Earth and Mars: A Reflection [1 ed.] 9780816532261, 9780816500383

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EARTH AND MARS

EARTH

& MARS

A Reflection

STEPHEN E. STROM AND BRADFORD A. SMITH

tucson

The University of Arizona Press www.uapress.arizona.edu

© 2015 The Arizona Board of Regents All rights reserved. Published 2015

Printed in the United States of America 20 19 18 17 16 15  6 5 4 3 2 1 ISBN-13: 978-0-8165-0038-3 (paper) Cover designed by Leigh McDonald Cover photo courtesy of NASA MRO HiRISE Library of Congress Cataloging-in-Publication Data Strom, Stephen, author, photographer. Earth and Mars : a reflection / Stephen E. Strom and Bradford A. Smith. pages cm Includes bibliographical references. ISBN 978-0-8165-0038-3 (pbk. : alk. paper) 1. Earth (Planet)—Surface—Pictorial works. 2. Mars (Planet)—Surface—Pictorial works. I. Smith, Brad, 1931– author. II. Title. GB403.S87 2015 550—dc23 2015001526 This paper meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper).

EARTH AND MARS

Overleaf, Global images of Mars and Earth (courtesy U.S. Geological Survey)

Introduction STEPHEN E. STROM I have spent most of my professional life as an astronomer searching out patterns encoded in the light from distant stars in the hope of understanding how our sun and solar system came to be. Over the past four decades, I have been perched on remote mountaintops for countless hours, looking upward mostly but also contemplating the desert below. During that time, I became drawn to and then seduced by the changing patterns of desert lands sculpted by the glancing light of the rising and setting sun, light that reveals forms molded both by millennial forces and yesterday’s cloudburst into undulations of shapes and colors. In response, I began what has become a threedecade-long devotion to capturing images of those patterns and the rich history they encode. These terrestrial images find resonance in the remarkable photographs taken by the Mars orbiters and landers that NASA and its European counterpart, the European Space Agency (ESA), have launched over the past decade. Tens of thousands of these images are available in digital form in public domain databases. As an experiment, I decided to examine them from the perspective of an artist rather than an astronomer. In doing so, I studied more than a thousand long “strip maps” of the martian surface taken by the Mars Reconnaissance Orbiter (MRO) as well as the panoramas taken by the Opportunity and Curiosity landers, and I searched these rich archives for patterns that evoke the same powerful emotional response as a tellurian landscape. I could not help but be drawn to the commonality of motifs manifest in the martian and terrestrial images. That these patterns are manifest on vastly different scales on different planetary surfaces speaks to the profound beauty inherent in forms that result from the action of universal physical laws over time and space and the interaction of the classical elements: earth, fire, air, and water. Why did these patterns call to me so strongly? Is it the rhythmic repetition manifest in the ripples that are the inevitable by-product of the motion of air over a sand surface? Is it the fractal character of the channels produced by ancient martian rivers or the spiderlike patterns produced by the interaction of carbon dioxide with the fractured martian regolith? Is it the simplicity or universality of these landscape patterns

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that are somehow imprinted in biological or cultural deep memory? I invite the reader to explore these images and the aesthetic questions they raise. Beyond contemplating the multilevel beauty of two worlds, the images, together with Brad Smith’s words, might well inspire reflection on another level. How and why did two planets, each born in a zone favorable to supporting life, evolve to the point where one is rich with complex life-forms and the other is apparently lifeless? As a guide to stimulate these aesthetic and philosophical explorations, Earth and Mars is presented in four sections. In the first section, “Earth,” the images provide visual introduction to the panoply of geological phenomena found on the surfaces of Earth and Mars, from volcanic landscapes to cratered plains, desert canyons to ice-dominated planetary poles, dry river beds to towering dunes. The reader will discover haunting similarities between the two planets, similarities that were built in at the time of formation. But as the planets evolved, one—ours—remained rich with water, the source and origin of life as we know it, while the less fortunate Mars lost nearly all of its surface water. The section ends with images that evoke the epoch when life began on Earth, while on Mars any primitive life-forms would find evolution toward greater complexity challenging if not impossible. Brad Smith’s essay complements this visual tour with an overview of our understanding of how both planets were formed and a tantalizing preview of the forces that determined their subsequent evolutionary paths. In the second section, “Fire,” Brad Smith looks more closely at the structure of each of the planets, from their molten interiors to their visible crusts, and paints the broad picture of how tectonic and volcanic activity on Earth and Mars shaped the surfaces we see today. The accompanying images lead the reader through eroded volcanic craters, fossil lava flows, and a chain of craters produced by passage of crust over hot subsurface layers. Close-ups of volcanic rocks and examples of viscous liquid flow on Earth provide a metaphor for the far grander-scale effects of “Fire.” In “Air,” rippled sand dunes, streaks of windblown minerals, the path of dust devils, and wind-eroded canyon walls all speak to the role of each planet’s atmosphere in continuously reshaping their surfaces. But although both planets exhibit similar wind-shaped features, their present-day atmospheres are vastly different: Earth has a dense oxygen- and nitrogen-rich atmosphere, and Mars has a thin and primarily carbon dioxide–based atmosphere. Brad Smith’s essay provides context for understanding how the atmospheres of Earth and Mars reached such vastly different endpoints, a lesson in how small differences can lead to enormous, life-altering consequences.

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“Water” is the last of our tetralogy of primitive elements. Because water no longer flows on the surface of Mars, we find evidence of its past abundance in the surface wrinkles produced by glaciers, surfaces fractured as mud dries and solidifies, and the molding of river channels, sediment, and canyons, features that are also part of ongoing rhythms of shaping and reshaping Earth’s surface. On Mars, we are treated to something unique: delicate patterns produced at its north and south poles as a thin layer of carbon dioxide ice evaporates as the spring or fall sun begins to heat one of the poles. The closest analogue on Earth is ice floes: blue-white complexes floating against the vivid blue of liquid water. Brad Smith’s final essay relates a story that might be called tragic were we to take a terracentric viewpoint: how water first emerged on the surface of each planet, later forming deep oceans and flowing rivers that on Mars were fated to disappear in less than a billion years but that survived on Earth and became the incubator of life. Again, the story reminds us of the delicate balances of physical forces that resulted in humanity’s shared good fortune: to be alive on a planet abundant with life-giving water. What Earth and Mars aspires to do is to create visual and textual imagery with power sufficient to stimulate contemplation of both the beauty inherent in images of the physical world and in our ability to coax deeper understanding of phenomena from these visual clues: a fusion of art and science. My hope is that the reader will find that the dialog between images and text and art and science will be a poetic evocation of what the late essayist Ellen Meloy described as a “geography of infinite cycles, of stolid pulses of emergence and subsidence, which, in terms geologic and human, is the story of the earth itself” and of Mars as well (Ellen Meloy, The Last Cheater’s Waltz: Beauty and Violence in the Desert Southwest, 1999).

