10
Outward and Onward
Beyond Earth, beyond the Present
The whole universe may be as one plain, the distance between planet and planet being only as the pores in a grain of sand, and the spaces between system and system no greater than the intervals between one grain and the grain adjacent.
But for a slice of luck, the whole world might have been made of sand.
PROBING
It was the early hours of December 25, 2004. In England, Christmas stockings were being surreptitiously left next to sleeping children; in the United States, Christmas Eve celebrations were still continuing. Far, far away, a bundle of spectacular technology, looking like an inverted salad bowl set in a large, upside-down patio umbrella and wrapped in gold foil, slipped quietly away from its moorings and began its final journey. Twenty-one days into that journey, as it hurtled into an increasingly dense orange-brown haze, the technology awoke, its parachute deployed, the umbrella jettisoned. And it began filming a new world.
As the haze thinned, blurred outlines became visible, slowly resolving into a desolate, fractured landscape. Jagged ridges appeared, river valleys, coasts, dusty plains—and sand dunes. It would be satisfying to think that, somehow or another, Christiaan Huygens was watching the scene unfold; he would have been, like us, riveted, astonished. For this new world was Titan, the largest of Saturn’s moons, first discovered by Huygens in 1655. It was in his honor that the bundle of technology, the Huygens probe, was named.
SCALE
As I suggested in the preface, because of the very nature of sand, this book is in many ways about scale, from the microscopic to, so far, the global. We have scrutinized sand in its many diverse roles—but only in the context of our planet and only in the present and the past. In this final chapter, we will broaden our horizons to look beyond the Earth and beyond the present. The first direction, as we will see, informs the second, and sand is there in both.
We met Huygens earlier in this book. As one of the luminaries in the 1600s, that extraordinary century of scientific inquiry and discovery, he moved in a broad but close community of illustrious colleagues. Huygens was a skilled card player and, with the encouragement of Blaise Pascal, wrote the first book on chance in gambling—or probability theory. He taught mathematics to Gottfried Leibniz, a man for whom scale was fundamental (chapter 1), was a family friend of René Descartes, visited Isaac Newton (with whom he disagreed on the theory of light), and worked on telescopes with the Dutch philosopher Benedict de Spinoza. As a glass grinder and maker of telescopes, Huygens was a friend of fellow Dutchman Antony van Leeuwenhoek, the microscope pioneer, who called Huygens “the great sky-inquirer.” In a time when science was doing one of the things it does best—shattering the limitations of scale—Huygens was discovering new worlds in one direction, van Leeuwenhoek in the other. The latter discovered bacteria, described in one of his many letters to the Royal Society as so small that “even if 100 of these very wee animals lay stretched out one against another, they could not reach to the length of a coarse grain of sand.” The former was revealing the enormity of the number of stars, so extensive that to count them “requires an immense Treasury, not of twenty or thirty Figures only . . . but of as many as there are Grains of Sand upon the shore. And yet who can say that even this number exceeds that of the fixt stars?” (Cosmotheoros, 1698). Oddly, even though Huygens was a notable mathematician, he makes no mention of Archimedes’ approach to this challenge (chapter 3).
The discovery of Titan, called by Huygens simply Saturni Luna, was made possible by the telescope technology he had developed. He also identified separate rings around Saturn and was the first to see and describe the Orion Nebula. There are now thirty-five named moons of Saturn; further ones were identified not long after Titan by Giovanni Domenico Cassini, a French-Italian astronomer whose telescope was even longer than that of Huygens. The “mother ship” from which the Huygens probe detached itself that Christmas day was named in honor of Cassini.
CHILD OF HEAVEN AND EARTH
The world of Titan revealed seemingly miraculously by Huygens is in many ways starkly alien, in others hauntingly familiar. Titan deserves to be a planet—it’s larger than Mercury and is the only satellite in the solar system to boast a proper, if exotic, atmosphere. In Greek mythology, the Titans, children of Uranus and Gaia, were powerful deities, and Saturn’s moon is a powerful and dynamic world, in spite of its temperature.
As Huygens plunged through the atmosphere of Titan, the shock wave generated temperatures of 12,000°C (22,000°F), but by the time it landed, the surface temperature was about minus 180°C (-290°F). The probe owed its survival in these extremes to the properties of silica-based insulation and ceramics (chapter 9). Huygens settled on the surface and sent information homeward for an hour and a half before it succumbed to its environment. Peering out into its new world, Huygens had a limited, but dramatic, view. Figure 44 shows what it was looking at. (Scale is important here—the objects are cobbles and pebbles, not boulders.)
It is a strange, but at the same time recognizable, world. The area around the pebble in the foreground has been scoured, a phenomenon one can see on any river-bank on Earth after a flood subsides or on the rock-strewn surface of a desert hamada. The pebbles are rounded and set on a bed of what looks like sand. Extending a sensor, Huygens dug into it, and it had the consistency of sand. It is sand (because of its size), but sand unlike any we have encountered, or could encounter, in our own world. Titan’s atmosphere is made up of over 98 percent nitrogen, the remainder being composed of methane (the simplest hydrocarbon), carbon dioxide, and complex carbon molecules. The sand grains and pebbles are made of rock-hard frozen hydrocarbon ice, and possibly some water ice. When Huygens landed, it set off a puff of gas, probably methane, from the sediment beneath it, melted by the probe.
FIGURE 44. Our first view of Titan’s surface, transmitted by the Huygens probe. (Not a fine-quality image, but impressive, considering where it was taken.) (Image © ESA/NASA-University of Arizona)
The ingredients may be exotic, but the processes clearly are not. Not only did Huygens collect images of ridges and valleys as it descended, along with a scoured bank of sand and pebbles, but its mother ship, Cassini, has continued to observe and map the surface of Titan, using radar to penetrate the thick hydrocarbon smog of the atmosphere. The landscapes that it has revealed appear to have been sculpted by the same physical processes as those on Earth—sinuous river valleys and tributaries, eroded hills, volcanoes—but the material for the sculpture is not granite or sandstone or limestone, but frozen hydrocarbons.
