Mars: dusty, barren, dry.
Oh, and also! Icy. Sometimes snowy.
Yep. We know that the Red Planet has ice under its rocky surface. And we know, or at least are pretty sure, that the Red Planet was once watery. Mars features riverbeds, after all, that were likely carved by liquid water. What we have been less sure about, however, has been where that liquid water came from. Did it bubble up from below the Martian surface? Or did it precipitate, falling onto the surface in the form of rain or snow?
A new study gives new evidence for option B. The research, led by the Brown University geologist Kat Scanlon, examined four different locations on Mars that feature marks consistent with the liquid runoff from orographic precipitation -- rain or snow that falls, in particular, when prevailing winds are pushed upward by mountain ridges. The new findings, Space Ref puts it, offer "the most detailed evidence yet of an orographic effect on ancient Mars."
In other words: Mars may once have been a regular little snow globe.
To come to that finding, the researchers relied on what's known as the general circulation model (GCM) for Mars, which takes what we know about the gas composition of the early Martian atmosphere and simulates air movement based on those gases. They then used another model -- this one for orographic precipitation -- to try to figure out where, given the prevailing winds they'd gotten from the GCM, precipitation would be likely to fall on Mars. They repeated this process for each of their four Martian study areas.
And: they found evidence of precipitation. And precipitation, in particular, that would have been heaviest at the heads of the densest valley networks. As Scanlon explained it: "Their drainage density varies in the way you would expect from the complex response of precipitation to topography. We were able to confirm that in a pretty solid way."
In honor of that finding -- and to remind you that there is, indeed, snow on Mars -- here are some images of a snowy and/or icy and/or moist Red Planet.