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.

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So Scanlon looked for similar patterns on Mars that might be fossilized signs of the same sort of thing happening on early Mars.

“That’s what immediately came to mind in trying to figure out if these valleys on Mars are precipitation related,” she said in Brown University press release.

The researchers, including professor Jim Head (the man behind Mars glaciers studies), found four locations where the valleys were carved along high mountain ridges or crater rims. Then they worked out the ancient prevailing wind directions at each location using a new general circulation model (GCM) for Mars. Finally, they applied another model to see if orographic precipitation would be likely to happen in those locations.

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Sure enough, their simulations showed that precipitation would have drenched the upland parts of the densest valley networks.

“We were able to confirm that in a pretty solid way,” Scanlon said.

The GCM also predicts a cold climate at the time of the precipitation, hence the snow fall. Episodic warm spells and rain would have melted the snow to carve the valleys.

“The next step is to do some snowmelt modeling,” she said. “The question is how fast can you melt a giant snowbank. Do you need rain? Is it even possible to get enough discharge with just the snowmelt?”