How can the processes of erosion work at such frigid temperatures, when there is no water, the dominant sculptor on Earth? There is good evidence that it rains on Titan, which has storm clouds (larger and more menacing than ours) that gather periodically around the polar regions. The rain falls rarely, but when it does there is an almighty deluge, scouring the landscape with flash floods that carry vast volumes of sediment along with them, potentially rounding the pebbles and cobbles. However, the rain is not water, but toxic liquid hydrocarbons. Some methane is included, along with other materials pulled out of the atmosphere where the Sun has stewed the methane into more complex molecules, possibly forms of plastic. A portion of Titan’s sand may even be made from these molecules, rained out of the atmosphere, the rest of it by more familiar processes of weathering and erosion. The pebbles in Figure 44 could be giant hailstones.
Large dark areas on Titan had originally been interpreted as seas of liquid methane, but Cassini’s radar gaze dispelled this view. The dark equatorial regions are filled with seas, but seas of sand dunes, extraterrestrial ergs (Figure 45). The dunes are long and linear, 100 meters (330 ft) high, extending over 1,500 kilometers (930 mi). They have been compared in appearance to satellite views of dunes in Namibia and Arabia.
Titan’s dramatic atmosphere appears to circulate from the polar regions toward and around the equator, concentrating the seas of sand dunes. Winds on Earth are driven by solar heating and cooling, and it had been thought that because Titan is so far removed from the Sun’s warmth, surface winds would be negligible. Titan’s gravity is low, its atmosphere thick, and its sand relatively light, all of which make moving sand quite easy. But the dune fields demonstrate that it is also a world with significant winds. The origin of the winds, as we now understand, is the overwhelming presence of Saturn—the winds are caused by atmospheric tides as the gravitational embrace of the planet waxes and wanes, creating the effect of a kind of giant bellows. The resulting winds are still gentle by our standards—around half a meter per second (1 mph), but this is sufficient to move sand grains in Titan’s environment, perhaps augmented by the periodic giant storms. By analogy with the formation of our own dunes, the linearity of those on Titan suggests variable wind directions; if we study the shapes of the dunes in Figure 45 as they wrap around the topography, we can begin to map wind directions on a moon over a billion kilometers away.
FIGURE 45. Dune fields on Titan. The area shown is approximately 200 kilometers (124 mi) across. (Image by NASA/JPL)
The mapping of Titan’s surface is extraordinary: it reveals a ledger of familiar forms, written with a completely different chemistry.
WORLD OF THE HOURGLASS SEA
A geologist stands on a rippled bed of loose sand, carefully observing the layered sandstone cliff, documenting thicknesses, forms, and relationships. Moving closer, the observer examines the details of layering and architecture, notes the varying grain sizes with a magnifying lens, scratching the surface of the outcrop to get a better look. Routine field practice for a geologist—but this investigator is a robot, 380 million kilometers (240 million mi) from Earth. The robot is Opportunity, one of the two durable and observant Mars rovers that continue to trundle around, sending their detailed documentation back to the human geologists far away. The outcrop is Burns Cliff, in the wall of Endurance Crater, and the stories it tells are truly amazing. We shall return there shortly.
In 1659, when the Red Planet was in one of its close approaches to Earth, Christiaan Huygens took time off from perfecting the pendulum clock to turn his telescope toward Mars. He observed and sketched a large, dark, V-shaped mark on the planet’s surface, noting its movement over a few days and calculating that the length of a day on Mars is the same as that of the Earth (the period is actually 24.6 hours). The dark feature that he watched, long referred to as the Hourglass Sea because of its shape, is Syrtis Major, a vast volcanic plateau. Huygens and others after him noticed that the appearance of the feature seemed to change with the seasons; with today’s intimate knowledge of the surface of Mars, we now understand why. Figure 46 shows an area of Syrtis Major imaged by the Mars satellite Odyssey; the plateau is streaked with deposits of ever-shifting windblown sand accumulating in the lee of crater rims.
Unlike on Titan, the atmosphere of Mars is thin (pressure at the surface is less than 1 percent of that on Earth) and it’s composed mainly of carbon dioxide, with a little nitrogen and argon and minute traces of other gases. But, also unlike Titan, Mars is exposed to the full effects of changes in solar radiation, its temperatures varying from comfortably above freezing in the summer to minus 140°C (-225°F) in the extremes of winter at the poles. This huge temperature range is more than sufficient to drive very strong winds that, in turn, are more than capable of moving the very fine grains of Martian sand. As today’s research continues to show, if we simply change the terms for gravity, air density, and wind speed in Ralph Bagnold’s equations, we can model the transport of sand anywhere. Indeed, in 1974, at the age of seventy-eight, Bagnold wrote a paper with Carl Sagan on aeolian transport on Mars.
In that paper, Bagnold and Sagan commented on the great Martian sandstorms of 1971, recorded by the Mariner 9 probe. Sandstorms are regular stars of the Mars drama, as are dust devils on a scale unimaginable on Earth. These whirling dervishes of sand can reach heights many times those on Earth and cover huge areas of the surface at their base. Thorough studies of their puny earthly relatives are currently underway so that any attempt to put humans on Mars can take such things into account. We know very well the limitations to our understanding of the strange behaviors of granular materials on Earth and must significantly expand our thinking if we are to deal with them on Mars. For handle them we must. Opportunity was stuck for weeks in a sand ridge that was 30 centimeters (1 ft) high—to the rover, as to Bagnold in the Egyptian desert, the treacherously soft sand in front of it had looked no different from the rest. Human transport on Mars will require ingenuity—or, at least, gigantic wheels.
FIGURE 46. Sand streaks in Syrtis Major, on Mars. The area shown is approximately 60 kilometers (37 mi) across. (Image by NASA/JPL/ASU)
For any mission to send people to Mars, the availability of life-supporting resources will be a key issue. We shall look at what Martian sand is made of in more detail later, but much of it is derived from broken-down volcanic rock that, as on Earth, contains silicate minerals—a possible source of oxygen? But in order to “mine” and utilize sand, as well as to construct facilities, a knowledge of the granular behavior of the sand is critical. In a discussion of the kinds of Martian engineering questions that need answering, NASA asks whether “we understand granular processing well enough to do it on Mars” (“The Sands of Mars,” 2005). The experts’ answer: we don’t yet know. Researchers at the University of Arizona, Cornell University, and the University of Colorado are designing innovative engineering approaches and methods of obtaining key data from the next generation of Mars missions. From June to November 2008, one of these missions explored the northern polar region. Phoenix was a hardy robotic field geologist that delivered extraordinary results. For much longer than was thought feasible, Phoenix dug, collected, sorted, imaged, and analyzed the sand (and dust and ice) of Mars. It discovered water ice in different forms. It found salts and calcium carbonate that support the case for liquid water existing on Mars, possibly relatively recently. It didn’t find little green men or even little green microbes, but it did uncover some of the ingredients necessary for life. And, peering down its microscope, Phoenix sent images of individual sand grains—Henry Clifton Sorby would have been delighted.
