is probably small: as we saw for Titan in chapter 7, Saturn’s great distance from the Sun and hazy atmosphere reduce the amount of solar energy reaching deep into the atmosphere and restrict the effects of seasonal fluctuations to the upper atmosphere. As with Titan, we can see to different depths in Saturn’s atmosphere by observing at different wavelengths of light. Unlike Titan, however, there is no solid surface to observe beneath the atmosphere. Heat from Saturn’s interior, however, produces a glow in the infrared part of the spectrum that can act as a backlight to illuminate clouds that would otherwise be obscured by haze from above (figure 8.7). By

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Figure 8.5. This true-color image from the equatorial plane of Saturn shows the yellower hue of the atmosphere in the Sunlit southern hemisphere and bluer tones in the wintry northern hemisphere. The gases in the atmosphere preferentially absorb red light, so the more air that sunlight passes through before being reflected back to Cassini’s cameras the bluer it appears. The Sun is shining on the planet’s southern hemisphere, so sunlight hits the northern atmosphere at an oblique angle, passing through more air before it is reflected off clouds. Image Credit: NASA/JPL-Caltech/SSI.

combining observations from Cassini’s cameras we’re able to piece together the structure of the atmosphere. A curious feature of Saturn’s northern polar region is a hexagonal feature in the clouds (figure 8.8). This feature was observed in Voyager images (Godfrey 1988, see also Del Genio et al 2009) and has remained stable since that time and may be much older. While it may seem odd to see an angular structure in the atmosphere of a spherical planet, the hexagonal appearance is a bit misleading. If we superimpose a circle on the hexagon and then unwrap it, we see that it is actually a sinusoidal wave wrapped around the planet (figure 8.9). This by itself does not explain the origin of the hexagonal feature, but makes its shape somewhat less confounding. There is a ‘north polar spot’ – a local storm – adjacent to one side of the hexagon that may be involved in its formation by diverting a zonal flow (like a terrestrial trade wind) around it (Allison et al 1990), or its roots may go deeper into the planet (Barbosa Aguiar et al 2009). In either event, the south pole does not have a matching feature (figure 8.10).

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Figure 8.6. This image taken from Saturn’s equatorial plane shows the dramatic difference in sunlight hitting the two hemispheres due to the shadow of the rings. In this view the Sun is high above the ring plane so the rings cast a broad shadow over the southern hemisphere. The shadow moves from the south to the north and back again over the course of a Saturn year, vanishing to a thin line at the equinoxes when the Sun shines on the rings edge-on. Image Credit: NASA/JPL-Caltech/SSI.

Figure 8.7. This false-color view of Saturn and its rings was produced from images taken with the Cassini VIMS instrument in three different infrared wavelengths. Blue and green are reflected sunlight, while the red indicates the glow from Saturn’s interior. Clouds in the southern hemisphere are seen in silhouette in front of this intrinsic glow. Image Credit: NASA/JPL-Caltech/Univ. Arizona.

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Figure 8.8. These four views of the north polar region reveal the hexagon at different wavelengths of light (from upper left, clockwise, the colors go from violet to infrared) and therefore different depths in the atmosphere. The hexagon and clouds associated with it are most prominent in the red and infrared which also reveal a dark spot at the pole indicating a cloud-free zone. Image Credit: NASA/JPL-Caltech/SSI.

The north pole itself is the location of a hurricane-like circulation with high cloud walls reminiscent of the eye-wall of a hurricane (figure 8.11). The atmospheric winds can be measured by looking at sequences of images and identifying small features and tracking them from frame to frame (figure 8.12). This provides the speed that the air is moving in an absolute sense, but to convert that to wind speeds we have to know how fast Saturn itself is rotating. When talking about a gas giant planet like Saturn, though, it’s sometimes difficult to know what is even meant by ‘Saturn itself’. The parts that we see – the clouds and various waves and disturbances in the atmosphere – are all moving, and their speeds with respect to the center of the planet are all different, varying primarily with latitude. We can’t see a surface to provide a reference frame for the wind speeds, so we have to turn to other observations to infer the rotation rate of the bulk planet. This is where the magnetic field comes in handy, as well as the gravity field measurements made during the Grand Finale. The rate that we get for the rotation of the planet shows that most of the atmosphere is super-rotating (winds blowing toward the east, in the same direction the planet is rotating) at speeds up to several hundred miles per hour at the equator.

