over tens of thousands of years had cut the whole length of the chasm.
Rather than driving up to the head of the valley, the explorers crossed to the western wall, aiming toward a transponder that had been included in a drop of ice-mining equipment. The sand dunes mid-chasm were low and regular, and the rovers rolled over the corrugated land, up and down, up and down. Then as they crested a sand wave they spotted the drop, no more than two kilometers from the foot of the northwest ice wall: bulky lime-green containers on skeletal landing modules, a strange sight in this world of whites and tans and pinks. “What an eyesore!” Ann exclaimed, but Phyllis and George were cheering.
During the long afternoon, the shadowed western ice side took on a variety of pale colors: the purest water ice was clear and bluish, but most of the hillside was a translucent ivory, copiously tinted by pink and yellow dust. Irregular patches of CO2 ice were a bright pure white; the contrast between dry ice and water ice was vivid, and made it impossible to read the actual contours of the hillside. And foreshortening made it hard to tell how tall the hill really was; it seemed to go up forever, and was probably somewhere between three and five hundred meters above the floor of Borealis.
“This is a lot of water,” Nadia exclaimed.
“And there’s more underground,” Phyllis said. “Our borings show that the cap actually extends many degrees of latitude farther south than we see, buried under the layered terrain.”
“So we have more water than we’ll ever need!”
Ann pursed her mouth unhappily.
• • •
The drop of the mining equipment had determined the site of the ice mining camp: the west wall of Chasma Borealis, at longitude 41°, latitude 83° N. Deimos had just recently followed Phobos under the horizon; they wouldn’t see it again until they returned south of 82° N. The summer nights consisted of an hour’s purple twilight; the rest of the time the sun wheeled around, never more than twenty degrees above the horizon. The six of them spent long hours outside, moving the ice miner to the wall and then setting it up. The main component was a robotic tunnel borer, about the size of one of their rovers. The borer cut into the ice, and passed back cylindrical drums one-and-a-half meters in diameter. When they turned the borer on it made a loud, low buzz, which was louder still if they put their helmets to the ice, or even touched it with their hands. After a while white ice drums thumped into a hopper, and then a small robot forklift carried them to a distillery, which would melt the ice and separate out its considerable load of dust, then refreeze the water into one-meter cubes more suitable for packing in the holds of the rovers. Robot freight rovers would then be perfectly capable of driving to the site, loading up and returning to base on their own, and base would then have a regular water supply, larger than they could ever use. Around four or five million cubic kilometers in the visible polar cap, Edvard calculated, though there were a lot of guesses in the calculation.
They spent several days testing the miner, and deploying an array of solar panels to power it. In the long evenings after dinner Ann would climb the ice wall, ostensibly to take more borings, although Nadia knew she just wanted away from Phyllis and Edvard and George. And naturally she wanted to climb all the way to the top, to get on the polar cap and look around, and take borings of the most recent layers of ice. So one day when the miner had passed all the test routines, she and Nadia and Simon got up at dawn— just after two a.m.— and went out into the supercold morning air and climbed, their shadows like big spiders climbing before them. The slope of the ice was about thirty degrees, steepening and then letting off time after time as they ascended the rough benches in the hill’s layered side.
It was seven a.m. when the slope lay back and they walked onto the surface of the polar cap. To the north was a plain of ice that extended as far as they could see, to a high horizon some thirty kilometers away. Looking back to the south they could see a great distance over the geometric swirls of the layered terrain; it