TECHNICAL NOTES Most of the images were selected from strip maps taken by the HiRISE camera designed by scientists at the University of Arizona’s Lunar and Planetary Laboratory and launched into orbit around Mars as part of NASA’s MRO mission. As the spacecraft orbits Mars, the camera maps “strips” along the martian surface through filters roughly approximating bands of colors we would call blue, green, and red (or in some cases a band of infrared colors of wavelength longer than that of the reddest light the eye can see). The detector is a high-quality charge-coupled device (CCD) similar to those in use on many digital cameras. Merged RGB (red-green-blue) or IGB (infrared-green-blue)

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color maps are stored on the HiRISE and NASA Jet Propulsion Laboratory websites. Typical pixel sizes for HiRISE images correspond to about one meter on the surface of Mars, and most images in this book derive from images 1000 × 1000 to 2000 × 2000 pixels in size cropped from the larger strip maps. In order to produce the colors shown in the martian images, I examined the histograms of intensity values (the number of pixels in the image having a particular intensity plotted against the intensity value) for individual blue, green, and red (or infrared) images, and I stretched the histograms so that the intensity values for each color span the range from minimum to maximum recordable value. I then combined the separate B, G, R (or I) images into a single “color” image. NASA uses a similar technique to produce illustrative images for public release. I refined my search of the extensive archive of Mars images by making use of the classifications provided by the HiRISE science team (e.g., images illustrating fluvial, eolian, or volcanic activity). This enabled the selection of martian images that both met my aesthetic criteria and illustrated the actions of a particular physical process in shaping the martian surface. I then searched through my archive of terrestrial landscape interpretations gathered over the past thirty-five years and chose images that appeared to pair well with the MRO images from both an aesthetic and illustrative standpoint. However, the reader should not expect all pairings to be didactic. For example, a pattern formed by martian cinder cones and one formed by bubbles in a terrestrial rock may be aesthetically similar and reference “fire” as shared origin but does not strictly reflect the action of identical physical processes. Similarly, motifs produced by carbon dioxide ice on Mars and water ice on Earth each illustrate the beauty of the solid frozen forms on the two planetary surfaces even though their chemical makeup differs. I selected a few of the martian images from panoramas taken by either the Opportunity or Curiosity landers. In these cases I cropped what I thought to be an aesthetically appealing image and again paired it with an appropriate terrestrial counterpart. Finally, I selected a small subset of terrestrial images from larger images or maps made from cameras aboard the International Space Station (ISS), NASA’s Landsat satellites, and NASA’s Earth Observing System (EOS) satellites. The patterns manifest in the images are often of vastly different scale, which to my mind adds to their visual mystery but may also raise questions in the mind of the discerning reader. In service of describing “how big it is,” I have provided an approximate scale for each of the images along with its provenance.

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How the images were selected

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Cratered surface of Mars as imaged by NASA’s Mars Global Surveyor (scale ~ 1,000 km) >

EARTH

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Earth BRADFORD A. SMITH To understand the origins of the planets Earth and Mars, we must look a long way back in cosmological time. Our narrative begins nearly five billion years ago with an enormous cloud of gas and dust spinning slowly in the darkness of space, its presence revealed only by feeble light from a few nearby stars. Unlike the many other interstellar clouds that populate our Milky Way galaxy, this one was crucial to our very existence—if it survived, it would become the birthplace of our solar system. At that critical moment, though, the cloud was unstable. Within it, each atom of gas and each particle of dust was exerting a gravitational tug on every other atom and particle, a universal force that would normally cause the cloud to collapse inward on itself. However, other influences were also at work—disruptive forces. Gas pressure as well as the turbulent motions of small blobs of gas and dust within the cloud resisted the pull of gravity, pushing persistently outward. For a while, the cloud’s future hung in the balance. Would it survive, or would it instead disintegrate, releasing its gas and dust back into interstellar space? Fortunately for us, fate intervened: in its death throes, a massive nearby star exploded as a supernova, emitting intense radiation that pressed against the outer layers of the cloud. This nudge gave gravity the assistance it needed to overcome the outward pressure. In the end, gravity became the ultimate winner, and the cloud began to collapse. To describe what happened next, we must introduce a concept called angular momentum. This is a property of an isolated object that is defined by the object’s rate of rotation and the distribution of its mass. Physicists say that the angular momentum of an isolated object is “conserved,” meaning that regardless of how the distribution of its mass might change, the total angular momentum will always remain the same. If mass spreads out, the system responds by rotating more slowly; if mass concentrates toward the center, the system responds by rotating more rapidly. For example, think of a slowly spinning ice skater with her arms extended. When she wraps her arms closely around her, she spins more rapidly. Now think about our slowly rotating cloud. As it collapsed, we might imagine all the gas and dust rushing toward its center, causing it to spin at a furious rate, but this is not what happens. While much of the cloud’s mass fell to the center to become a protostar—in this case, our proto-Sun—the rest ended up in an extended, flatted disk of gas and dust known variously as a protostellar or a protoplanetary disk.