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Figure 8.9. This schematic illustrates how a traveling sinusoidal wave (top) can become a hexagon when it is wrapped around a circle with a periodicity of six wavelengths around the planet.

Saturn’s atmosphere was imaged during some of the Grand Finale orbits when gravity measurements were not being made. As the spacecraft plunged through the gap between the planet and the rings, it snapped pictures of the planet beneath. These long strips of images were dubbed ‘noodles’ by the imaging team. As Cassini approached the equator, it got closer and closer to the planet, so the resolution of the images increased while the amount of real estate covered on the planet decreased (figure 8.13). Because of how fast the spacecraft was moving relative to the planet, the camera took cropped images one-quarter the normal size.

8.3 Magnetic field Like the Earth, Saturn has a magnetic field that to a first approximation is like that of a bar magnet. But while Earth’s magnetic field is tilted by a few degrees from the rotation axis, Saturn’s is nearly perfectly aligned with it: its north magnetic pole is in the same direction as its north rotational pole (see chapter 4, figure 4.13). However, it is asymmetric about Saturn’s equator as if the bar magnet were displaced to the north by 2800 km. We think planetary magnetic fields are generated by a form of dynamo in the interior of the planet. Electrical currents generate magnetic fields, so a planetary dynamo involves some bulk circulation of conducting material in a planet’s interior. In the Earth this is likely due to convection in the outer core of heavy metallic elements. At Saturn the field is generated in the metallic hydrogen 8-13

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Figure 8.10. These two views of the south pole show circular air flows around the pole, but no hexagonal feature like that seen at the north pole. Both images are taken in the infrared with the Cassini VIMS instrument. The lower panel is at 5.04 microns where clouds appear dark silhouetted against the thermal emission from the interior of the planet. The lower image makes up the red part of the upper false-color image where green and blue represent reflected sunlight at 4.08 and 3.08 microns, respectively. The redder appearance of the polar region indicates that it is relatively clear of the high-altitude hazes that reflect sunlight at more equatorial latitudes. Image Credit: NASA/JPL-Caltech/University of Arizona.

envelope outside the core. The precise mechanisms for generating the field and the nature of the flows in the interior are both complex and incompletely understood. The north–south asymmetry in Saturn’s magnetic field (and the tilts in those of Earth and Jupiter, e.g.) show that the generation of the field is not simply connected to rotation at the core of the planet. Cassini’s Grand Finale orbits provided the best measurements of the magnetic field due to proximity to the planet for the same basic reasons that they provided the best measurements of Saturn’s gravity. Amazingly, the magnetometer team found no tilt in Saturn’s magnetic field to within 0.01°. The theory of planetary dynamos predicts some asymmetry. Cassini measurements suggest that there is a deep dynamo surrounded by a stable layer (no convection) that is below the differentially rotating region described by the gravity measurements. Close to Saturn there are lots of fluctuations in the magnetic field in the north–south direction that may be originating in the outer quarter of the planet (Dougherty et al 2018). The magnetic field at the cloud tops of Saturn is comparable in strength to that at the surface of the Earth. However, since Saturn is roughly 10 times larger than the Earth, the total 8-14

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Figure 8.11. This dramatic image shows the circulation around the north pole of Saturn. Image Credit: NASA/ JPL-Caltech/SSI.

magnetic field density at Saturn is nearly 1000 times larger than that of the Earth (Gombosi et al 2009). Charged particles tied to the magnetic field can radiate at radio wavelengths, so any periodicity to these emissions is tied to the rotation of the magnetic field. At Jupiter, the tilt of the magnetic field from the rotation axis of the planet makes these radio emissions stand out like the beacon from a lighthouse. At Saturn, though the field is aligned, there is still a periodic signal in radio waves called Saturn kilometric radiation (SKR). Following the first measurements of this it was thought that, like at Jupiter, the period of the fluctuations in this signal indicated the period of rotation of the bulk of the planet. However, the period of the SKR has changed since the Voyager epoch, and it’s not possible for the bulk rotation of the planet to have changed that much so quickly. Even more curiously, the period of the SKR split during the Cassini mission indicated different emissions from the northern and southern hemispheres of Saturn (figure 8.14). This strange behavior of the SKR at Saturn is not fully understood, but its changing and bimodal nature means that it is not governed entirely by magnetic field generation deep within the planet. There is some modification of the emission that arises in its atmosphere, or at least less deep in the interior, that can undergo changes on timescales of years. 8-15

