When scientists try to describe what makes Earth special, they usually note that it's the "wet planet." Now they'll have to come up with some other distinction. Last week NASA confirmed that Mars, too, was once home to water--and the source may have looked a lot like a geological feature here on Earth. Mars's water, says Central Michigan University geologist Kathy Benison, probably existed in the form of "acid lakes," shallow beds of salty water found largely (until now) in Australia. Scientists' big break last week was finding jarosite. The mineral forms only in the presence of low-pH water. Acid-lake beds are also reservoirs of iron-rich compounds like hematite. Translation: they're Martian red. And Benison, a leading expert on ancient acid lakes, thinks the crystals recovered by the Mars rover resemble those she has found in Earthly sediments.

If the Red Planet did have acid lakes long ago, alien-life enthusiasts have a lot to be, well, enthused about. Although the lakes on Earth aren't home to many large flora and fauna, they teem with microbes, which scientists also hope to find on Mars. Acid lakes also tend to dry up seasonally, and since no one knows if Mars has seasons or even weather, the new sediments may provide clues there, too. "Do things work exactly the same on Mars as they do on Earth? Probably not," says Benison. For example, there's the small matter of gravity. But one thing is clear: there is, in fact, a place at least a little bit like home.

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