Martian orbit, which is extreme by Terran standards, for at perihelion Mars is about forty-three million kilometers closer to the sun than it is at aphelion, and thus receiving about 45 percent more sunlight. This fluctuation makes the southern and northern seasons quite unequal. Perihelion arrives every year at Ls = 250°, late in the southern spring; so southern springs and summers are much hotter than northern springs and summers, with peak temperatures as much as thirty degrees higher. Southern autumns and winters are colder, however, occurring as they do near aphelion—so much colder that the southern polar cap is mostly carbon dioxide, while the northern one is mostly water ice.
So the south was the hemisphere of extremes, the north that of moderation. And the orbital eccentricity caused one other feature of note; planets move faster the closer they are to the sun, so the seasons near perihelion are shorter than those near aphelion; Mars’s northern autumn is 143 days long, for instance, while northern spring is 194. Spring fifty-one days longer than autumn! Some claimed this alone made it worth settling in the north.
In any case in the north they were, and summer had passed. The days got shorter by a little bit every day, and the work went on. The area around the base got more cluttered, more crisscrossed with tracks. They had laid a cement road to Chernobyl, and the base itself was now so big that from the trailer park it extended over the horizon in all directions: the alchemists’ quarter and the Chernobyl road to the east, the permanent habitat to the north, the storage area and the farm to the west, and the biomed center to the south.
Eventually everyone moved into the finished chambers of the permanent habitat. The nightly conferences there were shorter and more routinized than they had been in the trailer park, and days went by when Nadia got no calls for help. There were some people she saw only once in a while; the biomed crew in its labs, Phyllis’s prospecting unit, even Ann. One night Ann flopped on her bed next to Nadia’s, and invited her to go along on an exploration to Hebes Chasma, some 130 kilometers to the southwest. Obviously Ann wanted to show her something outside the base area, but Nadia declined. “I’ve got too much work to do, you know.” And seeing Ann’s disappointment: “Maybe next trip.”
And then it was back to work on the interiors of the chambers, and the exteriors of a new wing. Arkady had suggested making the line of chambers the first of four, arranged in a square, and Nadia was going to do it; as Arkady pointed out, it would then be possible to roof the area enclosed by the square. “That’s where those magnesium beams will come in handy,” Nadia said. “If only we could make stronger glass panes….”
They had finished two sides of the square, twelve chambers entirely done, when Ann and her team returned from Hebes. Everyone spent that evening looking at their videotapes. These showed the expedition’s rovers rolling over rocky plains; then ahead there appeared a break extending all the way across the screen, as if they were approaching the edge of the world. Strange little meter-high cliffs finally stopped the rovers, and the pictures bounced as one explorer got out and walked with helmet camera turned on.
Then abruptly the shot was from the rim, a one-eighty pan shot of a canyon that was so much bigger than the sinkholes of Ganges Catena that it was hard to grasp. The walls of the far side of the chasm were just visible on the distant horizon. In fact they could see walls all the way around, for Hebes was an almost-enclosed chasm, a sunken ellipse about two hundred kilometers long and a hundred across. Ann’s party had come to the north rim in late afternoon, and the eastern curve of the wall was clearly visible, flooded by sunset light; out to the west the wall was just a low dark mark. The floor of the chasm was generally flat, with a central dip. “If you could float a dome over the chasma,” Ann said, “you’d have a nice big enclosure.”
“You’re talking miracle domes, Ann,” Sax said. “That’s about ten thousand square kilometers.”
“Well, it would make a good big enclosure. And then you could leave the rest of the planet alone.”
“The weight of a dome would collapse the canyon walls.”
“That’s why I said you’d have to float