FROZEN DESERTS
The atmospheric circulation of Mars tends to move sand toward the poles, and it is in these regions that the sands of the planet become really spectacular. It is hardly surprising that, given the ample supplies of sand and the dynamic atmosphere, there are sand dunes on Mars. It is also not surprising that the rules of the aeolian game are the same as on Earth. Figure 47 shows dunes in the Syrtis Major region; if these features look familiar, compare them with the dunes in Plate 11. Barchan dunes are barchan dunes, whatever planet you find them on. This particular colony of dunes is marching toward the top left of the picture, the direction in which their steep, curving slip faces, their lee sides, are pointing. But when we look at the seas of dunes in the polar regions, something different seems to be happening; the asymmetry of the dunes tends to be less well developed, and evidence of significant migration is not easy to find.
The polar ergs of Mars are huge features—one sand sea is the size of Texas, and one dune is 475 meters (1,500 ft) high, the tallest known dune in the solar system. These sand seas occupy large topographical depressions (often craters) some 5 kilometers (3 mi) below the average surface elevation of the planet and are in places embraced by the towering walls of the polar ice caps. In the winter, as the ice sucks out more and more of the atmosphere, the ice caps expand and cover the dunes.
That the sand is seasonally buried by ice accounts for what would appear to be the violent harbingers of spring. As pressurized layers of ice and sand beneath the surface begin to melt, they break through the overlying ice, and jets of sand and carbon dioxide fire high into the atmosphere like giant alien geysers, showering the surface of the ice with falling sand.
FIGURE 47. Barchan dunes in the equatorial region of Mars. The area shown is approximately 3 kilometers (2 mi) from top to bottom. (Image by NASA/JPL/Malin Space Systems)
The interplay between ice and sand creates extraordinary dune forms that differ from the typical denizens of Earth’s great sand seas. Figure 48 shows some of the most studied dunes in the solar system—those in the Proctor Crater in the southern Martian Highlands. The image—a work of art in its own right—was taken in winter, and the bright areas are possibly ice or snow (carbon dioxide or water?)—or sand. The forms of the dunes are unusual. On a close look, they seem to have two slip faces, and some of the crests appear to have gullies cut in them, in contrast to the usual avalanche shapes. But they clearly are fresh dunes, with areas between that are sculpted by large ripples (left of the image). There is some evidence that the bright material moves backward and forward over the crests with the seasons and wind changes, but the appearance of the avalanches and the occasional gullies are odd. Elsewhere in the polar regions, the dunes have no obvious slip faces at all, appearing as domes of sand with rounded crests, along which gullies have clearly been eroded. But eroded by what? It seems highly likely that many of these dunes are, in a way, lithified, hardened and stabilized by frozen liquid between the grains. Melting during the summer can cause slurry avalanches and the erosion of gullies. These dunes may be akin to those, saturated with water, that buried the Mongolian dinosaurs, as we saw in chapter 7, and are similar to dunes in Antarctica that seasonally freeze and thaw. It’s also interesting to note that, perhaps analogously, in the days when Arabian dunes were slowed down by spraying them with oil, their slip faces disappeared; fresh sand continued to blow over them, but their shapes were domes rather than dunes.
FIGURE 48. Winter dunes, Proctor Crater, in the southern Martian Highlands. (Image by NASA/JPL-Caltech/University of Arizona)
The possibility that significant quantities of water, as well as frozen carbon dioxide, glue together the polar dunes of Mars is of enormous importance, and the fieldwork that Phoenix conducted provided critical evidence of this.
That Mars once had plentiful water, including oceans, is clear—and sand tells the story. Valleys contain meandering channels and point bars, deltas have formed at the mouths of canyons, and highlands show the hallmarks of water erosion. “Before” and “after” images show some signs of possible continuing, sporadic debris slides that could be liquid or dry granular flows. To see more of the testaments of sand to the early environment of the planet, we must return to the work of our robot geologist.
A FEELING WE’RE NOT IN KANSAS ANYMORE—OR ARE WE?
Plate 16 is a picture of an outcrop on Mars, Burns Cliff, assembled from many of Opportunity‘s carefully documented images. There is no question that we can apply a sort of planetary uniformitarianism to interpreting it. The cliff is built up of layers of sedimentary rock totaling around 7 meters (23 ft) in thickness, which can be divided into three general groups. The lowest section shows the massive internal features of sand dunes—cross-bedding—exactly as we see on Earth (left detail in the plate).
The middle section is made up of roughly parallel layers of sand and gravel. While the sands of Mars are typically composed of volcanic rock fragments, these are different. Yes, among the sands there are silicates, but many of the grains are formed of minerals that could only have been deposited in water. Among these are sulfate minerals, which would seem to make up much of the sand in Burns Cliff and elsewhere in this region of Mars. The sand dunes at White Sands National Monument in New Mexico are built of grains of gypsum (calcium sulfate), formed as desert lakes dried out and the wind picked up the crystals (chapter 1). Were there once dunes on Mars that looked like those at White Sands? Embedded in the sand are tiny spheres that have earned the affectionate name “blueberries,” for they make the rock look like a fruit-studded muffin (right detail in the plate); they are made of the iron oxide mineral hematite. You have only to go to Zion National Park and look closely at the spectacular ancient dunes of the Navajo Sandstone there to see essentially the same things. In the Navajo Sandstone, they formed as part of the iron-rich diagenesis of the sandstone; on Mars, they seem to have formed in association with the evaporation of sulfate-rich water. The exotic mineral jarosite has also been identified, a watery sulfate of iron and potassium, found on our planet where waters are acidic.
The uppermost layers at Burns Cliff (the broken and jumbled rocks in the image) are different again. They contain small-scale cross-bedding and ripples, indications of a flowing current, and mud cracks, typical of periods when the sediment dries out.
We have ample evidence for the existence of water in the early days of Mars, but what would the scene have looked like? Kathleen Benison at Central Michigan University has put together a compelling story that relates the character of Burns Cliff to the environment 270 million years ago in Kansas and in modern acidic salt lakes in Australia. The situation in Kansas not long before the great Permian extinction began was unpleasant, to say the least. The rocks there record a desert environment, dunes surrounding ephemeral acidic salt lakes, the lakes drying out before being inundated by the next torrential rains. Such would seem to have been the conditions on Mars a long time ago—it probably hasn’t rained there for a few billion years.