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This disk absorbed most of the cloud’s angular momentum that otherwise would have torn the infant Sun apart. From our personal perspective, though, the disk played another but equally important role. It became the birthplace of all the planets, moons, asteroids, and comets that make up the solar system we see today. But how could the planets, including Earth and Mars, be created out of something as tenuous as dust? About 30 million years after the interstellar cloud began its collapse, the proto-Sun had settled down and was shining as a real star. Yet long before the young Sun reached this stage of maturity, the dusty disk surrounding it had already begun to evolve. As the tiny dust grains—about the size of smoke particles—collided, some stuck together to form larger clumps that then grew to the size of grains of sand, then to pebble size, then to boulder size, and finally to planetesimals—orbiting bodies ranging in size from a few kilometers to nearly a thousand. Those planetesimals that formed in the fiery environment close to the Sun were composed mostly of rocky material. Those farther out were made up of rock, various ices, and organic materials. In everyday language, the term organic make us think of life, but that is not the way astronomers and chemists typically use it. An organic molecule is simply one that contains the element carbon (along with other elements such as hydrogen, nitrogen, oxygen, sulfur, and phosphorus). This is true whether or not it is associated with living organisms. Similarly, when we think of ice, frozen water comes to mind, but astronomers also include frozen methane, ammonia, carbon dioxide, and other frozen gases as “ices.” The young solar system was a place of extreme violence, with swarms of orbiting bodies smashing into one another, sometimes coalescing into larger bodies, sometimes destroying themselves in the process. Ultimately, a small number of larger planetesimals swept up most of the remaining bodies in their vicinity, forming objects we call protoplanets. In one such collision, a Mars-size protoplanet hit proto-Earth with a glancing blow that destroyed the impacting body and sent debris flying into orbit around proto-­ Earth. This debris quickly coalesced to form Earth’s only known natural satellite, the Moon. Eight of the surviving protoplanets eventually became the planets that we know today. Remarkably, this entire process of turning tiny grains of dust into huge planets took no more than five million years. Closest to the Sun are the rocky terrestrial planets: Mercury, Venus, Earth, and Mars. More distant are the gas-rich giant planets: Jupiter, Saturn, Uranus, and Neptune. But not all of the planetesimals were caught up in the planet-building process. What happened to the survivors? Ultimately, these leftover chunks of primitive material suffered various fates. Some found themselves in orbits that were highly sensitive to the gravitational pull of the larger planets and were launched outward,

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escaping from the solar system altogether. Others spiraled inward toward the Sun to meet a fiery death. Still others ended up as an assortment of dwarf planets, rocky asteroids, and icy comet nuclei. Most of the rocky asteroids now occupy the wide space between the orbits of Mars and Jupiter. The icy comet nuclei live on today, orbiting in the dark, frigid outer reaches of the solar system beyond the orbit of Neptune, the most distant planet. Five of these remaining planetesimals are so large that we call them dwarf planets. They include the rocky object Ceres, formerly known as the largest asteroid, and the icy body Pluto, once known as the ninth planet. With the formation of its eight planets, the solar system was now complete, but the planets themselves had yet to settle down. Gravitational tugging and pulling among the four giant planets caused their orbits to migrate inward and outward before settling down into the locations that we find today. During this period of readjustment, Jupiter disturbed the orbits of a large number of asteroids, causing them to drift inward and rain down on the terrestrial planets. The energy of the impacts during this era was so great that it kept the interiors of the terrestrial planets in a completely molten state for hundreds of millions of years. As the mayhem finally came to an end, the young terrestrial planets cooled enough to become encrusted with dark veneers of solidified rock—barren worlds orbiting a young Sun. Over the next four billion years, both Earth and Mars would evolve along sometimes similar, sometimes divergent paths. Earth, with a diameter of 7,900 miles (12,700 km), is the larger of the two planets. Mars, with a diameter of 4,200 miles (6,800 km), is only slightly more than half as large as Earth. At first, both planets acquired substantial atmospheres and oceans of liquid water. Our more massive Earth, however, would retain most of its atmosphere and water, while less massive Mars would lose much of its atmosphere and most of its water. Over time, both Earth and Mars would experience a series of surface-altering processes, including impact cratering, volcanism, tectonics, and erosion by wind and water—but on vastly differing scales. These divergent processes have been responsible for the two completely dissimilar planets we see today, one water rich and life bearing, the other cold, dry, and forbidding.

Overleaf left, Volcanic cinder cones, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Volcan Villarica, Chile (NASA EOS, scale ~ 2.5 km)

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< Impact craters and dry river beds, Mars (ESA Mars Express, scale ~ 100 km)

Overleaf left, Crater with CO2 ice, Mars (NASA MRO HiRISE, scale ~ 600 m) Overleaf right, Ross Kamm impact crater, Namibia (NASA ISS, scale ~ 15 km)

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< Dune field, Rub’ al Khali, Arabian empty quarter (NASA EOS, scale ~ 50 km)

Overleaf left, Dune field, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Mesquite dunes, Death Valley, California (Strom, scale ~ 200 m)

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< Dune fields, Mars (NASA MRO HiRISE, scale ~ 500 m)

Overleaf left, Dune field, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Dunes, White Sands National Monument, New Mexico (Strom, scale ~ 50 m)

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< Grand Canyon of the Colorado (NASA EOS/Terra, scale ~ 300 km)

Overleaf left, Water-eroded channels, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, M’Zab Valley River channels, Algeria (NASA EOS, scale ~ 10 km)

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< Water-eroded badlands, Beautiful Valley, south of Chinle, Arizona (Strom, scale ~ 200 m)

Overleaf left, Water-eroded gullies, Mars (NASA MRO HiRISE, scale ~ 1 km) Overleaf right, River channel, Erg Issaouane, Algeria (NASA ISS, scale ~ 30 km)

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< Fjord complex, Norway (NASA EOS/Terra, scale ~ 60 km)

Overleaf left, North polar ice cap, Mars (ESA Mars Express, scale ~ 500 km) Overleaf right, Ice pattern, Ragnhild Ali, Norway (NASA EOS, scale ~ 10 km)

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< Grand Canyon of the Colorado (NASA Landsat, scale ~ 300 km)

Overleaf left, Water-eroded channels, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Ice and river channels, Hindu Kush (NASA EOS/Terra, scale ~ 500 km)

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< Badlands with rocks, north of Many Fields, Arizona (Strom, scale ~ 30 m)

Overleaf left, Meridiani Planum, Mars (NASA Opportunity rover, scale ~ 50 m) Overleaf right, South Desert, Cathedral Valley, Utah (Strom, scale ~ 100 m)

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< Badlands, south of Round Rock, Arizona (Strom, scale ~ 100 m)

Overleaf left, Meridiani Planum, Mars (NASA Opportunity rover, scale ~ 50 m) Overleaf right, Rock, dry river bed, Sunset Crater National Monument, Arizona (Strom, scale ~ 200 m)