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Figure 8.12. This false-color mosaic of Saturn’s north pole shows the hexagon, the north polar white spot, and the polar storm. This time lapse covers 10 h or about one full rotation of Saturn and is in a frame rotating with Saturn so all motions here are due to atmospheric winds. The air in the hexagon is like a jet stream that prevents air from crossing it. Image Credit: NASA/JPL-Caltech/SSI/Hampton University. Movie available at https://iopscience.iop.org/book/978-1-64327-714-1.

As mentioned above, the rings provide another piece of information about the internal structure and period of the planet. As we saw in chapter 5, the rings are a dynamically quiet system that is very sensitive to even very weak perturbations, such as the gentle gravitational resonant tugs on the particles from nearby small moons. Any mass that has a motion with a period that is resonant with one of the natural frequencies of ring particles can launch a wave at the location in the rings where that resonance occurs. Voyager data showed a handful of unusual waves in the C ring, and Cassini data revealed many more. These waves do not correspond to any resonances with known moons, and in some instances seem to propagate in a direction that indicates they are caused by a perturber interior to them, rather than outside the rings as with the rest of the known waves. It was recognized that these may be caused by natural oscillations within Saturn itself (Marley & Porco 1993), and the abundance and quality of measurements made by Cassini has confirmed this to be the case. The rings are ringing, for lack of a better word, in resonance with bulk oscillations within the planet or the motion of some asymmetric distribution of

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Figure 8.13. This mosaic of 137 images, nicknamed the noodle, was captured on the first Grand Finale orbit on 26 April 2017. The mosaic begins at the north pole and extends to 18° north of the equator. The first image is taken from a distance of 72 400 km, while the last is from only 8400 km above the atmosphere with a resolution of 1 km/pixel in the last frame. Image Credit: NASA/JPL-Caltech/SSI. Movie available at https:// iopscience.iop.org/book/978-1-64327-714-1.

Figure 8.14. This figure is a spectrogram where time is on the horizontal axis and the frequency of the Saturn kilometric radiation (SKR) is plotted vertically. Colors indicate the intensity of the radio emission at each frequency. SKR emission was used to determine the rotation period (length of day) of Saturn until it was seen to change, and then a second component appeared. Observations made since this observation show a merging of the two components (Fischer et al 2015) followed by a separation with the north component slower than the south (Ye et al 2016). Image Credit: NASA/JPL-Caltech/University of Iowa.

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Figure 8.15. Ultraviolet movie of Saturn’s north polar aurora from the Cassini UVIS and the Hubble Space Telescope. The intensity and configuration of the aurorae change with the strength of the solar wind. Image Credit: NASA/JPL-Caltech/University of Colorado/Central Arizona College and NASA/ESA/University of Leicester and NASA/JPL-Caltech/University of Arizona/Lancaster University. Movie available at https:// iopscience.iop.org/book/978-1-64327-714-1.

Figure 8.16. This animation shows Saturn’s northern aurorae in visible light. The timelapse spans 81 h and shows aurorae in false color extending up to 1200 km above the visible edge of the planet. The glow at left is scattered sunlight from the upper atmosphere on the dayside of the planet. Image Credit: NASA/JPL-Caltech/ SSI. Movie available at https://iopscience.iop.org/book/978-1-64327-714-1.

matter in the planet, or both (Hedman & Nicholson 2014). Understanding the forcing for these waves, combined with the magnetic field and gravitational field measurements performed during the Grand Finale, remains an active area of research. The magnetic field is a barrier to the charged particles streaming out from the Sun in the solar wind. Some of these electrons and protons become trapped by the magnetic field. As discussed in chapter 4, these particles spiral along the magnetic 8-18

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Figure 8.17. These Cassini UVIS images show Saturn’s aurorae and a spot of emission that maps directly to Enceladus (boxes). Image Credit: NASA/JPL-Caltech/Univ. Colorado/Central Arizona College.