SLINGSHOTS AND ARROWS
All interplanetary probes rely on help in their journeys. The route to their destination is a complex one, as they swing around the solar system, employing the slingshot principle to steal energy from the gravity of planets to speed them on their way. Cassini reached Titan after two swings by Venus, a shot back past Earth, and a final boost from Jupiter. We shall continue our extraterrestrial sand quest in a kind of reverse slingshot: having looped around Mars, we shall now swing around Venus before returning to our home planet.
Venus was the destination of Russian choice in the early days of planetary expeditions, and, with some disasters along the way, it was the place where exploration records were set. In 1967, Venera 4 was the first man-made object to enter another planet’s atmosphere, and in 1970 Venera 7 was the first to make a (relatively) soft landing and send information back home. Then on October 22, 1975, Venera 9 sent us the first astonishing picture of a planet other than our own. It was a picture of rocks and sand.
Venera 9 was stalwart: it survived for nearly an hour on the truly evil surface of the planet. The composition of Venus’s atmosphere is very similar to that of Mars—mainly carbon dioxide—but there’s a lot more of it. The atmosphere is close to a hundred times as dense as ours, and the pressure on the surface would crush an automobile. The temperature is around 460°C (860°F), hot enough to melt lead, and one of the minor components of the atmosphere, sulfur dioxide, creates swirling clouds of sulfuric acid. Images from recent missions whose radar has peered through the turbulent toxic soup of Venus’s atmosphere show mountain peaks covered in what looks like frost—obviously not frost as we know it, but perhaps metal snowflakes.
With an atmosphere this thick, it’s probably difficult to stir up a good wind. But there is dust in the upper reaches, and Carl Sagan, following up on his work with Bagnold on Mars, evaluated the mechanics of Venusian aeolian transport in a paper in 1975. Radar images show wind streaks on the surface like those on Mars and features that can be interpreted as sand dunes—and the Russian images clearly show a sandy environment. Something is going on, but exactly how it all works remains to be seen. Venus is a strange place, and we shall return to it later as we reflect on our own planet.
As we turn homeward, we are reminded that we are not alone, and that the term slingshot is appropriate in another sense. Space is “full” of particles, many of them the size of sand, each on its own individual journey. Full, of course, is a relative term—what we encounter is hardly a cosmic sandstorm. But that there is debris flying around the solar system is no surprise; our own planet is a daily target. Four million extraterrestrial dump trucks shower their loads onto the Earth every year. Occasionally, a rather large chunk will have unfortunate consequences (as we saw in chapter 7), but most of the debris is sand and dust. The extraterrestrial components of sediments were first identified during the expedition of HMS Challenger in the late nineteenth century; tiny spherical grains, black and magnetic, were found on the ocean floor, with a character and composition unlike anything originating on Earth. Fine cosmic sand grains, micrometeorites, are found in polar ice (quite a few of them from Mars), and sand-sized debris from what may have been the largest collision in the history of the solar system is found entombed in 500-million-year-old limestones in Sweden. In the eastern United States, on the night of November 12, 1833, many witnesses thought that the end of the world had arrived. The sky was filled with fireballs and flaming arrows—Native Americans described the event as “the night the stars fell.” As was later understood, the Earth was on its annual encounter with the cosmic debris shed from the comet Tempel-Tuttle. This comet slingshots through the solar system every thirty-three years, renewing its cloud of rubbish, which, when the Earth encounters it in November every year, creates the Leonid meteor shower. The best performances are immediately after the comet itself has visited, and 1833 was one such event: up to five thousand fiery flashes were to be seen every hour during the night. Much of the cloud is made up of cosmic sand grains, hitting Earth’s atmosphere at speeds seventy times that of a bullet.
Comets are cosmic vacuum cleaners, collecting debris from the outer reaches of the solar system, but they are also very inefficient vacuum cleaners, spewing out clouds of material behind them. The chances of our encountering a comet on our journey back home are remote; if we do, the outcome could be catastrophic—but very interesting. Comets and their rubbish tell some extraordinary stories.
DEEP TIME, DEEP IMPACTS
In 1986, a veritable armada of spacecraft approached that most famous of comets, Halley’s. The probe Giotto, on the first deep-space expedition of the European Space Agency (ESA), traveled in the company of a probe from NASA, two Russian craft, and two from Japan. The armada was in for a rough ride, but a productive one. The debris cloud around a comet, its coma, covers a tremendous volume of space; Halley’s coma extends over 20 million kilometers (12 million mi) from the comet itself. It was, nevertheless, something of a surprise when one of the Japanese craft was hit by two particles the size of very coarse sand when it was 150,000 kilometers (90,000 mi) from the comet; fortunately, there was no serious damage. However, Giotto’s subsequent closer approach was compared by ESA’s scientific program director to a game of Russian roulette: “You may survive, but one shot will kill you.” The analogy was accurate: as it got nearer to the comet, Giotto ran into a “wall of dust the size of grains of sand.” The craft was knocked out of alignment, communications were lost, and its camera was destroyed. But it did survive, and, along with the rest of the fleet, it sent back the first direct data from a comet.
Halley’s nucleus is blacker than coal, but it’s “fluffy “—around one-third the density of water. It is made up of water (80 percent), plus dust, sand, and other debris, and some very interesting minor ingredients—carbon monoxide, carbon dioxide, and two other carbon compounds, methane and ammonia. But much of it is empty space.
To get a better look at what a rock is made of, a geologist will crack it open with a trusty hammer and look inside. This was the next important step in revealing the secrets of comets. On July 4, 2005, NASA’s Deep Impact project dropped a battery-powered “impactor” onto an innocent, unremarkable, comet, Tempel 1, selected simply because of its proximity to Earth. The impactor’s weight was equivalent to three heavyweight sumo wrestlers, and when it slammed into the small comet’s nucleus it caused quite a stir—the event looked something like the impact sequence in Figure 13. Two hundred and fifty thousand tons of water and ten thousand dump truck loads of dust were ejected from a crater 100 meters wide and 30 meters deep (330 by 100 ft). The material that exploded from Tempel 1 is the stuff of the dawn of the solar system. Only the very coarsest particles reached the size of fine sand grains, most of them being finer dust; nonetheless, their composition has not only shed light on our system’s origins but demonstrated that they are more complicated than once thought. In addition to water and gases similar to those around Halley, there were silicates, some primitive and unstructured, but some of them crystalline, including olivine, the same green mineral that populates some of Hawaii’s famous beaches. The standard thinking about comets had been that, since they were formed in the outer reaches of the solar system, their material would be primitive, unprocessed by proximity to the cooking of the Sun. But this is clearly not the case—olivine and other silicates can be formed only by the solar stew. The solar system obviously has a much stronger circulation going on than had been thought.
