A region of nearly pure silica on Mars hints at the ancient existence of water, according to a study published in the May 23 Science. The Mars rover Spirit found the silica – the main component in sand on Earth – in the EasternValley of the Gusev Crater more than a year ago.The research confirmed earlier suspicions that 4 billion years ago, the planet hosted hydrothermal sites, places where geothermal activity and water meet, such as the hot springs and geysers in YellowstoneNational Park. “In places like Yellowstone, these hydrothermal sites are very luxuriant places for life,” says Mike Carr of the U.S. Geological Survey in Menlo Park, Calif., who was not involved in the study. “So finding these things on Mars is pretty exciting.” While no one knows if microbes ever got going on Mars, the region had the two ingredients, heat and water, that could support life, he says. What’s more, the silica deposits could encase any possible remains, increasing the chance of finding preserved bacteria. Though Spirit accidentally kicked up the telltale deposits more than a year ago, further analysis verified its significance, says Steve Squyres, an astrophysicist at CornellUniversity who conducted the study. Squyres and his team used spectroscopy — analyzing the light reflected from the soil —and other methods to determine the chemical composition of the soil, which could provide clues about the source of the silica. Two scenarios could create silica-rich soil. “One way to do it is a hot spring,” Squyres says. In hot springs, geothermal activity heats up water, making it lighter. As the hot water rises, silica from nearby rocks flakes off and dissolves into the water. But once the water reaches the surface, it quickly cools. Because silica doesn’t dissolve as well in colder water, the element settles out of the water and builds up on nearby rock The deposits could also have formed through a leaching process. Around volcanic regions, steam is often chock-full of sulfur, and therefore very acidic. In this scenario, steam rises through cracks in the rock, dissolving everything but silica. What’s left is almost pure silica. Right now, the scientists have no plans to have Spirit collect more samples. The rover doesn’t have the equipment needed to test for microbial life, but researchers hope to one day bring some rocks back to Earth for a better look, Squyres adds.