The first close-up images of Mars, captured in 1972 by the probe Mariner 9, were a planetary scientist’s dream: they revealed networks of valleys that looked uncannily like drainage basins and streambeds back here on Earth and thus implied that there had once been water freely flowing over the surface of Mars. The images also implied that Mars had once had an atmosphere. Our planet is blessed with liquid water on its surface only because it has an atmosphere to maintain a high pressure and trap the sun’s heat. So planetary scientists proposed that when Mars formed 4.6 billion years ago, it too had a dowry of a heat-trapping atmosphere, composed of carbon dioxide and water vapor. With warmth, water, and air, they speculated, Mars might once have been a garden world, a paradise among planets.
But, as they also discovered, the garden didn’t last long. None of the streambeds were younger than 3.7 billion years. Something happened to Mars, something that stripped its atmosphere, killed its streams, and froze the garden forever.
Researchers have suggested many scenarios for the Martian apocalypse. Some have proposed that the sun gradually whittled away Mars’ atmosphere with its wind of charged particles. Others have hypothesized that the planet itself absorbed its atmosphere, turning carbon dioxide into carbonate rocks. For the past seven years, however, Ann Vickery and Jay Melosh, two planetary scientists from the University of Arizona, have been exploring a far more spectacular ending: Mars’ atmosphere, they suggest, was blasted away by a succession of asteroids and comets.