field lines. Depending on the energy of the particles and their location within the magnetic field, they can impact Saturn’s atmosphere creating aurorae, the same process that produces the aurorae on Earth. When electrons molecules in Saturn’s upper atmosphere (mostly molecular hydrogen), they can cause the hydrogen to glow. The emission from hydrogen is primarily in the ultraviolet, so we typically don’t see the aurorae in our visible light images. It is quite prominent in the ultraviolet images taken by Cassini’s UVIS (figure 8.15) and also by the Hubble Space Telescope. Visible light images of the aurorae have been captured looking at the nightside of the planet. Saturn’s vertically extended atmosphere, a product of its low overall density, means that the electrons interact with atmospheric hydrogen over a large vertical extent, producing auroral ‘curtains’ that are the tallest in the solar system (figure 8.16). In addition to the solar wind, two of Saturn’s moons make significant contributions to the population of charged particles in the magnetosphere. As we saw in chapter 7, methane in the upper atmosphere of Titan is destroyed by sunlight and the solar wind producing hydrogen that escapes into space, becoming part of the magnetosphere. Meanwhile the geysers of Enceladus launch water molecules directly into the magnetosphere, and water is dissociated into hydrogen and oxygen by energetic solar photons. Some of these atoms become ionized, but there is also a large population of neutral atoms in Saturn’s magnetosphere from these two sources. Electrons produced by ionization of the water vapor from Enceladus can precipitate directly along the magnetic field lines that pass by Enceladus and impact Saturn’s atmosphere. The Cassini UVIS has observed a spot in the aurora that corresponds to the field lines that pass by Enceladus, directly demonstrating this link (figure 8.17) (Pryor et al 2011). However, the spot appears only occasionally, and the origin of this fluctuation is uncertain. Enceladus’ water vapor production appears to have remained fairly constant over the course of the Cassini mission, but only a few

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direct measurements of this output have been made (Hansen et al 2017) so temporary increases in activity cannot be ruled out. Many of the atoms and molecules fed into the magnetosphere by Titan and Enceladus remain unionized, giving Saturn large belts of neutral gas in the magnetosphere, a characteristic that is unique to Saturn in the solar system. Saturn’s interior, both deep within the planet where the rain-out of helium provides an internal heat source and at shallower levels where inhomogeneities may launch waves in the rings and slippage between the outer and inner layers of the planet alters the SKR, remains a mystery wrapped in tens of Earth masses of hydrogen and helium. Following the Grand Finale, Saturn’s interior has revealed enough to show us that it is much more complicated than we had imagined.

References Allison M et al 1990 Science 247 1061–63 Barbosa Aguiar A C et al 2009 Icarus 206 755–63 Del Genio A D et al 2009 Saturn from Cassini-Huygens ed M Dougherty, L W Esposito and S Krimigis (Berlin: Springer) and references therein Dias R P and Silvera I F 2017 Science 355 715–8 Dougherty M et al 2018 Science 362 eaat5434 Fischer G et al 2015 Icarus 254 72–91 Genio D 2009 Saturn Atmospheric Structure and Dynamics, Saturn from Cassini-Hugens ed M Doughert, L W Esposito and S Krimigis (Berlin: Springer) and references therein for details on Saturn’s atmosphere discussed in this chapter Godfrey D A 1988 Icarus 76 335–56 Gombosi T I et al 2009 Saturn from Cassini-Huygens ed M Dougherty, L W Esposito and S Krimigis (Berlin: Springer) and references therein Hansen C J et al 2017 Geophys. Res. Lett. 44 672–7 Hedman M M and Nicholson P D 2014 Mon. Not. Royal Astron. Soc 444 1369–88 Iess L et al 2019 Science 364 eaat2965 Li C and Ingersoll A P 2015 Nat. Geoscience 8 398–403 Marley M S and Porco C C 1993 Icarus 106 508–24 Pryor W R et al 2011 Nature 472 331–3 Ye S-Y et al 2016 J. Geophys. Res. 121 11714–28

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IOP Concise Physics

The Ringed Planet (Second Edition) Cassini’s voyage of discovery at Saturn Joshua Colwell

Chapter 9 Future exploration of the Saturn system

The Cassini mission’s exploration of the Saturn system spanned more than 13 years, making Saturn the most comprehensively studied planet in the solar system after the Earth and Mars (figure 9.1). Exploration of Jupiter by the Galileo mission, a scientific sibling to Cassini in many ways, was hampered by the failure of its highgain radio antenna to deploy, greatly reducing the amount of data it could transmit to Earth. Jupiter is far from neglected, however, with the Juno mission exploring its interior using the same techniques of Cassini’s Grand Finale orbits at Saturn. NASA’s plans for exploration of the outer solar system are focused on ocean worlds and astrobiology, and it is currently planning a Jupiter orbiter that will be focused on detailed studies of Europa, an icy moon with a subsurface ocean of its own.