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< View from crater rim, Meridiani Planum, Mars (NASA Opportunity rover, scale ~ 50 m)

Overleaf left, Meridiani Planum, Mars (NASA Opportunity rover, scale ~ 30 m) Overleaf right, Volcanic hillsides, Atacama Desert, Chile (Strom, horizontal scale ~ 300 m)

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< Ice floes, Kamchatka coastline (NASA ISS, scale ~ 300 km)

Overleaf, Ocean views, Big Sur coast, California (Strom, horizontal scale ~ 100 m)

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< Tidal flats, mid-Oregon coast (Strom, horizontal scale ~ 50 m)

Overleaf left, Incoming waves, Big Sur, California (Strom, scale ~ 30 m) Overleaf right, Ocean and sand, Oregon coast south of Point Arago (Strom, scale ~ 200 m)

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< Intertidal zone tide pools and foliage, Face Rock Beach, Oregon (Strom, scale ~ 20 m)

Overleaf left, Tilted rocks, Ona Beach, Oregon (Strom, scale ~ 30 m) Overleaf right, Tide pool, Point Lobos State Reserve, California (Strom, scale ~ 30 m)

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Hillsides, Mauna Loa Volcano, Hawai‘i, Hawai‘i (Strom, scale ~ 300 m) >

FIRE

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Fire BRADFORD A. SMITH We return now to Earth and Mars a few hundred million years after their births. At this time they existed only as thin-encrusted, molten bodies pulled into spherical shape by the force of their own self-gravity. In this early molten state, the interiors of the two planets became differentiated, meaning that heavier, denser materials tended to sink toward the center while lighter, less dense materials rose toward the surface. Dense metals, mostly iron and nickel, formed the cores of both planets. Lighter materials such as silicates formed lower-density shells, called mantles, that surrounded the cores. The very lightest materials migrated to the surface, where they cooled to form a rocky crust. Although cool on the outside, the two planets seethed within. As we noted earlier, Mars is just slightly more than half as large as Earth, and its internal structure is proportionally smaller. Earth’s metallic core measures about 4,300 miles (7,000 km) in diameter, a little more than half the size of the planet. The diameter of the martian core is about 2,200 miles (3,600 km), also slightly more than half the size of the planet. The thickness of the silicate mantle surrounding Earth’s core is about 1,800 miles (2,800 km), while that of Mars is about 960 miles (1,500 km), roughly the same ratio of mantle to core as Earth’s. Mars, however, has a thicker crust, 30 miles (50 km) compared to Earth’s 20-mile-thick (35 km) crust, and three times as thick relative to the sizes of the two planets. Today, the interiors of both planets remain heated by residual warmth left over from the time of their creation and from thermal energy supplied by the decay of certain radioactive elements called radioisotopes. The decay of these radioisotopes, including thorium-232 (232Th), uranium-238 (238U), and potassium-40 (40K), continues to replace the heat lost into space by radiation from the surface. Crustal material is not a very good conductor of heat, though, so much of the internal heat that reaches the surface is released through volcanism. Both Earth and Mars have been volcanically active throughout their entire geological history, although Mars appears far less active today than in its distant past. Over the short term, the rocky silicate material in the mantle acts like a solid, just as we would expect. Over very long timescales, however, this otherwise solid material behaves as an extremely viscous fluid. Consequently, the mantle is subject to slow

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convective motions: fluid heating and rising at the bottom but cooling and descending at the top. As the convective motion of the fluid mantle rises and falls, it causes a slow but steady deformation of the overlying crust, a process called tectonism. Earth’s crust is fractured into seven major rigid “plates” and several smaller ones. Driven by mantle convection, these plates slide around over Earth’s lower crust—sometimes colliding, sometimes tearing apart—in a process called plate tectonics. When plates collide, they shove the crust upward, folding it and forming mountain chains. For example, Earth’s Indo-Australian Plate is now colliding with the Eurasian Plate at a rate of a little less than two feet, or half a meter, per century. As the two plates yield to the crushing force, they respond by pushing up the massive Himalayas, the tallest of which is Mount Everest, the highest mountain on Earth. When plates separate, as is happening now on the Atlantic and Pacific Ocean seafloors, lavas flow upward to fill the gap, creating fresh surface rock. When the spreading seafloor approaches continental margins, it slides under the more buoyant continental plates, accompanied by strong frictional heating and ensuing volcanism. Most of Earth’s volcanoes are found in a “ring of fire” that coincides with the rim of the Pacific Plate. Thus, Earth’s surface remains in a constant state of upheaval, but as we will now see, the surface of Mars is relatively tranquil. Unlike Earth, the crust of Mars is not broken into individual plates. Nevertheless, mantle convection in the interior of Mars has its tectonic consequences. In places where convection is pushing the crust upward, the surface inflates. These surface bulges are found primarily in the Tharsis and Elysium regions of Mars, where the planet’s largest volcanoes are also located. In those places where the mantle is sinking, support of the crust falls away, fracturing the surface into long cracks and fissures called faults. The unsupported ground between the faults then collapses, creating canyon-like features. Valles Marineris, the largest known canyon in the solar system, is an extreme example of this tectonic surface readjustment. On both Earth and Mars, small pockets of magma—molten rock with dissolved gases—are found just beneath the crust near the upper parts of the mantle. As a viscous liquid, magma is less dense than its more solid surroundings, and it occasionally works its way to the surface, where it bursts forth, creating volcanoes and surface lava flows. Over time, volcanism has dramatically shaped the landscape of both planets. The largest of Earth’s volcanoes is Hawai‘i’s Mauna Kea, which rises six miles (10 km) above the Pacific floor. But Mauna Kea is tiny compared to Olympus Mons, whose summit towers fifteen miles (25 km) above the martian landscape. How could smaller

