• NASA/JPL/University of Arizona

Phil Plait's Bad Astronomy blog flags a photo from the UA's HiRISE camera aboard the Mars Reconnaissance Orbiter to note that it may have rained on Mars a long time ago:

This fan is on the inside of the rim of Mojave Crater, a 60-kilometer-wide (40-mile-wide) impact crater near the equator of Mars. The structure matches an Earthly alluvial fan almost perfectly. Larger boulders are heavier and can’t be carried as easily by floodwaters, so they tend to stop soon after the terrain levels out. Smaller rocks can travel farther, which appears to be the case here. The branches, the shape, the direction: Everything indicates a flash flood on Mars.

What could have caused it? This part surprised me: It may have been due to rain, water rain, that could occur after an asteroid or comet impact. For example, ice under the surface could be melted by the impact, which would then rain down over a large area. This would be a temporary and local event, but could spark flash floods something like rainstorms do here on Earth.

But after that, gravity and terrain did the rest, on Mars as it is on Earth. That’s actually rather astonishing: Given some basic and fundamental principles, you can actually figure out how weather and erosion processes work on another planet. And when you look at it, it actually kinda reminds you of home.