So he and Kasei began driving round the clock, taking shifts of three hours at the wheel. Another day and they were down the Compton Break, and into tight-walled Ius Chasm, and Michel relaxed.

Ius was the narrowest of all the canyons in the Marineris system, only twenty-five kilometers wide when it left the Compton Break, dividing Sinai Planum from Tithania Catena. The canyon was a deep slash between these two plateaus, its side cliffs a full three kilometers high; a long, narrow, giant of a rift. But they only saw the walls in glimpses, through bubbles of open air in the blowing dust. They continued to follow a level but rockstrewn route, making good progress through all of a long dim day. It was quiet in the car, the radio turned down to decrease the irritation of the static. The cameras’ views, higher than the windows, were of dust whipping past them so that it seemed they hardly moved. Often it looked as if they were slewing sideways. It was hard driving, and Simon and Sax spelled Michel and Kasei, following their directions. Ann was still not talking, and they did not ask her to drive. Sax drove with one eye on his AI screen, which was giving him atmospheric readouts. She could tell from across the car that the AI was indicating that the impact of Phobos was thickening the atmosphere a great deal, projected to as much a fifty millibar addition, an extraordinary amount. And the newly smashed craters were still outgassing. Sax noted this change with his owlish satisfaction, oblivious to the death and destruction that came with it. He noticed her glare and said, “Like the Noachian Age, I suppose.” He nearly added more, but Simon silenced him with a look, and changed the subject.

In the next car Maya and Frank passed the hours by calling over and asking Michel questions about the hidden colony, or discussing with Sax the the physical changes occurring, or speculating about the war. Hashing it all over endlessly, trying to make sense of it, to figure out what had happened. Talking talking talking. On Judgement Day, Ann thought, as all the quick and the dead staggered around together, Maya and Frank would still be talking, trying to figure out what had happened. Where they had gone wrong.

Their third night out, the two cars ran down the lower end of Ius, and came to a long lemniscate fin dividing the canyon. They followed the official trans-Marineris Highway down the south fork. In the last hour before dawn, they caught sight of some clouds overhead, and the dawn was much lighter than those of the previous days. It was enough to send them to cover, and they stopped in a fall of boulders stacked against the foot of the canyon’s south wall, and gathered in the lead car to wait out the day.

Here they had a view out over the broad expanse of Melas Chasma, the biggest canyon of them all. Ius’s rock was rough and blackish in comparison to the smooth red floor of Melas; it seemed to Ann possible that the two canyons were made of rock from ancient tectonic plates, once moving past each other, now juxtaposed forever.

They sat through a long day, talked out, tense, exhausted, their hair oily and uncombed, their faces grimy with the ubiquitous red fines of a dust storm. Sometimes there were clouds, sometimes haze, sometimes sudden pockets of clarity.

In mid-afternoon, without any warning at all, the rover rocked on its shock absorbers. Startled to attention, they jerked up to look at the TVs. The rover’s rear camera was pointed back up Ius, and suddenly Sax tapped the screen displaying its view. “Frost,” he said. “I wonder…”

The camera showed the frost steam thickening, moving downcanyon toward them. The highway was up on a bench above the main floor of Ius’s south fork; and this was lucky, because with a roar that shook the rover, that main floor disappeared, overwhelmed by a low wall of black water and dirty white mush. It was a juggernaut of ice chunks, tumbling rocks, foam, mud and water, a slurry throwing itself down the middle of the canyon. The roar was like thunder, even inside the car; it was too loud to talk, and the car trembled under them.

Below their bench, the canyon floor proper was perhaps fifteen kilometers across. The flood filled this whole expanse in a matter of minutes, and promptly began to rise against a long talus slope that ran out from the cliff downcanyon from them. The surface of the flood settled as it pooled against this dam, and froze solid as they watched: a lumpy discolored chaos of ice, strangely stilled. Now they could hear themselves shout over the cracks and booms and omnipresent roaring, but there was nothing to say; they only stared out the low windows or at the TVs, stunned. The frost steam coming off the flood’s surface lessened to a light fog. But no more than fifteen minutes later the ice lake burst at its lower end, rupturing in a surge of black steaming water that tore the talus dam away, with an explosive roar of avalanching rock. The flood poured downcayon again, its leading edge beyond their view, down the great slope from Ius into Melas Chasma.

Now there was a river running down Valles Marineris, a broad, steaming, ice-choked deluge. Ann had seen videotape of the outbreaks in the north, but she hadn’t been able to get to one, to see it in person. Here in the flesh, she found it almost impossible to grasp. The landscape itself was now speaking a kind of glossolalia. The inchoate roar smashed at the air, and quivered their stomachs like some bass tearing of the world’s fabric; and it was visual chaos as well, a meaningless jumble that she couldn’t seem to focus on, to distinguish near from far, or vertical from horizontal, or moving from still, or light from dark. She was losing the ability to read meaning from her senses. Only with great difficulty could she understand her companions in the car. She wasn’t sure if it was her hearing or not. She couldn’t stand to look at Sax, but then Sax she at least understood. He was trying to hide it from her, but it was clear he was excited by what was happening. That calm dead exterior had always been a mask over a passionate nature, and she had always known it. Now he was high-colored as if he had a fever, and he never met her eye; he knew that she knew what he felt. She despised his shirking inability to confront her, even if it did arise from some kind of consideration for her. And the way he stayed always busy at his screen. She had never seen him get down and actually look out the low floor windows of the rover, to see the flood with his own eyes. The cameras have a better view, he would say mildly when Michel urged him to have a look. And after only a half hour of watching the first arrival of the flood on the TVs, he had gone to his AI screen to work out what it might mean to his project. Water rushing down Ius, freezing, breaking up and rushing down again; certainly into Melas; whether there would be enough water to make it into Coprates, and then down into Capri and Eos, and then down into the Aureum chaos… it seemed unlikely on the face of it, but the Compton Aquifer had been big, one of the biggest ever found. Marineris very likely owed its existence to outbreaks from earlier incarnations of the same aquifer, and the Tharsis bulge had never stopped outgassing… She found she was lying on the floor of the rover, watching the flood, trying to comprehend it. She tried to calculate its flow in her head, just as a way to focus better on what she saw, to bring it back out of the meaninglessness that threatened to overwhelm her. Despite herself she felt the fascination of the calculation, and of the view, and even of the flood itself; this had happened on Mars before, billions of years ago, and probably just like this. There were signs of catastrophic floods all over, beach terraces, lemniscate islands, channel beds, scablands… And the old broken aquifers had refilled, from the Tharsis upwelling and all the heat and outgassing that that engendered. It would have been slow, but give it two billion years…