The cliff face disappeared in the rising cloud of dust, which billowed up and out as if a giant bomb had gone off. A distinct boom was followed by a faint roaring, like distant thunder. She checked her wrist; a little under four minutes. Speed of sound on Mars was 252 meters per second, so the distance of sixty kilometers was confirmed. She had seen almost the very first moment of the fall.
Deep in the embayment a smaller piece of cliff gave way as well, no doubt triggered by shock waves. But it looked like the merest rockfall compared to the collapsed headland, which had to be millions of cubic meters of rock. Fantastic to actually see one of the big landslides— most areologists and geologists had to rely on explosions, or computer simulations. A few weeks spent in Valles Marineris would solve that problem for them.
And here it came, rolling over the ground by the edge of the glacier, a low dark mass topped by a rolling cloud of dust, like time-lapse film of an approaching thunderhead, sound effects and all. It was really quite a long way out from the cape. She realized with a start that she was witnessing a long runout landslide. They were a strange phenomenon, one of the unsolved puzzles of geology. The great majority of landslides move horizontally less than twice the distance they fall; but a few very large slides appear to defy the laws of friction, running horizontally ten times their vertical drop, and sometimes even twenty or thirty. These were called long runout slides, and no one knew why they happened. Cape Solis, now, had fallen four kilometers, and so should have run out no more than eight; but there it was, well across the floor of Melas, running downcanyon directly at Ann. If it ran only fifteen times its vertical drop, it would roll right over her, and slam into the Geneva Spur.
She adjusted the focus of her binoculars for the front edge of the slide, just visible as a dark churning mass under the tumbling dustcloud. She could feel her hand trembling against her helmet, but other than that she felt nothing. No fear, no regret— nothing, in fact, but a sense of release. All over at last, and not her fault. No one could blame her for it. She had always said that the terraforming would kill her. She laughed briefly, and then squinted, trying to get a better focus on the front edge of the slide. The earliest standard hypothesis to explain long runouts had been that the rock was riding over a layer of air trapped under the fall; but then old long runouts discovered on Mars and Luna had cast doubts on that notion, and Ann agreed with those who argued that any air trapped under the rock would quickly diffuse upward. There had to be some form of lubricant, however, and other forms proposed had included a layer of molten rock caused by the slide’s friction, acoustic waves caused by the slide’s noise, or merely the extremely energetic bouncing of the particles caught on the slide’s bottom. But none of these were very satisfactory suggestions, and no one knew for sure. She was being approached by a phenomenological mystery.
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 dustcloud rolled over the low dike, there was a wind—