She got out of her car. It was almost sunset, the sky a great arch of violent colors, the sun low in the hazy west. The light would be behind her, making her very hard to see. She wound between dunes, then carefully made her way to the crest of one, and crawled the final meters of the way, and looked over the crest at the tower, now only a kilometer to the east. When she saw how close its base was she kept her chin right on the ground, among ejecta the size of her helmet.
It was some kind of mobile drilling operation, a big one. The massive base was flanked by giant caterpillar tracks, like those used to move the largest rockets around a spaceport. The drill tower rose out of this behemoth more than sixty meters, and the base and lower part of the tower clearly contained the technicians’ housing and equipment and supplies.
Beyond this thing, a short distance down a gentle slope to the north, was a sea of ice. Immediately north of the drill, the crests of the great barchan dunes still stuck out of the ice— first as a bumpy beach, then as hundreds of crescent islands. But a couple of kilometers out the dune crests disappeared, and it was ice only.
The ice was pure, clean— translucent purple under the sunset sky— clearer than any ice she had ever seen on the Martian surface, and smooth, not broken like all the glaciers. It was steaming faintly, the frost steam whipping east on the wind. And out on it, looking like ants, people in walkers and helmets were ice-skating.
• • •
It came clear the moment she saw the ice. Long ago she herself had confirmed the big impact hypothesis, which accounted for the dichotomy between the hemispheres: the low smooth northern hemisphere was simply a superhuge impact basin, the result of a scarcely imaginable collision in the Noachian, between Mars and a planetesimal nearly as big as it. The rock of the impact body that had not vaporized had become part of Mars itself, and there were arguments in the literature that the irregular movements in the mantle that had caused the Tharsis bulge were late developments resulting from perturbations originating with the impact. To Ann that wasn’t likely, but what was clear was that the great crash had happened, wiping out the surface of the entire northern hemisphere, and lowering it by an average of four kilometers relative to the south. An astonishing hit, but that was the Noachian. An impact of similar magnitude had in all probability caused the birth of Luna out of Earth. In fact there were some anti-impact holdouts arguing that if Mars had been hit as hard, it should have had a moon as big.
But now, as she lay flat looking at the giant drilling rig, the point was that the northern hemisphere was even lower than it first appeared, for its floor of bedrock was amazingly deep, as much as five kilometers beneath the surface of the dunes. The impact had blown that deep, and then the depression had mostly refilled, with a mixture of ejecta from the big impact, windblown sand and fines, later impact material, erosional material sliding down the slope of the Great Escarpment, and water. Yes, water, finding the lowest point as it always did; the water in the annual frost hood, and the ancient aquifer outbreaks, and the outgassing from the blistered bedrock, and the lensing from the polar cap, had all eventually migrated to this deep zone, and combined to form a truly enormous underground reservoir, an ice and liquid pool that extended in a band all the way around the planet, underlying almost everything north of 60° north latitude, except, ironically, for a bedrock island on which the polar cap itself stood.
Ann herself had discovered this underground sea many years before, and by her estimates between sixty and seventy percent of all the water on Mars was down there. It was, in fact, the Oceanus Borealis that some terraformers talked about— but buried, deeply buried, and mostly frozen, and mixed with regolith and dense fines; a permafrost ocean, with some liquid down on the deepest bedrock. All locked down there for good, or so she had thought, because no matter how much heat the terraformers applied to the planet’s surface, the permafrost ocean would not thaw much faster than a meter per millennium— and even when it did melt it would remain underground, simply as a matter of gravity.
Thus the drilling rig before her. They were mining the water. Mining the liquid aquifers directly, and also melting the permafrost with explosives, probably nuclear explosives, and then collecting the melt and pumping it onto the surface. The weight of the overlying regolith would help push the water up through pipes. The weight of water on the surface would help push up more. If there were very many drilling rigs like this one, they could put a tremendous amount on the surface. Eventually they would have a shallow sea. It would refreeze and become an ice sea again for a while, but between atmospheric warming, sunlight, bacterial action, increasing winds— it would melt again, eventually. And then there would be an Oceanus Borealis. And the old Vastitas Borealis, with its world-wrapping black garnet dunes, would be sea bottom. Drowned.