laugh was scary. “Welcome to Mars,” he said.
Ann Clayborne was driving down the Geneva Spur, stopping every few switchbacks to get out and take samples from the roadcuts. The Transmarineris Highway had been abandoned after ’61, as it now disappeared under the dirty river of ice and boulders covering the floor of Coprates Chasma. The road was an archaeological relic, a dead end.
But Ann was studying the Geneva Spur. The Spur was the final extension of a much longer lava dike, most of which was buried in the plateau to the south. The dike was one of several—the nearby Melas Dorsa, the Felis Dorsa farther east, the Solis Dorsa farther west—all of them roughly parallel, all perpendicular to the Marineris canyons, and all mysterious in their origin. But as the southern wall of Melas Chasma had receded, by collapse and eolian removal, the hard rock of one dike had been exposed, and this was the one named the Geneva Spur, which had provided the Swiss with a perfect ramp to get their road down the canyon wall, and was now providing Ann with a nicely exposed dike base. It was possible that it and all its companion dikes had been formed by concentric fissuring resulting from the rise of Tharsis; but they could also be much older, remnants of a basin-and-range type spread in the earliest Noachian, when the planet was still expanding from its own internal heat. Dating the basalt at the foot of the dike would help answer the question one way or the other.
So she drove a little boulder car slowly down the frost-covered road. The car would be quite visible from space, but she didn’t care. She had driven all over the southern hemisphere in the previous year, taking no precautions except when approaching one of Coyote’s hidden refuges to resupply. Nothing had happened.
She reached the bottom of the Spur, only a short distance from the river of ice and rock that now choked the canyon floor. She got out of the car and tapped away with a geologist’s hammer at the bottom of the last roadcut. She kept her back to the immense glacier, and did not think of it. She was focused on the basalt. The dike rose before her into the sun, a perfect ramp to the clifftop, some three kilometres above her and fifty kilometres to the south. On both sides of the Spur the immense southern cliff of Melas Chasma curved back in huge embayments, then out again to lesser prominences—a slight point on the distant horizon to the left, and a massive headland some sixty kilometres to the right, which Ann called Cape Solis.
Long ago Ann had predicted that greatly accelerated mass wasting would follow any hydration of the atmosphere, and on both sides of the Spur the cliff gave indications that she had been right. The embayment between the Geneva Spur and Cape Solis had always been a deep one, but now several fresh landslides showed that it was getting deeper fast. Even the freshest scars, however, as well as all the rest of the fluting and stratification of the cliff, were dusted with frost. The great wall had the coloration of Zion or Bryce after a snowfall—stacked reds, streaked with white.
There was a very low black ridge on the canyon floor a kilometre or two west of the Geneva Spur, paralleling it. Curious, Ann hiked out to it. On closer inspection the low ridge, no more than chest-high, did indeed appear to be made of the same basalt as the Spur. She took out her hammer, and knocked off a sample.
A motion caught her eye and she jerked up to look. Cape Solis was missing its nose. A red cloud was billowing out from its foot.
Landslide! Instantly she started the timer on her wristpad, then knocked the binocular hood down over her faceplate, and fiddled with the focus until the distant headland stood clear in her field of vision. The new rock exposed by the break was blackish, and looked nearly vertical; a cooling fault in the dike, perhaps—if it too was a dike. It did look like basalt. And it looked as if the break had extended the entire height of the cliff, all four kilometres of it.
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, almost subsonic, 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 metres per second, so the distance of sixty kilometres 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 metres 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 timelapse film of an approaching thunderhead, sound effects and all. It was really quite a long way out from the cape. She realised 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 kilometres, 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 travelled at 120 kilometres 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
