First it was fresh toboggan tracks on Mars, now it's evidence that it snowed there, albeit a billion years ago.
Rather than water bubbling up from the ground or raining down in drops, a new study based on Hawaiian precipitation patterns supports the idea that the water came down as snow and then melted and ran off to create the branching valley networks that remain visible on Mars today.
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Those valley networks are old news, of course, but the source of the water has never been settled. This new study, which appears in Geophysical Research Letters, points to four particular locations where the valleys appear to have been caused by runoff from what's called orographic precipitation. That's snow or rain that is squeezed out when moist air has to rise (therefore cool and lose capacity to hold water) over a mountain or range of mountains.
The new study was led by geology graduate student Kat Scanlon of Brown University, who studied meteorology in Hawaii were the orographic precipitation makes the islands habitable. What you see on every island is an eastern, lush tropical wet side where the easterly winds hit the islands and drop their moisture, and a western desert side where the wrung-out air continues on its journey across the ocean.