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Mars build bigger volcanoes than Earth? As we have pointed out, Earth’s upper crust is fragmented into a series of rigid “plates” that float around on a quasi-fluid upper mantle. Imagine a pocket of hot magma flowing up through Earth’s upper crust to form a volcano. In time, the plate on which the volcano formed moves away from the volcano’s magma source, and the mountain-building process comes to an end. The crust of Mars, however, is not fractured into moving plates. Why this is so remains an unanswered question, but it may be due to the relative thickness of the martian crust. The magma source that created Olympus Mons has remained beneath it throughout the lifetime of the mountain. It seems likely that Olympus Mons is continuing to build but may be dormant at this time. Olympus Mons is only one of several large volcanoes in the Tharsis region, all ranging in height between 8.5 and 11 miles (14–18 km) above their surroundings, and all far taller than Earth’s largest volcano. Volcanoes are not the only manifestation of volcanism on Earth and Mars. Both planets have experienced enormous outpourings of relatively fluid lavas that have inundated the surrounding landscape. Among the larger terrestrial lava flows are the Columbia River basalts that now cover much of the states of Washington, Oregon, and Idaho. The amount of lava that flowed onto the surface of the Pacific Northwest some 15 million years ago is estimated to be an impressive 42,000 cubic miles (175,000 km3). Mars has also seen its share of huge surface flows. These regions of extensive flood basalts are analogous to the dark basins on the Moon. Magmas, as we mentioned earlier, contain dissolved gases, and as ancient volcanoes spewed out their lavas, they also released huge amounts of these gases, primarily carbon dioxide and water vapor. The atmospheres of Earth and Mars came about in this early era of superactive volcanism.

Overleaf left, Cinder cones, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Volcan El Misti, Peru (NASA ISS, scale ~ 10 km)

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< Fossil lava flows, Mars (NASA MRO HiRISE, scale ~ 1 km)

Overleaf left, Eroded cinder cones, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Eroded volcanic rock, Johns Island, Washington (Strom, scale ~ 1 m)

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< Fossil lava flow, Mars (NASA MRO HiRISE, scale ~ 500 m)

Overleaf left, Chain of cinder cones, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Weathered volcanic rock, Craters of the Moon National Monument, Idaho (Strom, scale ~ 1 m)

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< Cinder cone complex, El Pinacate, Sonora, Mexico (NASA Landsat, false color, scale ~ 50 km)

Overleaf left, Complex of small volcanic cinder cones, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Weathered volcanic rock, Craters of the Moon National Monument, Idaho (Strom, scale ~ 0.5 m)

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< Weathered volcanic rock, Smith Rock State Park, Oregon (Strom, scale ~ 1 m)

Overleaf left, Fossil lava flow, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Mud flow, Harris Beach, Oregon (Strom, scale ~ 1 m)

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< Crater interior, Haleakalaˉ National Park, Maui, Hawai‘i (Strom, scale ~ 500 m)

Overleaf left, Fossil lava flow, Mars (NASA MRO HiRISE, scale ~ 1 km) Overleaf right, Fossil lava flow, Mars (NASA MRO HiRISE, scale ~ 500 m)

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Dune field with CO2 ice, Mars (NASA MRO HiRISE, scale ~ 1 km) >

AIR

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Air BRADFORD A. SMITH To understand the origin of the atmospheres of Earth and Mars, we need to look back to the earliest moments in their histories. As protoplanets, they were surrounded by the disk of gas and dust that gave birth to the solar system, a disk that still contained enormous amounts of hydrogen (H) and helium (He) gas. All eight young planets were immersed in this abundant supply of gas, and all eight captured some of it to form primary atmospheres. The strong gravitational grasp of the massive giant planets enabled them to form permanent atmospheres by capturing and then holding on to large amounts of these lightweight gases. This was not the case for the terrestrial planets, though. Hydrogen and helium are the lightest of the elements, and the weaker gravity of the less massive terrestrial planets could not prevent these gases from escaping back into space. Within a few million years, the primary atmospheres of Earth and Mars had completely vanished. Yet other processes were already at work, processes that would produce permanent secondary atmospheres composed of more massive molecules such as water (H2O) and carbon dioxide (CO2). As lavas erupted onto the surfaces of Earth and Mars, they released massive amounts of dissolved gases, primarily carbon dioxide and water vapor, which then became the main constituents of their early permanent atmospheres. At about this same time, Jupiter and Saturn were stirring up icy planetesimals in the outer solar system and ice-rich asteroids in the inner solar system. Many of these icy objects drifted inward, raining down on Earth and Mars and adding additional water, along with methane (CH4) and ammonia (NH3), to their atmospheres. Both methane and ammonia, however, are unstable under the influence of intense solar ultraviolet radiation. Ammonia quickly breaks down into its component parts of hydrogen and nitrogen (N). The lighter hydrogen atoms then escaped to space, leaving behind the heavier nitrogen atoms, which promptly combined in pairs to form nitrogen molecules (N2). Nitrogen now makes up most of Earth’s atmosphere, but it remains only as a minor constituent in the martian atmosphere. With its smaller size and weaker gravity, Mars soon lost much of this early atmosphere as the gases escaped into space. Carbon dioxide, whose relatively massive molecules tend to resist loss, now makes up most

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of today’s martian atmosphere. Some water vapor has also survived, but most of the water on Mars is tied up as subsurface water ice. Today, molecular nitrogen makes up 78 percent of Earth’s atmosphere and molecular oxygen (O2) about 21 percent. Trace amounts of argon, water vapor, and carbon dioxide constitute most of the remaining 1 percent. Earth’s abundant atmospheric oxygen is unique among all known planets—both within and beyond our solar system. It is maintained solely by terrestrial and aquatic plant life, which consumes carbon dioxide and releases oxygen as a waste product. If plant life were to vanish, so would Earth’s atmospheric oxygen and all animal life, ourselves included. Most of Earth’s water exists as oceans, lakes, and the enormous continental ice sheets in Greenland and Antarctica. You might ask what happened to all that carbon dioxide, once a major constituent of Earth’s early atmosphere. It still exists today, but not as a gas. Instead, nearly all of it is locked up in the form of carbonates such as marble, limestone, and chalk, most of it buried far out of sight. Chemical interactions with surface rocks have been a powerful mechanism in removing much of the carbon dioxide from our atmosphere and storing it as solid carbonates. Plant life and ocean invertebrates, such as coral, are also responsible for keeping carbon dioxide from being more than a minor constituent in our terrestrial atmosphere. The early removal of carbon dioxide from Earth’s atmosphere has had a profound effect on the evolution of our planet. Carbon dioxide is a powerful greenhouse gas, raising temperatures by trapping solar radiation. Had most of this gas not been removed by chemical and biological processes, the surface temperature of Earth would likely be far above the boiling point of water. There would be no oceans, no life. Earth would be an inferno much like Venus, whose CO2-rich atmosphere has kept surface temperature at a sizzling 860°F (460°C), hot enough to melt lead. Yet a modest greenhouse effect is necessary for our survival. In the absence of all greenhouse gases, Earth’s average global temperature would be about 0°F (–18°C), and our planet would be a frozen ball of ice. Earth’s atmosphere surrounds us like an ocean of air. It is responsible for our blue skies and for all of our weather. It supplies us with oxygen, on which all animal life, ourselves included, must depend. It also contains small amounts of ozone, a molecule consisting of three oxygen atoms (O3). Ozone is found high up in Earth’s stratosphere, and there it forms a protective layer, absorbing intense and deadly ultraviolet radiation from the Sun. Our atmosphere is also a dynamic force, affecting the landscape that we see around us. Over time, its winds have contributed to the erosion that wears down