9.1 Ocean worlds Cassini has provided clear evidence that both Enceladus and Titan are ocean worlds. Enceladus’s ocean may be much closer to the surface than Titan’s or Europa’s and, in any event, unlike those moons, it provides us with free samples of the ocean by spitting water vapor and ice crystals out into space. A focused mission to Enceladus could analyze these ocean samples in much greater detail than Cassini’s instruments. If the ingredients for life are present, such a mission could find them. While Titan’s subsurface ocean1 is not accessible, Titan itself would be a planet in its own right if it were in orbit around the Sun. It is larger than the planet Mercury. Cassini’s cameras and radar have done much to reveal Titan’s surface, Doppler tracking has shown us much about the interior structure, and the Huygens lander provided a tantalizing and detailed glimpse at one moment in time and one spot on this world (figure 9.2). But there are lakes, vast fields of sand dunes, mountain ranges, and a weather cycle whose daily and seasonal patterns remain incompletely 1 Not to be confused with its large lakes of liquid methane and ethane on the surface, which are ripe for exploration.

doi:10.1088/2053-2571/ab3783ch9

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ª Morgan & Claypool Publishers 2019

The Ringed Planet (Second Edition)

Figure 9.1. This computer rendition shows the entire Cassini ‘tour’ of the Saturn system. Saturn is too small to see, at the center of the action. The colors indicate the phase of the mission, ranging from gray for the initial four-year prime mission to the final orbits in green. Image Credit: NASA/JPL-Caltech. Movie available at https://iopscience.iop.org/book/978-1-64327-714-1.

Figure 9.2. The Huygens probe operated on the surface of Titan after landing. This is its view of the horizon. The rocks in the foreground are likely made of water ice and are about 10 cm across. Image Credit: ESA/NASA.

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understood. Fortunately, Titan does provide an atmosphere that would enable global exploration via balloon or airship. That is a mission I would love to see. NASA decided in June 2019 to give us all the chance to see just that. Dragonfly, a large, drone-style aircraft with eight rotors, was selected to be the next New Frontiers mission. These are the largest class of planetary science missions that are led by what is known as a Principal Investigator (PI). The PI is usually a scientist from outside NASA who assembles a team to propose a mission in response to a solicitation for mission ideas from NASA. There have been four New Frontiers missions, beginning with the New Horizons mission to Pluto led by Dr Alan Stern of the Southwest Research Institute. Dragonfly is led by Dr Elizabeth Turtle of the Applied Physics Laboratory at Johns Hopkins University. Dr Turtle worked on the imaging team of Cassini studying Titan (figure 9.3). Dragonfly will land in the equatorial dunes on Titan. Most of the time it will make measurements from the surface. Titan’s day is 16 Earth-days long. It will charge its battery during the long Titan night and use the battery to power the rotors to hop to a new location during the daytime. It will be powered by the same type of radioisotope thermoelectric generator that powers the Mars Curiosity and Mars 2020 rovers. This will keep it warm and power science operations and communications during the daytime with an estimated power output of only 70 W, about the level from a typical incandescent light bulb (Lorenz et al 2018). Dragonfly will cover more distance than all Mars rovers combined during its two-and-a-half year mission. Initially it will stay close to its first landing site, making forays of increasing distance up to a maximum of about 8 km in a single flight to explore the region. It will fly at an altitude of 500 m over the surface, and like any self-respecting drone has a

Figure 9.3. This artist’s rendering shows different phases of the Dragonfly mission to Titan. It will be slowed by a parachute before releasing its backshell and flying to the surface under the power of its eight rotors. Image Credit: Johns Hopkins APL.