It was well known that comets and asteroids contain organic material (compounds containing carbon, not necessarily produced by life), but Tempel 1 was a gold mine, a wide selection of complex organic molecules. These included methanol (CH3OH), methyl cyanide (CH3CN), acetylene (C2H2), and ethane (C2H6)—the kinds of compounds that can form the building blocks of life. Plentiful water and complex organic molecules were undoubtedly delivered to the early Earth by extraterrestrial bombardment. Tempel 1 also, incredibly, contained carbonate and clay minerals that generally require liquid water to form—where did they come from?
All this extraordinary information emerged from analysis of the effects of the impactor, without our ever laying hands on the stuff of comets. But that changed in January 2006, when a barbecue-sized device thudded into the Utah sand after a journey of more than 3 billion kilometers (1.9 billion mi). The Stardust project had sent a probe to encounter comet Wild 2, which was born beyond Neptune and is a recent visitor to the inner solar system. On the probe was aerogel, the lightest material known, made, through the wonders of nanotechnology, from silica (chapter 9). As the spacecraft flew through the comet’s coma and particles slammed into it, the aerogel acted as a highly efficient shock absorber, capturing the particles and eventually returning them to Earth. This cornucopia of comet material, made up of very fine sand grains and smaller particles, continues to be analyzed by 150 researchers around the world, but some results are already clear. Crystalline olivine appeared again, showing, in the words of Scott Sandford of NASA’s Ames Research Center, that “when the solar system formed, the solar nebula had to have been mixing like a son of a gun”—highly inconvenient for the traditional, simple theories of the origin of our solar system. Stardust’s treasures also included polycyclic aromatic hydrocarbons, something of a molecular mouthful and called PAHs in the trade. PAHs are even more complex hydrocarbons, found in barbecue soot and automobile exhaust and as products of the incomplete combustion of materials such as wood, coal, and fat; their structure is made up of hexagonal rings, like a cross section of a honeycomb. They had been spotted before in interstellar space, but were not known to occur in comets. The PAHs from Stardust seem to be associated with oxygen and nitrogen—in terms of the way life might have started via incoming grains, the plot thickens.
A FLOATING RUBBLE PILE
Comets originate in the remote vastness of the solar system, in the Kuiper Belt beyond Mercury and the ominously named Oort Cloud far beyond Pluto. The Oort Cloud is a place of numerical excess—7.5 trillion kilometers (4.6 trillion mi) from the Sun and home to perhaps 10 trillion comets. Some of these icy, sandy, dusty, fluffy comets swing through the inner solar system and then disappear for sometimes thousands of years. Asteroids, on the other hand, come from closer to home, between Mars and Jupiter, originating from collisions early in the history of the solar system, a failed planet perhaps. The traditional distinction between asteroids and comets was their orbits and their composition, but the latter distinction is becoming less clear. Given their position closer to the Sun, asteroids do not contain the ice of comets, but they are not necessarily solid rock either. In late 2005, the Japanese probe Hayabusa succeeded in actually landing on an asteroid, took pictures and samples, and took off again. In spite of engine problems, it will return to Earth in 2010. The asteroid in question, Itokawa, only a few hundred meters long, is a strange-looking character (Figure 49), resembling a dirt-covered potato in space. As documented by close-up images by the spacecraft, it’s actually made up of dust, sand, gravel, boulders, and nothing—40 percent of it is nothing, in fact. It is simply a floating pile of rubble, held loosely together by its own gravity.
FIGURE 49. Itokawa, the floating rubble pile. (Image courtesy of Japan Aerospace Exploration Agency)
Did a huge colleague of this bizarre object wipe out the dinosaurs?
HOME: THE PALE BLUE DOT
In the quest for extraterrestrial sand, our journey has taken us vast distances. We may have occasionally strayed from the immediate material of interest, but it is always in the vicinity: sand and dust are the stuff the universe is made from. Peering into space, the Hubble telescope has seen “proto-planetary discs” in the Orion Nebula. Earthbound observatories are now confirming clouds of primordial sand, including silicates, swirling around young solar systems, slowly but surely clustering and coagulating into larger and larger pieces until planets are born. These views are being amplified by NASA’s Space Infrared Telescope, “Spitzer,” which, among other tasks, analyzed the results of the Deep Impact impactor’s plunge. Spitzer is looking at already-formed planets and proto-planets light years away from Earth, detecting silicates and PAHs, watching births and deaths.
For the significance of scale from a human point of view, there is no image more iconic than that of our planet taken from the fringes of the solar system by the Voyager 1 spacecraft on Valentine’s Day 1990. Carl Sagan called the Earth in that image “a pale blue dot” this became the title for one of his books and has been used ever since as a touchstone for reflection upon our home and our place in the universe. The “pale blue dot” provides a powerful image of scale, the minuteness of what is everything to us—and also a sense of fragility and vulnerability. We are fixated on fragility on our own scale, understandably, but as we look back over the 4.6-billion-year history of that pale blue dot, as we ponder its future and contemplate how our views are informed by our understanding of the universe, a broader perspective arises. One element of that perspective is how different our planet is—unique in our solar system, and arguably unique on a larger scale. Another element is how convenient for us that uniqueness is—but also how brief, in the grand scheme of things, that period of convenience really is.
And then as we begin to see home in more detail, we pass its sterile moon. We have much to be grateful to our moon for, but from the point of view of our topic, it holds little interest. Yes, there is sand on the Moon, but it has no life and is constantly pulverized over and over again by impacts in the absence of an atmosphere to protect and enliven it.
As we return to the embrace of our pale blue dot, it is its life that gives us the last element of perspective, not just life in the organic sense, but the life in its atmosphere and oceans, the vigor of its coasts and mountains, wet and arid lands, volcanoes and storms, and all the continuing, wonderful activity.