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the topography built up by volcanism and plate tectonics. The weight of all that air above us creates an average atmospheric pressure of 1,013 millibars (mb) at sea level. (A millibar is a thousandth of a bar, a standard unit of pressure.) Atmospheric pressure decreases with height. At the summit of Mauna Kea, 13,800 feet (4,200 m) above sea level, the average pressure is 600 mb, and at the very top of Mount Everest, more than 29,000 feet (8,800 m) above sea level, the atmospheric pressure is only 315 mb. Yet, even at this lofty height, the atmospheric pressure is far higher than the highest that now exists anywhere on the surface of Mars. While the atmosphere of Mars is much thinner than Earth’s, its winds can be strong enough to create huge planetwide dust storms, towering dust devils, and large fields of sand dunes. As a result of these winds, the martian atmosphere is so full of dust that its skies are more likely to have a light brown or orange color rather than the blue skies that we enjoy. The average atmospheric surface pressure on Mars is only 6 mb, but it varies between 12 mb in the depths of Hellas Planitia (the equivalent of Earth’s ocean basins) to 0.3 mb at the summit of Olympus Mons. Since there is no oxygen in the martian atmosphere, there can be no ozone, and so deadly ultraviolet solar radiation reaches all the way to the surface. Any life-forms that might exist today on Mars would have to be living in protected subsurface habitats. Earth’s atmospheric composition continues to evolve today, but at a rate that is unprecedented over its long geological history. The Industrial Revolution, which began about two hundred years ago, launched the current era in which humans have been burning ever-increasing amounts of fossil fuels—coal, oil, and natural gas. Concentrations of atmospheric carbon dioxide, the powerful greenhouse gas that has historically prevented Earth from being a giant snowball, is now increasing in lockstep with the burning of these fossil fuels. While a nominal amount of carbon dioxide has kept our planet from freezing over, increasing amounts of the gas are leading toward worrisome global warming and the prospect of severe climate change. Moreover, global warming is releasing frozen methane from the Arctic tundra and elsewhere. Methane is an even stronger greenhouse gas, which then accelerates the rate of global warming. By continuing to release carbon dioxide in large quantities, we are playing dangerously with the health of our atmosphere—and possibly with life itself. Overleaf left, Dune complex, Mars (NASA MRO HiRISE, scale ~ 1 km) Overleaf right, Dust devil tracks, Mars (NASA MRO HiRISE, scale ~ 1 km)

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< Dune detail, Mesquite Flat Sand Dunes, Death Valley National Park, California (Strom, scale ~ 10 m)

Overleaf left, Dune complex, Mars (NASA MRO HiRISE, scale ~ 1 km) Overleaf right, Dune, Mesquite Flat Sand Dunes, Death Valley National Park, California (Strom, scale ~ 20 m) ,

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< Dune detail, Mesquite Flat Sand Dunes, Death Valley National Park, California (Strom, scale ~ 5 m)

Overleaf left, Dune complex, Mars (NASA MRO HiRISE, scale ~ 1 km) Overleaf right, Dune detail, Great Sand Dunes National Park, Colorado (Strom, scale ~ 5 m)

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< Windblown dunes, Clam Beach, near Arcata, California (Strom, scale ~ 3 m)

Overleaf left, Dune field, Mars (NASA MRO HiRISE, scale ~ 1 km) Overleaf right, Wind-eroded sandstone wall, Canyon de Chelly National Monument, Arizona (Strom, scale ~ 3 m)

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< Windblown sand features, Harris Beach, Oregon (Strom, scale ~ 3 m)

Overleaf left, Windblown streaks of subsurface martian material (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Windblown sand, Harris Beach, Oregon (Strom, scale ~ 1 m)

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< Windblown dune, Clam Beach, near Arcata, California (Strom, scale ~ 3 m)

Overleaf left, Dune field, Mars (NASA MRO HiRISE, scale ~ 1 km) Overleaf right, Dune pattern, White Sands National Monument, New Mexico (Strom, scale ~ 5 m)

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< Dune patterns, White Sands National Monument, New Mexico (Strom, scale ~3 m)

Overleaf left, Elongated dune pattern, Mars (NASA MRO HiRISE, scale ~ 1 km) Overleaf right, Dune pattern, Saudi Arabian desert (NASA EOS/Terra, scale ~ 50 km)

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San Juan River, Utah and Colorado (NASA ISS, scale ~ 70 km) >

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Water BRADFORD A. SMITH By the time the solar system was about a hundred million years old, both Earth and Mars had acquired secondary atmospheres that contained abundant amounts of carbon dioxide and water vapor. Still, no liquid water flowed over their barren, rocky surfaces because the ground remained too hot for water to exist in a liquid state. But the hot ground did heat the cooler air just above it. This caused atmospheric convection, which carried water vapor upward to heights where it was cold enough to condense out as droplets of liquid water. When they had grown large enough, these droplets then fell as rain. Yet even as the rain hit the scalding surface, it instantly evaporated and returned to the atmosphere once again as water vapor. This condensation-evaporation cycle continued until the ground became cool enough for liquid water to survive. Studies of ancient zircon crystals indicate that some liquid water must have existed on Earth’s surface when our planet was only 150 million years old. Because Mars was smaller and therefore cooled faster, we can assume that some liquid water also existed on the martian surface at least this early. At an age of 200 million years or so, the surfaces of Earth and Mars had cooled sufficiently to allow the accumulation of substantial amounts of liquid water. Rainwater flowed over the landscape and cascaded into streams and rivers where it eventually reached deep basins in the crust. This created vast oceans that covered the surfaces of both planets. In its journey from the highlands, liquid water acted as a powerful erosive force that redistributed rock and soil by carving away at surface topography and depositing layers of silt in lake beds and ocean floors. Today, the surfaces of Earth and Mars bear the scars of widespread erosion caused by water and wind. Water is one of the few materials that expands when it freezes. When water seeps into tiny cracks in rocks and then freezes, this expansion forces the cracks apart, and the rocks break down into smaller pieces. These smaller pieces are ground down still further to become fine sand as they tumble along in fast-moving streams. In this way, entire mountains can be eroded away and carried to the sea. Today, oceans cover nearly three-quarters of Earth’s surface at an average depth of 12,100 feet (3,700 m). Lakes and the great ice sheets of Antarctica and Greenland make up the rest of Earth’s water supply. There is evidence that nearly a third of Mars was

