Nothing about the mass approaching her under the dustcloud indicated one theory over another. Certainly it wasn’t glowing like molten lava, and though it was loud, there was no way of judging whether it was loud enough to be riding on its own sonic boom. On it came in any case, no matter what the mechanism. It looked as though she was going to get a chance to investigate in person, her last act a contribution to geology, lost in the moment of discovery.

She checked her wrist, and was surprised to see that twenty minutes had passed already. Long runouts were known to be fast; the Blackhawk slide in the Mojave was estimated to have traveled at 120 kilometers per hour, going down a slope of only a couple of degrees. Melas was in general a bit steeper than that. And indeed the front edge of the slide was closing fast. The noise was getting louder, like rolling thunder directly overhead. The dustcloud reared up, blocking out the afternoon sun.

Ann turned and looked out at the great Marineris glacier. She had almost been killed by it more than once, when it was an aquifer outbreak flooding down the great canyons. And Frank Chalmers had been killed by it, and was entombed somewhere in its ice, far downstream. His death had been caused by her mistake, and the remorse had never left her. It had been a moment of inattention only, but a mistake nevertheless; and some mistakes you never can make good.

And then Simon had died too, engulfed in an avalanche of his own white blood cells. Now it was her turn. The relief was so acute it was painful.

She faced the avalanche. The rock visible at the bottom was bouncing, it seemed, but not rolling over itself like a broken wave. Apparently it was indeed riding over some kind of lubricating layer. Geologists had found nearly intact meadows on top of landslides that had moved many kilometers, so this was confirmation of something known, but it certainly did look peculiar, even unreal a low rampart advancing across the land without a rollover, like a magic trick. The ground under her feet was vibrating, and she found that her hands were clenched into fists. She thought of Simon, fighting death in his last hours, and hissed; it seemed wrong to stand there welcoming the end so happily, she knew he would not approve. As a gesture to his spirit she stepped off the low lava dike and went down onto one knee behind it. The coarse grain of its basalt was dull in the brown light. She felt the vibrations, looked up at the sky. She had done what she could, no one could fault her. Anyway it was foolish to. think that way; no one would ever know what she did here, not even Simon. He was gone. And the Simon inside her would never stop harassing her, no matter what she did. So it was time to rest, and be thankful. The dustdoud rolled over the low dike, there was a wind — Boom! She was thrown flat by the impact of the noise, picked up and dragged over the canyon floor, thrown and pummeled by rock. She was in a dark cloud, on her hands and knees, dust all around her, the roar of gnashing rock filling everything, the ground tossing underfoot like a wild thing….

The jostling subsided. She was still on her hands and knees, feeling the cold rock through her gloves and kneepads. Gusts of wind slowly cleared the air. She was covered with dust, and small fragments of stone.

Shakily she stood. Her palms and knees hurt, and one kneecap was numb with cold. Her left wrist felt the stab of a sprain. She walked up to the low dike, looked over it. The landslide had stopped about thirty meters short of the dike. The ground in between was littered with rubble, but the edge of the slide proper was a black wall of pulverized basalt, sloping back at about a forty-five-degree angle, and twenty or twenty-five meters tall. If she had stayed standing on the low dike, the impact of the air would have thrown her down and killed her. “Goddamn you,” she said to Simon.

The northern border of the slide had run out onto the Melas glacier, melting the ice and mixing with it in a steaming trough of boulders and mud. The dustdoud made it hard to see much of that. Ann crossed the dike, walked up to the foot of the slide. The rocks at the bottom of it were still hot. They seemed no more fractured than the rock higher in the slide. Ann stared at the new black wall, her ears ringing. Not fair, she thought. Not fair.

She walked back to the Geneva Spur, feeling sick and dazed. The boulder car was still on the dead-end road, dusty but apparently unharmed. For the longest time she could not bear to touch it. She stared back over the long smoking mass of the slide — a black glacier, next to a white one. Finally she opened the lock door and ducked inside. There was no other choice.

Ann drove a little every day, then got out and walked over the planet, doing her work doggedly, like an automaton.

To each side of the Tharsis bulge there was a depression. On the west side was Amazonis Planitia, a low plain reaching deep into the southern highlands. On the east was the Chryse Trough, a depression that ran from the Argyre Basin through the Margaritifer Sinus and Chryse Planitia, the deepest point in the trough. The trough was an average of two kilometers lower than its surroundings, and all the chaotic terrain on Mars, and most of the ancient outbreak channels, were located in it.

Ann drove east along the southern rim of Marineris, until she was between Nirgal Vallis and the Aureum Chaos. She stopped to resupply at the refuge called Dolmen Tor, which was where Michel and Kasei had taken them at the end of their retreat down Marineris, in 2061. Seeing the little refuge again did not affect her; she scarcely remembered it. All her memories were going away, which she found comforting. She worked at it, in fact, concentrating on the moment with such intensity that even the moment itself went away, each instant a burst of light in a fog, like things breaking in her head.

Certainly the trough predated the chaos and the outbreak channels, which were no doubt located there because of the trough. The Tharsis bulge had been a tremendous source of outgassing from the hot center of the planet, all the radial and concentric fractures around it leaking volatiles out of the hot center of the planet. Water in the regolith had run downhill, into the depressions on each side of the bulge. It could be that the depressions were the direct result of the bulge, simply a matter of the lithosphere bent down on the outskirts of where it had been pushed up. Or it could be that the mantle had sunk underneath the depressions, as it had plumed under the bulge. Standard convection models would support such an idea — the upwelling of the plume had to go back down somewhere, after all, rolling at its sides and pulling the lithosphere down after it.

And then, up in the regolith, water had run downhill in its usual way, pooling in the troughs, until the aquifers burst open, and the surface over them collapsed: thus tfie outbreak channels, and the chaos. It was a good working model, plausible and powerful, explaining a lot of features.

So every day Ann drove and then walked, seeking confirmation of the mantle convection explanation for the Chryse trough, wandering over the surface of the planet, checking old seismographs and picking away at rocks. It was hard now to make one’s way north in the trough; the aquifer outbreaks of 2061 nearly blocked the way, leaving only a narrow slot between the eastern end of the great Marineris glacier and the western side of a smaller glacier that filled the whole length of Ares Vallis. This slot was the first chance east of Noctis Labyrinthus to cross the equator without going over ice, and Noctis was six thousand kilometers away. So a piste and a road had been built in the slot, and a fairly large tent town established on the rim of Galilaei Crater. South of Galilaei the narrowest part of the’ slot was only forty kilometers wide, a zone of navigable plain located between the eastern arm of the Hydaspis Chaos and the western part of Aram Chaos. It was hard to drive through this zone and keep the piste and road under the horizons, and Ann drove right on the edge of Aram Chaos, looking down onto the shattered terrain.