HITCHED TO EVERYTHING ELSE IN THE UNIVERSE
In 1911, in My First Summer in the Sierra, John Muir, the naturalist, geologist, and founder of the Sierra Club, wrote: “When we try to pick out anything by itself, we find it hitched to everything else in the Universe.” I hope that this book illustrates that picking up a few grains of sand connects us not only to a particular beach or riverbank, but to the most recent journey that those grains have made, to the mountains from which they originated, to countless older journeys and cycles, and to the history of our planet. Muir was ahead of his time in many ways, this being one of them. It is only recently, perhaps facilitated by our views of the pale blue dot and other worlds, perhaps by radical cross-disciplinary conversations, that we have come to see the Earth as a system. We cannot, as Muir said, pick out the ocean, the atmosphere, or the continents and understand them independently, in the same way that we cannot look at a camshaft and understand how the internal combustion engine works. Reading the stories that a sand grain has to tell can only be accomplished through understanding the context, the system, in which that grain has played its role.
The uniquely dynamic character of our planet is a result of this system, the internal engine, the atmosphere, oceans, rocks, and living things constantly interacting in an immense, and immensely complicated, game. Scientists of all stripes attempt to understand the rules of this game, we hope with the same sense of wonder that Muir had. Sand has always been a major player in this game, not so much as a maker of the rules, but as the ground troops in it—the ones that live to tell the tale. As we have seen, sand bears witness to erosion and sea level change, landscape evolution and tectonics, the positions of continents, and climate—the vigor and character of the oceans and atmosphere—and the conditions preferred by life.
The complexity of the rules is nowhere better illustrated than in our climate. It is our climate that has enabled our existence and in whose turbulent embrace the future of our planet lies. Once again, in looking for the rules, we encounter the challenge of scale, not only the scale of the processes themselves, but the scale of change.
The words climate and change are inseparable. The Earth’s climate has changed dramatically over the past 4.6 billion years, swinging between icehouse and hothouse conditions, cycling back and forth on slow and relatively rapid time scales. These changes occur on a scale of hundreds of millions and hundreds of thousands of years—when small changes happen on a scale of days, we call them “weather.” On a human scale, we’re in a warm period, but on a larger scale, this is simply an-other interglacial period of an ice age that began several million years ago.
The reason that forecasting (on whatever scale) is a challenge is simply the number of variables, the components that are interacting, and the fact that all are interacting as part of a complex system. The rules of the game are characterized by feedback, change causing change. Some feedback is positive, change augmenting change; some feedback is negative, change counteracting change. And all this is going on in a system that, at any given moment and any given place, is in a state of imbalance, ready to be tilted in one direction or another.
So far, we have seen the roles and testaments of our ground troops in the great game that is our Earth system today and in the past. What of the future? Feedbacks, positive and negative, conspire to regulate a system—if either type becomes dominant, then the condition of the system escalates. In the Earth system, we have two dominant components that are intimately linked regulators—plate tectonics and the composition of the atmosphere, both generals for the ground troops. We cannot help but notice that no other planet at the present time has plate tectonics or an atmosphere like ours—perhaps this can tell us something.
BACK TO THE FUTURE
It seems very possible that Venus, and perhaps Mars, had some form of plate tectonics way back in their history, that both had early atmospheres different from today’s, and that something catastrophic happened to cause the change: crustal activity on both planets all but ceased, and their atmosphere was either largely lost (Mars) or turned into a toxic, escalating greenhouse (Venus). We also can’t help but notice that our own planet seems to be fortuitously placed in what has been called the “Goldilocks zone” for the entire diversity of life over the history of the planet, its setting has been “just right.” Escaping from its early inferno, generously bequeathed water and complex organic molecules by comets and meteorites, relieved of a large portion of its lighter rocky materials by the formation of the Moon, Earth developed its dynamic tectonic and biological systems in a place that had just the right relationship with its star.
The single frame of the epic movie in which we feature today also happens to be “just right” for Homo sapiens—which is one reason our species has evolved the way it has. Today’s environment is atypical—there are immense periods of the Earth’s history that would have been intolerable for humans, and there will be a future that is equally so. At present, we are justifiably obsessed with one particular set of short-term feedbacks, ones that, as we shall see, have had a huge impact in the past, and will even more profoundly influence the Earth’s future in the long term. Those feedbacks are, of course, between the atmosphere and surface temperature and, specifically, concern the role of carbon dioxide as a temperature regulator through the “greenhouse effect.” An extraordinary fact is that the total amount of carbon dioxide in the Venus system is about the same as in the Earth system—the planets had similar origins. But Venus has suffered the nightmare of “runaway greenhouse,” whereas we as yet have not. Why this crucial difference? The answer is that virtually all of the carbon dioxide of Venus has remained in its atmosphere, whereas on Earth most of it is locked away, or “sequestered,” thanks to plate tectonics, the Earth system, and feedbacks. Soil contains twice the carbon that is currently in the atmosphere; the oceans contain fifty times as much. The Earth’s crust holds, locked away over its history, 100,000 times the carbon that is in today’s atmosphere.
In the (geologically) short term, sands will respond to a warming of the atmosphere in predictable ways. The physical games that sand plays will continue unchanged, but the pace of the games and their locations will shift. Rising sea level (as we have seen from the end of the last ice age episode) will move barrier islands and shorelines, rivers will adjust their profiles, and the balances between sand transport and deposition will adjust, both locally and globally. Migrating and vanishing beaches will be even more fickle in terms of supporting the tourism economy. Warming temperatures will change the levels of biological activity in the oceans and thus affect the formation of biogenic sands. Greater precipitation in some parts of the world will increase erosion rates and the sedimentary load of rivers—and the delivery of sand to changing littoral cells, whose sediment budgets will be revised. In other parts of the world, aridity will spread and relict ergs will be reactivated, increasing the transport of windblown sand. This, incidentally, might trigger its own example of feedback: if warming increases hurricane activity in the Atlantic, higher atmospheric levels of windblown sand and dust may, in turn, dampen the potency of those hurricanes—the jury is out (on both counts), because we don’t yet understand the system fully.
