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Drawing on soil data from the five missions to Mars before the current Phoenix Lander and comparing it to information collected in Earth's driest places, the scientists concluded that water must have fallen from above, not welled up from below, as has been thought.

"The soil acts as a sort of an imperfect record of climate change," said Ronald Amundson, UC Berkeley professor of ecosystem sciences and the study's lead author. "We can study the chemistry of the soil and extract information about climate history."

Amundson's key observation is that Martian soil has a layer of sulfates sitting on top of chlorides. That's a pattern consistent with water moving downward from the atmosphere to the regolith in places on Earth.

Though he can't say for sure whether the precipitation on Mars fell as snow, sleet or rain, the evidence of reacting with rocks suggests that the water was liquid on the ground.