PART THREE The Crucible

       It formed with the rest of the solar system, around five billion years ago. That’s fifteen million human generations. Rocks banging together in space and then coming back and holding together, all because of the mysterious force we call gravity. That same mysterious warp in the weft of things caused the pile of rocks, when it was big enough, to crush in on its center until the heat of the pressure melted the rock. Mars is small but heavy, with a nickel-iron core. It is small enough that the interior has cooled faster than Earth’s: the core no longer spins inside the crust at a different speed and so Mars has practically no magnetic field. No dynamo left. But one of the last internal flows of the molten core and mantle was in the form of a huge anomalous lumping outward on one side, a shove against the crust wall that formed a continent-sized bulge, eleven kilometers high: three times as high as the Tibetan plateau is above its surroundings. This bulge caused many other features to appear: a system of radial fractures covering an entire hemisphere, including the largest cracks of all, the Valles Marineris, a lace of canyons that would cover the United States coast to coast. The bulge also caused a number of volcanoes, including three straddling its spine, Ascraeus Mons, Pavonis Mons and Arsia Mons; and off on its northwest edge, Olympus Mons, the tallest mountain in the solar system, three times the height of Everest, one hundred times the mass of Mauna Loa, the Earth’s largest volcano.

      So the Tharsis Bulge was the most important factor in shaping the surface of Mars. The other major factor was meteor fall. In the Noachian Age, three to four billion years ago, millions of meteors were falling on Mars at a tremendous rate, and thousands of them were planetesimals, rocks as big as Vega or Phobos. One of the impacts left behind Hellas Basin, 2000 kilometers in diameter, the largest obvious crater in the solar system; although Daedalia Planum appears to be the remains of an impact basin 4500 kilometers across. Those are big; but then there are areologists who believe that the entire northern hemisphere of Mars is an ancient impact basin.

       These big impacts created explosions so cataclysmic that it is hard to imagine them; ejecta from them ended up on the Earth and the Moon, and as asteroids in Trojan orbits; some areologists think that the Tharsis Bulge started because of the Hellas impact; others believe that Phobos and Deimos are ejecta. And these were only the largest impacts. Smaller stones fell every day, so that the oldest surfaces on Mars are saturated with cratering, the landscape a palimpsest of newer rings obscuring older ones, with no patch of land untouched. And each of these impacts released explosions of heat that melted rock: elements were broken out of their matrix and fired away in the form of hot gases, liquids, new minerals. This and the outgassing from the core produced an atmosphere, and lots of water; there were clouds, storms, rain and snow, glaciers, streams, rivers, lakes, all scouring the land, all leaving unmistakable marks of their passage – flood channels, stream beds, shorelines, every kind of hydrologic hieroglyphic.

       But all that went away. The planet was too small, too far from the sun. The atmosphere froze and fell to the ground. Carbon dioxide sublimed to form a thin new atmosphere, while oxygen bonded to rock and turned it red. The water froze, and over the ages seeped down through the kilometers of meteor-broken rock. Eventually this layer of regolith became permeated with ice, and the deepest parts were hot enough to melt the ice; so there were underground seas on Mars. And water always flows downhill; so these aquifers migrated down, slowly seeping, until they pooled behind some stoppage or another, a rib of high bedrock or a frozen soil barrier. Sometimes intense artesian pressures built against these dams; and sometimes a meteor would hit, or a volcano appear, and the dam would burst apart and a whole underground sea would spew over the landscape in enormous floods, floods ten thousand times the flow of the Mississippi. Eventually, however, the water on the surface would freeze and sublime away in the ceaseless dry winds, and fall on the poles in every winter’s fog hood. The polar caps therefore thickened, and their weight drove the ice underground until the visible ice was only the tip of two world-topping lenses of underground permafrost, lenses ten and then a hundred times the visible caps’ volume. While back down toward the equator, new aquifers were being filled from below, by outgassing from the core; and some of the old aquifers were refilling.

      And so this slowest of cycles approached its second round. But as the planet was cooling, all of it happened more and more slowly, in a long ritard like a clock winding down. The planet settled into the shape we see. But change never stops; the ceaseless winds carved the land, with dust that grew finer and finer; and the eccentricities of Mars’s orbit meant that the southern and northern hemispheres traded the cold and warm winters in a 51,000 year cycle, so that the dry ice cap and the water ice cap reversed poles. Each swing of this pendulum laid down a new stratum of sand, and the troughs of new dunes cut through older layers at an angle, until the sand around the poles lay in a stippled cross-hatching, in geometrical patterns like Navajo sand paintings, banding the whole top of the world.

       The colored sands in their patterns, the fluted and scalloped canyon walls, the volcanoes rising right through the sky, the rubbled rock of the chaotic terrain, the infinity of craters, ringed emblems of the planet’s beginning … Beautiful, or harsher than that: spare, austere, stripped down, silent, stoic, rocky, changeless. Sublime. The visible language of nature’s mineral existence.

       Mineral; not animal, nor vegetable, nor viral. It could have happened but it didn’t. There was never any spontaneous generation out of the clays or the sulphuric hot springs; no spore falling out of space, no touch of a god; whatever starts life (for we do not know), it did not happen on Mars. Mars rolled, proof of the otherness of the world, of its stony vitality.

       And then, one day …

      She hit the ground with both feet solid, nothing tricky about it, the g familiar from nine months in the Ares; and with the suit’s weight, not that much different from walking on Earth, as far as she could remember. The sky was a pink shaded with sandy tans, a color richer and more subtle than any in the photos. “Look at the sky,” Ann was saying, “look at the sky.” Maya was chattering away, Sax and Vlad spun like rotating statues. Nadezhda Francine Cherneshevsky took a few more steps, felt her boots crunch the surface. It was salt-hardened sand a couple of centimeters thick, which cracked when you walked on it; the geologists called it duricrust or caliche. Her boot tracks were surrounded by small systems of radial fractures.

      She was out away from the lander. The ground was a dark rusty orange, covered with an even litter of rocks the same color, although some of the rocks showed tints of red or black or yellow. To the east stood a number of rocket landing vehicles, each one a different shape and size, with the tops of more sticking over the eastern horizon. All of them were crusted the same red-orange as the ground: it was an odd, thrilling sight, as if they had stumbled upon a long-abandoned alien spaceport. Parts of Baikonur would look like this, in a million years.

      She walked to one of the nearest landing vehicles, a freight container the size of a small house, set on a skeletal four-legged