But, as the world warms, other key feedbacks will continue in the background, as they always have, albeit on longer time scales than those in which we are currently and nervously interested. One feedback of global significance is weathering. As we saw in the first chapter, most sand is born from the decay and disintegration of rocks exposed to the ravages of the atmosphere. The chemistry of that process is vital and unique to our planet. The dominant mineral constituents of those rotting rocks are silicates, commonly combinations of calcium, sodium, and potassium with silicon and oxygen; formed at high temperatures and pressures within (and below) the crust, they are unstable at the surface. The carbon dioxide in the rain that falls on them and the moisture of the atmosphere that surrounds them are acidic, attacking these vulnerable minerals. When the carbon dioxide teams up with the calcium to form calcium carbonate—limestone—the silica is released to dissolve or re-form into clay minerals. Rivers carry the calcium (or in some cases, potassium or sodium) carbonate down to the oceans as their dissolved load. And on the shallow shelves of the world’s oceans, life voraciously exploits those dissolved minerals to build shells. The carbon dioxide has been effectively removed from the atmosphere—potentially for very long periods of time as the shells fall to the sea floor and accumulate to build great thicknesses of limestone. Continually buried, those limestones will remain there until exhumed or subducted by the forces of plate tectonics.
This process, the carbon-silicate cycle, has been a crucial negative feedback mechanism and regulator. The warmer the surface of the Earth, the faster these chemical reactions of weathering take place and the more carbon dioxide is drawn down from the atmosphere—the greenhouse effect is reduced and the planet cools. Cooler conditions result in less vigorous weathering and less sequestering of carbon dioxide—and things start to warm up again. And it’s not just temperature; the total area of rocks exposed to weathering is also critical. As plate tectonics steered India ponderously but dramatically into its collision with Asia over a long period around fifty million years ago, the towering Himalayas rose from the closing jaws of the vise. This monumental increase in fresh rock exposed to weathering reduced the carbon dioxide levels in the atmosphere to the point where the resulting cooling was the precursor of the ice ages. Current levels of carbon dioxide in our atmosphere are perhaps ten times less than they were when the dinosaurs were wiped out.
Temperature driving carbon dioxide levels driving temperature—a complex but elegant example of the mechanisms driving the Earth system. Except that, unfortunately, it’s not that simple. We have neglected a couple of further factors and feedbacks—one negative, one positive. The first is life, on all scales: even simple bacteria living on desert sand grains have a significant effect on atmospheric chemistry. Vitally, we owe much of the historic stability, such as it is, in the level of atmospheric carbon dioxide to the emergence and activities of plants, which soak up the gas as part of photosynthesis and breathe oxygen into the atmosphere. And not only do plants directly soak up carbon dioxide, but their chemical activity modifies soil and increases rates of weathering, further drawing down atmospheric levels. By about 300 million years ago, Earth had become a fecund place for plants—great global forests developed, and these would form the raw material for the global coal beds after which the Carboniferous chapter of the Earth’s history was named. But those forests caused carbon dioxide levels in the atmosphere to plummet, helping plunge the world into a major ice age. The increased ice coverage reflected more of the Sun’s energy, another positive feedback for cold conditions. Did the dramatic reduction in weathering rates allow carbon dioxide levels to build slowly back up and eventually bring the ice age to a close? Did this greenhouse effect swing the Earth into the subsequent desert conditions of the Permian and Triassic, playing a part in the great Permian extinction? There are too many feedback mechanisms going on, too many possible smoking guns for the forensics to be definitive.
The other factor we have neglected is the temperature of the oceans. As we have seen, calcium carbonate, oddly, is more soluble in cold water than warm: warm waters have less capacity to store carbon dioxide than cold. So, as temperatures increase and weathering and the delivery of calcium carbonate to the oceans increases, warming ocean waters have to give up carbon dioxide, returning it to the atmosphere. This is a positive feedback, reinforcing the effectiveness of the greenhouse. Cycles, feedbacks, causes and effects, checks and balances, budgets—a complex system.
And it is a system that becomes more complicated the closer we look. The ice age at the end of the Carboniferous period was probably helped along by yet another factor—the position of the continents. Or “continent,” for it was one of those times (the sequence of events has often repeated itself) when all the Earth’s land-masses had gone into a huddle, having drifted together into the megacontinent of Pangea. The intimate interplay between land and oceanic and atmospheric circulation was dramatically affected; ocean currents were rerouted and bodies of warm and cold air repositioned. Furthermore, Pangea lay close to the South Pole, a land-mass that created a perfect platform for thicknesses of ice to accumulate. Changes in weathering patterns resulting from a single landmass fed back into changes in the chemistry of the oceans. Plate tectonics, in a minor way through volcanic activity returning gases to the atmosphere and belching out radiation-reflecting dust and smoke, and in a major way through relocation of the continents, has a fundamental influence on climate. And plate tectonics, as long as it keeps operating, will continue to shape the evolution of our planet.
ENDGAMES
As we have seen, sand, the sawdust of plate tectonics and Earth’s “grinding economy,” is diagnostic of the growth and dynamics of the continents. The composition and character of sand have evolved over time as plate tectonic processes first got underway and have since developed. But why do we—uniquely and fortunately—live in a world of plate tectonics at all? The crust of Venus is nothing but a rigid, thick shell, a “stagnant lid” on the hot interior; molten material from the interior still occasionally breaks through to create volcanic eruptions, but it cannot drive plates. One of the fundamental differences between our dynamic planet and stagnant Venus is water. Water, absorbed into the minerals of the Earth’s plates, significantly changes their physical properties. Water makes rocks less rigid; it weakens them and reduces their melting points. All this makes the slabs of the crust and upper mantle easier to bend, easier to subduct, easier to move. Subduction, in turn, carries water-rich minerals into the mantle, enabling the slow but enormously powerful driver of convection. Plate tectonics drives water around the Earth system, and water helps drive plate tectonics. For Venus, it remains a chicken-and-egg problem: did it lose its water because of the absence of plate tectonics, or did plate tectonics cease because of the loss of water?
Another difference between ourselves and Venus is our moon. Regardless of the precise sequence of events that formed the Moon, one effect was to remove a large proportion of the lighter components of Earth’s early crust. This changed the makeup of our planet and, literally, left room for the continents to move about. Put the contents of the Moon back into the Earth and the surface of our planet would be gridlocked with continental crust.
As long as our planet has plate tectonics, its dynamism will continue—but for how long? In the longer term (and now we are talking about a geological time scale), a number of different routes to a couple of different endgames can be foreseen. A future extraterrestrial astrobiologist landing a probe on Earth would reconstruct some of that history from sand—yes, it will still be here.
In the immediate (geological) future, the next few thousand years or so, the ice ages that began a few million years ago will likely reassert themselves. Negative feedbacks to the atmospheric carbon dioxide levels could become dominant, resulting in things cooling off markedly. In the meantime, warmer oceans may increase the rate of atmospheric transport of moisture to the polar regions, increasing precipitation and the buildup of snow and ice. If we are around to witness this, will we, like Ozymandias, look on our works and despair as the ice grinds our cities back into sand?