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once covered with water; deeply cut canyons and river channels provide still further proof that running water once flowed over the martian surface. Water now exists on Mars primarily as seasonal snows that accumulate in the planet’s polar caps and as ice buried in soils beneath the surface. Although small streams of briny liquid water have occasionally been observed by orbiting spacecraft, the frigid climate of Mars today keeps nearly all water in the solid state. What happened to all that water that once existed as oceans on Mars? Most of it, along with much of the planet’s original atmosphere, has been lost to space. And why did this occur on Mars but not, it seems, on our own planet? The answer lies in the dynamics of atmospheric gases. All of the gas molecules in a planet’s atmosphere are constantly in motion. A law of physics called the equipartition of energy requires that all molecules in a given volume of gas have the same average kinetic energy, that is, all have the same energy of motion. The kinetic energy of an individual molecule is proportional to its mass times the square of its velocity. If all kinds of molecules are obliged to have the same energy, it means that lighter molecules must be moving faster than their heavier neighbors. This was the case for water and carbon dioxide in the early martian atmosphere. Carbon dioxide molecules are more than twice as massive as water molecules and therefore move about more slowly. Picture a water molecule high in the atmosphere of Mars, where it might have enough speed to escape the planet’s relatively weak gravity and disappear into nearby space. A slower carbon dioxide molecule in a similar situation might also escape, but the probability of this happening would be lower. Consequently, the rate of water vapor loss on Mars has been greater than that of carbon dioxide. Over the past four billion years, Mars has lost most of its water and much of its carbon dioxide atmosphere. Of course, this same loss mechanism applies to our planet as well, but Earth’s stronger gravity has enabled it to hold onto most of its original air and water. The details of how planets lose their atmospheric gases is a subject far more complex than we have described here, and it remains an active topic of ongoing scientific research. Water is essential to all forms of life as we know it. It is the medium in which prebiotic molecules can evolve to achieve self-replication, a process that, once started, can eventually lead to the creation of living organisms. It seems likely that terrestrial life originated in Earth’s oceans, either in sunlit tide pools or in the warm, nutrient-­ rich waters surrounding deep geothermal vents.

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Think of an alien astronomer observing Earth and Mars from some distant planetary system. The Red Planet would appear as a mostly dead and nearly airless world, while the Blue Planet would be seen as possessing a thick atmosphere containing water vapor and an abundance of molecular oxygen. Knowing that oxygen is a highly reactive gas that cannot be maintained as a major atmospheric component without an active replacement source, the alien astronomer would likely conclude that some form of life might exist on the Blue Planet but not on the Red Planet. We, of course, have much closer access to Mars than does the alien astronomer. We can ask whether similar prebiotic processes might have taken place on Mars long ago when water covered much of its surface and whether liquid water survived long enough for primitive life to evolve. Searching for answers to these questions has become a fundamental scientific objective in the ongoing robotic exploration of Mars. As yet, no clear biological markers have been found in the soils of Mars. Martian life, if it exists, has so far stubbornly failed to reveal itself, and so the search continues. Returning now to our own planet, what is the source of our atmospheric oxygen? It has its origins in Earth’s oceans. More than 3.5 billion years ago, one of the earliest lifeforms, an ancient type of cyanobacteria, created oxygen as a biological waste product. (A present-day form of cyanobacteria, blue-green algae, is the bane of swimming pool owners.) Oxygen, as we have pointed out, is a highly reactive gas, easily combining with and oxidizing most materials that it comes in contact with. The earliest oxygen released into the terrestrial atmosphere by living organisms oxidized exposed metals and other minerals, removing the oxygen as quickly as it was created and changing the very chemistry of Earth’s surface. When these surface materials finally became completely oxidized, the oxygen content of the atmosphere began to increase, reaching the roughly 20 percent that we see today. If such biologically generated oxygen had ever existed on Mars, it would have been lost long ago when liquid water disappeared. Today, no oxygen also means no ozone, and that means deadly solar ultraviolet radiation can reach all the way to the martian surface. Any organisms surviving today on Mars must have found a habitat beneath a protective covering of surface soil. Mars, with its windy barren surface, is now seemingly devoid of living things. Earth, conversely, teems with diverse forms of life, perhaps providing just a hint of what early Mars might have been like. Overleaf left, Swiss cheese pattern, CO2- and water-ice layers, south pole of Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Sandstone layers, near Hite Crossing, Utah (Strom, scale ~ 1 m)

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< Ice breakup, near Akpatok Island, Ungava Bay, Quebec (NASA ESO/Terra, scale ~ 50 km)

Overleaf left, Swiss cheese pattern, CO2- and water-ice layers, south pole of Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Ice pattern, Canyon de Chelly, Arizona (Strom, scale ~ 0.5 m)

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< Polygonal patterns, Salar de Uyuni, Bolivia (Strom, horizontal scale ~ 10 m)

Overleaf left, Swiss cheese pattern, CO2- and water-ice layers, south pole of Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Sandstone layers, Escalante, Utah (Strom, scale ~ 1 m)

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< Layered sandstone deposits, White Pocket, Utah (Strom, scale ~ 0.5 m)

Overleaf left, Cross section of CO2-ice layers, north pole of Mars (NASA MRO HiRISE, scale ~ 1 km) Overleaf right, Sandstone layers, White Pocket, Utah (Strom, scale ~ 0.5 m)

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< Ice breakup, Bear Glacier, Kenai Peninsula, Gulf of Alaska (IKONOS satellite, scale ~ 10 km)