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downward-facing camera that will provide stunning views of this strange new world. At its landing locations it will have the ability to drill into the surface, and its scientific instruments will be focused on studying the chemistry of the surface and the atmosphere. After exploring the dune regions, it will head to one of the relatively rare impact craters on Titan, the Selk crater, which is surrounded by channels that may have funneled liquid into it.

9.2 Rings The end of the Cassini mission featured the highest-resolution images of the rings, and it will take a dedicated mission to the rings themselves with a spacecraft that tracks along with the ring particles to improve on these and show us individual particles. The end-of-mission images are already stunning (figure 9.4). Some highlighted the ubiquitous nature of clumps or aggregates of ring particles extending many kilometers in length, with a puzzling distribution and change of character from one region of the rings to another (figure 9.5). We have seen objects down to the resolution limit of the Cassini cameras, both within the rings and among the moons, and this tells us that there is more to see if we can get closer. Mission concepts have been developed both for spacecraft that would follow individual ring

Figure 9.4. This image from the start of Cassini’s final mission phase shows the Mimas 5:3 bending wave (see also chapter 5). The planet is to the right. Image Credit: NASA/JPL-Caltech/SSI.

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Figure 9.5. This region of the outer B ring was taken during the ring-grazing orbits in early 2017 with a resolution of 0.61 km/pixel. In addition to much variation in brightness with increasing distance from left to right, the image shows a streaky texture indicating clumping of particles in the direction of their orbits around Saturn. Such structure appears to be ubiquitous (Tiscareno et al 2019). Image: NASA/JPL-Caltech/SSI.

particles to see what a day or year in the life of a ring particle is like as well as those that would repeatedly skim over the rings, covering more real estate but not following individual particles. These mission concepts take advantage of both new technologies and clever innovations in orbital mechanics, some of which were developed by and for the Cassini project. The next mission to Saturn, though, is Dragonfly to Titan. Given the time it takes to develop planetary missions and get them to the outer solar system, we are unlikely to have better views of Saturn’s rings before 2040. The F ring has shown us accretion and fragmentation processes playing out before our eyes at the boundary of Saturn’s Roche zone. This provides a tantalizing link to suggestions that many of the moons in the system may be young, either fragments of larger objects or perhaps born from the outer edge of the rings themselves. The mission has not yet conclusively resolved the questions of the age and origin of the rings and moons—science takes time to get things right—but the data collected over the course of the mission have already rewritten our understanding of the history of Saturn.

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9.3 Saturn The final proximal orbits of Cassini, with the spacecraft threading the gap between the cloudtops and the inner edge of the rings, provided the highest-resolution images of the atmosphere and the inner rings. Stellar occultations probed the atmosphere of Saturn at new latitudes, helping untangle Saturn’s weather. In addition to getting the mass of the rings, we have discovered that the interior of Saturn is qualitatively different than that of Jupiter. It has a fairly massive core, more than 15 times the mass of the Earth, and its atmosphere seems to be more deeply coupled to the interior than anticipated, with differential rotation extending down into the planet by about 15% of its size. Its magnetic field, meanwhile, shows perfect alignment with its rotation, but with a large offset to the north and lots of ripples in the north–south direction. All of these phenomena require further modeling and may require data from future missions not yet on the drawing board. Each time we looked more closely we have seen something new. On 15 September 2017, on its 293rd orbit, Cassini plunged into Saturn’s atmosphere, its large antenna pointing Earthward and transmitting data until friction with the atmosphere caused the spacecraft to turn away from home one last time (figure 9.6). Thus ended the data-collection phase of one of the most successful missions of scientific exploration of the cosmos, but the discoveries will continue for years. In our focus and determination to collect the best data and maximize the return from the mission, we have often not had as much time as we need to process, analyze, model, and understand the wealth of observations. As we build and fly the next generation of planetary probes, including Dragonfly, we will continue the journey of discovery at Saturn with the wealth of data from Cassini.

Figure 9.6. This artist’s rendition of Cassini entering Saturn’s atmosphere at the end of its mission indicates those instruments that were collecting data up to the very last moment. The RSS antenna is pointed at the Earth, and Cassini’s thrusters are shown here firing to try to maintain its communication lifeline to Earth. Shortly after the spacecraft lost radio contact, it broke apart and was vaporized by friction with Saturn’s atmosphere. Image Credit: NASA/JPL-Caltech.

