‘Drizzle once fell on Martian soil, according to a new geochemical analysis by Berkeley scientists, though the rain probably stopped several billion years ago.
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. [..]
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.’