Green Mars - Kim Stanley Robinson [241]
“No.” Almost no water, she told Maya, was located under the basin floor itself. The floor had been desiccated by the original impact, and now it consisted of about a kilometer’s depth of eolian sediment, underlain by a hard cake of brecciated rock, formed during the brief but stupendous pressures of the impact. These same pressures had also caused deep fracturing all around the rim of the basin, and it was this fracturing that had allowed unusually large amounts of outgassing from the interior of the planet. Volatiles from below had seeped up and cooled, and the water portion of the volatiles had pooled in liquid aquifers, and in many zones of highly saturated permafrost.
“Quite an impact,” Maya observed.
“It was big all right.” As a general rule, Diana said, impactors were about one-tenth the size of the crater or basin they made (like historical figures, Maya thought); so the impacting planetesimal in this case had been a body about two hundred kilometers in diameter, coming down on ancient cratered highland terrain. Signature traces of it indicated it had probably been an ordinary asteroid, carbonaceous chondrite for the most part, with lots of water and some nickel-iron in it. It had had a speed on arrival of about 72,000 kilometers per hour, and had hit at a slightly eastward angle, which explained the huge devastated region east of Hellas, as well as the high, relatively well-organized concentric ridges of the Hellespontus Montes to the west.
Then Diana described another rule of thumb which caused Maya to free-associate analogies to human history: the bigger an impactor, the less of it survived the impact. Thus almost every bit of this one had vaporized in the cataclysmic strike— though there was a small gravitational bolide under Gledhill Crater, which some areologists claimed was almost certainly the buried remainder of the planetesimal, perhaps one ten-thousandth of the original or less, which they claimed would supply all the iron and nickel that they would ever need if they cared to go digging for it.
“Is that feasible?” Maya asked.
“Not really. Cheaper just to mine the asteroids.”
Which they were doing, Maya thought darkly. That was what a prison sentence meant now, under the latest UNTA regime— years in the asteroid belt, operating the very strictly circumscribed mining ships and robots. Efficient, the Transitional Authority said. Prisons that were both remote and profitable.
But Diana was still thinking about the basin’s awesome birth. The impact had occurred about three and a half billion years before the present, when the planet’s lithosphere had been thinner, and its interior hotter. Energies released by the impact were hard to imagine: the total energy created by humanity through all history was as nothing to it. And so the resulting volcanic activity had been considerable. Surrounding Hellas were a number of ancient volcanoes, which just postdated the impact, including Australis Tholus to the southwest, Amphitrites Patera to the south, and Hadriaca Patera and Tyrrhena Patera to the northeast. All of these volcanic regions had been found to have liquid water aquifers near them.
Two of these aquifers had burst onto the surface in ancient times, leaving on the eastern slope of the basin two characteristic sinuous water-carved valleys: Dao Vallis, originating on the corrugated slopes of Hadriaca Patera; and farther south, a linked pair of valleys known as the Harmakhis-Reull system, which extended for a full thousand kilometers. The aquifers at the heads of these valleys had refilled over the eons since their outbreaks, and now big construction crews had tented Dao and were working on Harmakhis-Reull, and were letting the water from the aquifers run down the long enclosed canyons, to outlets on the basin floor. Maya was extremely interested in these big new additions to the habitable surface, and Diana, who knew them well, was going to take her to visit some friends in Dao.
Their train glided