Overleaf left, Water-eroded channels, Mars (NASA MRO HiRISE, scale ~ 1 km) Overleaf right, Water-eroded sand pattern, Smelt Sands Beach, Yachats, Oregon (Strom, scale ~ 0.5 m)

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< Ice and mud pattern, Canyon de Chelly, Arizona (Strom, scale ~ 0.5 m)

Overleaf left, Water-eroded channels, bajada, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Water-eroded gullies, rim of Ubehebe Crater, Death Valley National Park, California (Strom, scale ~ 200 m)

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< Water-eroded gullies, Mars (NASA MRO HiRISE, scale ~ 1 km)

Overleaf left, Polygonal patterns formed by frozen CO2, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Drying mud, Dirty Devil River, Utah (Strom, scale ~ 20 m)

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< Polygonal patterns, drying river bank, Hite Crossing, Utah (Strom, scale ~ 50 m)

Overleaf left, Polygonal ice patterns, Mars (NASA MRO HiRISE, scale ~ 500 m) Overleaf right, Drying mud, southern Utah (Strom, scale ~ 3 m)

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< “Spider” patterns formed by CO2 ice, Mars (NASA MRO HiRISE, scale ~ 1 km)

Overleaf left, Fissure patterns near the south pole of Mars (NASA MRO HiRISE, scale ~ 1 km) Overleaf right, Drying mud, near Escalante, Utah (Strom, scale ~ 2 m)

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< Ice-formed polygonal patterns near the north pole of Mars (NASA MRO HiRISE, scale ~ 500 m)

Overleaf left and right, Ice patterns, crater bottom near the north pole of Mars (NASA MRO HiRISE, scale ~ 500 m)

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About the Authors Stephen E. Strom spent his professional career as an astronomer. Born in 1942 in New York City, he graduated from Harvard College in 1962. In 1964 he received his master’s and PhD in astronomy from Harvard University. From 1964 to 1968 he held appointments as lecturer in astronomy at Harvard and astrophysicist at the Smithsonian Astrophysical Observatory. He then moved to the State University of New York at Stony Brook and served 4 years as coordinator of astronomy and astrophysics. In 1972 he accepted an appointment at the Kitt Peak National Observatory in Tucson, Arizona, where he served as chair of the galactic and extragalactic program. The following 15 years were spent at the University of Massachusetts, Amherst; from 1984 to 1997 he served as chairman of the Five College Astronomy Department. In 1998 he returned to Tucson as a member of the scientific staff at the National Optical Astronomy Observatory (NOAO), where he carried out research directed at understanding the formation of stars and planetary systems and served as an associate director. He retired from NOAO in May 2007. Asteroid 4604 (Stekarstrom) is named for him and his late wife Karen. He began photographing in 1978. He studied both the history of photography and silver and nonsilver photography in studio courses with Keith McElroy, Todd Walker, and Harold Jones at the University of Arizona. His work, largely interpretations of landscapes, has been exhibited widely throughout the United States and is held in several permanent collections, including the Center for Creative Photography in Tucson, the Fred Jones Jr. Museum of Art at the University of Oklahoma, the Mead Art Museum in Amherst, Massachusetts, and the Museum of Fine Arts, Boston. His photography complements poems and essays in three books published by the University of Arizona Press: Secrets from the Center of the World (1989), a collaboration with Muscogee poet Joy Harjo; Sonoita Plain: Views of a Southwestern Grassland (2005), a collaboration with ecologists Jane and Carl Bock; and Tséyi' (Deep in the Rock): Reflections on Canyon de Chelly (2005), coauthored with Navajo poet Laura Tohe; as well as in Otero Mesa: America’s Wildest Grassland (2008), with Gregory McNamee and Stephen Capra, for University of New Mexico Press. Earth Forms, a monograph comprising 43 images, was published in 2009 by Dewi Lewis. His most recent publication is Sand Mirrors (2012), a collaboration with Zen teacher and poet Richard Clarke, for Polytropos Press.

Bradford A. Smith was born in 1931 in Cambridge, Massachusetts. He graduated from Northeastern University with a BS degree in chemical engineering in 1954 and a PhD in astronomy from New Mexico State University in 1972. He has served as associate professor of astronomy at New Mexico State University; as professor with joint appointments in planetary sciences and astronomy at the University of Arizona; as research astronomer at the Institute for Astronomy, University of Hawai‘i at Maˉnoa; and as a visiting associate professor in the Division of Geological and Planetary Sciences at the California Institute of Technology. As director of planetary research at New Mexico State University, he established a program of systematic planetary photography in 1958, the first such program to photograph the brighter planets on an uninterrupted routine schedule. In early 1976 Smith and his coworkers were the first to use a charge-coupled device (CCD) detector on an astronomical telescope, yielding the first high-resolution infrared images of Uranus and Neptune. Turning later to space research, Smith participated in a number of U.S. and international space missions, including Mars Mariner 6, 7, and 9, the Mars Viking mission, the Soviet Vega mission to Halley’s Comet, the Soviet Phobos mission to Mars, and the Wide Field/Planetary Camera team for the Hubble Space Telescope. He was the deputy team leader of the imaging team on the Mariner 9 Mars orbiter and was chosen by NASA to lead the camera team on the Voyager missions to Jupiter, Saturn, Uranus, and Neptune. Later, Smith’s interests turned to other planetary systems. He codiscovered a circumstellar disk around the nearby star, β Pictoris, the first direct evidence of a planetary system beyond our own, and continued these studies as a member of the infrared camera (NICMOS) experiment on the Hubble Space Telescope. Smith has four times been awarded the NASA Medal for Exceptional Scientific Achievement. Asteroid 8553 (Bradsmith) is named for him. Smith has served as the president of International Astronomical Union (IAU) Commission 16 for the physical studies of planets and satellites and as a member of the IAU Working Group for Planetary System Nomenclature, where he is also the chair of the Mars Task Group. He has been a coauthor on four editions of 21st Century Astronomy (2010–13), published by W. W. Norton, and two editions of Understanding Our Universe (2012–15), also published by Norton. He has authored popular articles in National Geographic magazine and Sky & Telescope.

Back cover, Sunset on Mars (NASA Spirit rover; the rim of Gusev Crater, seen in silhouette, is 160 km across)