Why is Mars such a rusty red? For decades, scientists have assumed the answer was as simple as rust on Earth. The Mars rovers, Spirit and Opportunity, established that the planet once was probably warm and wet, with standing pools of brackish water, and scientists believed the soil literally became rusty, with iron oxide forming from iron in the soil and oxygen in the water -- not far different from the process that ruins a lawn chair if you leave it out too often. Or maybe not -- at least according to Jonathan Merrison of Aarhus University in Denmark, who made a presentation at the European Planetary Science Conference. "Mars should really look blackish, between its white polar caps, because most of the rocks at mid-latitudes are basalt," said Merrison in a release. Basalt, the kind of rock produced by volcanoes and other sources of earthly heat, is almost always dark. For illustration, they offer the Hubble telescope image above. The globe on the left is real; the one on the right is doctored. Merrison and his team did a little experiment. They filled flasks with sand, and sealed them so they would stay as dry as -- well, as dry as dust. They put the vials in mechanical tumblers, turning them over ten million times to simulate the kind of erosion that goes on when rock particles are blown around or grind against each other over time. After several months, Merrison reports, the flasks (see picture) contained reddish dust. No water required. He's had a paper on this approved in the scientific journal Icarus. All this may throw a minor wrench into the thinking of other Mars explorers, who've believed that the way to understand the red planet -- and perhaps find evidence of life there -- was, in a mantra used at NASA, to "follow the water." But how much water was there, asks Merrison, and how important was it?