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9.4 Other worlds We can learn a lot not only about our own solar system but about the entire observable universe with telescopes based at Earth, both on the ground and in space. Different telescopes are optimized for different kinds of science. NASA’s next spacebased great observatory, the James Webb Space Telescope (JWST), will discover and characterize worlds around other planets as well as peer into the ancient history of the Universe (figure 9.7). Planets around other stars have been discovered in rapidly increasing numbers with planet-hunting, space-based telescopes such as Kepler and the Transiting Exoplanet Survey Satellite (TESS). Kepler observations have led to the discovery of thousands of exoplanets. JWST, scheduled for launch in 2021, will be able to study and characterize the atmospheres of some planets by studying the spectra of the light that shines from the star through the atmosphere as the planet passes in front of it. Such observations may reveal a planet with an atmosphere like that of the Earth. The signature of large amounts of oxygen in a

Figure 9.7. This artist’s rending of the James Webb Space Telescope (JWST) shows its primary mirror composed of 18 hexagonal segments and the large, five-layer sunshield that keeps the sensitive infrared detectors cool. JWST will orbit the Sun in tandem with the Earth at a distance of 1.5 million km. Unlike the Hubble Space Telescope, it will not be serviceable. Observing in the infrared requires cold detectors, and eventually it will be unable to continue to maintain low enough temperatures for science operations. The anticipated mission lifetime is 5–10 years. Image credit: Northrup Grumman.

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planet’s atmosphere would be a strong indication of life. Observing at infrared wavelengths, JWST will be able to peer inside the protoplanetary disks of forming planetary systems around other stars. JWST will also train its 6.5 m mirror on the Saturn system and other objects in the outer solar system, including the distant

Figure 9.8. This mosaic of Saturn from Cassini was taken from Saturn’s shadow (top). The background light from the different images making up the mosaic has not been removed, leading to the pattern of flares in the background. The Sun is blocked by Saturn, and the side of the planet facing us is illuminated by light reflected onto the atmosphere from the rings themselves. The equator of the planet is completely dark because the rings are so thin that from the equator they are virtually invisible and reflect no light back to it. The north pole is dark because the rings are below the horizon from that vantage point, while the rings themselves obstruct our view of portions of the southern hemisphere. The lower detail shows the Earth, visibly bluish, seen through the gap between the G and E rings. Examined in detail (upper left), the Moon is visible. At left, Enceladus can be seen in the E ring with tendrils of ice crystals spreading out to form the ring. Image Credit: NASA/JPL-Caltech/SSI.

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Kuiper Belt of comets. JWST will extend the time baseline of our observations of Saturn, Titan, and its rings. With a 30-year seasonal cycle, JWST will bridge the gap between Cassini and Dragonfly for observations of Titan, and perhaps show us another global storm on Saturn, the seasonal cycle of the ring spokes, and new discoveries to further our understanding of the history and evolution of our solar system. Perhaps the greatest legacy of Cassini–Huygens is the new perspective it provides us on our place in the solar system and the history of our little corner of the Galaxy (figure 9.8). On more than one occasion Cassini peered back toward Earth when Saturn protected its instruments from the heat and glare of the Sun. In modern parlance, humanity took a selfie. We are, as Carl Sagan said, living on a pale blue dot, barely visible from even within the confines of our own solar system. The Cassini–Huygens mission was a joint venture of 17 nations, led by NASA, the European Space Agency, and the Italian Space Agency. It was a purely scientific undertaking involving more than 5000 individual scientists and engineers, paid for by the people of the nations involved2. Its results are freely available to the world and are a testament to human curiosity and what we can accomplish with even modest resources when we work together.

Further reading The Saturn.jpl.nasa.gov website is an excellent portal to images and graphics from the mission, as well as detailed information about the spacecraft and the mission itself. NASA maintains the Planetary Data System (PDS), a free and publicly accessible data repository for all planetary missions. All Cassini data has been archived at various nodes of the PDS (pds.nasa.gov). Information on the James Webb Space Telescope can be found at jwst.nasa.gov.

References Lorenz R D et al 2018 Johns Hopkins APL Technical Digest vol 34, number 3 www.jhuapl.edu/ techdigest Tiscareno M S et al 2019 Science 364 eaau1017

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The U.S. share over the course of the mission was about 30 cents per person per year.

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