Red Mars - Kim Stanley Robinson [46]
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. 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 cycle of 51,000 years, 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