“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 along the northern rim of Hellas for all the first day, with the ice in view on the basin floor almost continually. They passed a little hillside town called Sebastopol, its stone walls Florentine yellow in the afternoon, and after that came to Hell’s Gate, the town at the bottom end of Dao Vallis. They walked out of the Hell’s Gate train station late in the afternoon, and looked down into a big new tent town, located under an enormous suspension bridge. The bridge supported the train piste, spanning Dao Vallis just up from the canyon’s mouth, so that its towers were over ten kilometers apart. From the canyon rim by the bridge, where the train station was, they could see down the widening mouth of the canyon onto the basin floor, stretching out under a lattice of kinky sun-stained clouds. In the other direction there was a view well up into the steep narrow world of the canyon proper. As they walked down a staired and switchbacked street into the town, the new tenting over the canyon was visible only as a certain red haze to the color of the evening sky, the result of a dusting of fines on the tenting materials. “We’ll go upstream tomorrow by way of the rim road,” Diana said, “and get an overview. Then come back down on the canyon floor, so you can see what it’s like down there.”
They descended the street, which had 700 numbered steps. In Hell’s Gate’s downtown they walked around and had dinner, and then climbed back up to the Deep Waters office, which was on the valley wall just under the bridge. They stayed in rooms there, and next morning went to a garage by the train station and borrowed a small company rover.