Researchers have been debating for years whether the water that once flowed on the red planet's surface formed by either water bubbling up from the ground or precipitation. (Also see "Mysterious Martian Grooves Carved by Dry Ice Chunks?")

Now powerful new computer models of wind circulation and precipitation patterns in the Martian atmosphere around four tall Martian mountains and crater rims show the strongest evidence yet for ancient snow or rainfall down their steep slopes.

The team's simulations indicate significant orographic precipitation—that is, where snow or rain falls due to moist winds blowing up mountainsides—were particularly heavy at the heads of valley networks. (Read "Mars Snow Falls Like Dry Ice Fog.")

"We found that crater rims and other topographic highs