Over the next thousands, or hundreds of thousands, of years, there are, of course, other factors beyond carbon dioxide levels that will in all likelihood help foster this renewed frigidity. Variations in the Earth’s tilt and its rotation around the Sun have always contributed to cyclic fluctuations of temperature and climate, on a variety of time scales, and will continue to do so. There is also a suggestion that every few hundred thousand years our orbit may carry us through a particularly concentrated cloud of interplanetary debris, reducing the effect of the Sun’s radiation.
In the still longer term, the influence of that radiation will become increasingly important. The Sun is brighter and warmer now than it was earlier in the Earth’s history and, as we see through our telescopes has happened elsewhere, it will eventually flare up and burn out—in perhaps a further 3.5 billion years. But in the meantime, the Earth will have changed; of the various paths this change may take, the following is a likely one. Continuing solar warming will feed carbon dioxide drawdown until it reaches the point where land plants can no longer survive. Plants have already adapted their methods of photosynthesis to adjust to the decrease in carbon dioxide since the heady days of the dinosaurs, but there will be a limit to such adaptation. And that limit will mark a “tipping point” for the planet. As plants die off, the land surface will be exposed to rampant erosion; stable meandering rivers will disappear and be replaced by braided torrents, which will pour huge volumes of sand and mud into the oceans (as happened on a lesser scale as a result of the great Permian extinction). The ocean basins will fill with sand, and the land will be covered by it. The continents, swathed in windblown sand and dust, will wear down, and the ocean water, displaced by the volumes of sediment and swollen by the melted ice caps, will all but cover them. The Earth will become a water world, looking very much as it did a billion years ago.
What happens next will depend on a number of factors, particularly plate tectonics. If the engine of the Earth’s internal radioactive heat diminishes, the point will be reached when there is no longer sufficient energy to drive the plates. Or if the increasing temperatures of the Sun drive most of the water of the oceans irrevocably into the atmosphere, the loss of its “lubricant” will also spell the inevitable end of plate movement. Or a feedback in the plate-tectonics game will itself bring things to a halt: through the activities of subduction and volcanism today, huge volumes of entirely new material are added each year to the continents—could continental growth reach the point where room for maneuver is so limited that the process shuts down?
There is yet another intriguing possibility for the future of plate tectonics. Adrian Lenardic at Rice University in Houston and Australian and Canadian colleagues investigate the metabolism of planets. They have recently reported that increasing surface temperatures would slow down the cooling rate of the interior, which would become hotter and therefore less viscous—to the point where it would lose the strength to move plates (this is what may have happened to Venus). Their work indicates that the effect would start to be noticeable when surface temperatures exceed 60°C (140°F); this is indeed hot, but the highest temperature so far recorded on our planet was in Libya in 1922—58°C (136°F). This is feedback, yet again: climate changes influence plate tectonics and the resulting changes modify the climate.
Regardless of which mechanisms cause it, the cessation of plate tectonics will herald the end of the world. Any hope of a thermostat, of regulating feedbacks, will be gone. Whatever feedbacks remain in the Earth’s system will determine whether the future is simply a “moist greenhouse” or, like Venus, a “runaway greenhouse,” as the Sun’s increasing inferno cooks the surface of the planet.
SURVIVORS
The diversity of today’s life-forms will be long gone, but chances are that life—as we are beginning to know it—will survive into some phases of the final endgame. It will do so in the sheltered worlds between grains of sand.
We revise our understanding of life today on a daily basis. As we expand our awareness of what constitutes the habitats of living things and the requirements, the fundamental nutrients, of life, we begin to question the very definition of life itself. Is it a valid assumption that life has to be based on water and carbon? Can exotic metabolisms formed from, for example, silicon, exist in environments very different from our own?
In chapter 2, we saw the extraordinary diversity of hitherto unknown microscopic organisms that inhabit the sand of an ordinary beach, communities with a diversity as great as, or greater than, that of any other environment on the planet. Among these creatures are the rotifers and the tardigrades. Rotifers thrive everywhere, from the poles to the tropics, and tardigrades are extraordinarily hardy creatures, capable, as we would never have thought possible, of surviving in extreme physical and chemical conditions. Tardigrades can withstand temperatures from -200 to +120°C (-330 to +250°F)—from the unimaginably cold to temperatures higher than the boiling point of water. Put a tardigrade under pressures hundreds of times that at the Earth’s surface or place it in a vacuum, dowse it in carbon dioxide, carbon monoxide, nitrogen, and hydrogen sulfide, and it will survive. Under “normal” conditions, tardigrades are known to live for more than a hundred years, entering a state of suspended animation when necessary. But even tardigrades don’t rank as genuine extremophiles, forms of life that require bizarre conditions. We are constantly discovering new microbes that thrive under extremes of pressure, temperature, acidity, alkalinity, salinity, toxicity, and radiation; creatures that have no need of oxygen and derive their energy from, among other nutrients, sulfur. Extremophiles are found in the super-hot springs of Yellowstone National Park, in the depths of the ocean trenches, in the volcanic vents on the mid-ocean ridges, in between the grains of sandstones in Antarctica, and at great depths in the Earth’s crust. They give us a glimpse of our primitive past—and our primitive future.
Vulcano Island rears out of the Mediterranean to the north of Sicily. It was home to Aeolus, the Greek god of the winds, and today is home, as its name implies, to an active volcano. The geothermal heat provides, as in Japan, a thriving tourist business based around therapeutic mud baths and “hot water beaches.” The temperature in parts of the sand in those beaches is close to the boiling point of water, and the grains are home to an extraordinary little extremophile, the delightfully named Pyrococcus furiosus. It is a simple spherical microbe that forms colonies around the sand grains. It thrives in boiling water (and “freezes” if the temperature drops), requires no oxygen, and exotically incorporates the metal tungsten into its metabolism. Its relatives are found in the deep-sea vents where the plates spreading apart produce superheated toxic environments and weird metabolisms.
As future conditions on our planet deteriorate, the microscopic spaces between sand grains will provide safe havens for life, almost to the end. It is not difficult to imagine the descendants of Pyrococcus furiosus representing the last of life on Earth—or their relatives thriving today deep in the sands of Mars. It is possible that life on Earth began in the spaces between grains of sand, and that this is also where it may end.