Ancient snowfall cause valleys on Mars

Some valleys on the surface of Mars appear to have been caused by ancient runoff from melting snow, US researchers said.
While the Martian valley networks provide strong evidence that water once flowed on the Red Planet, the source of that ancient water is still debated by scientists. Some of them think water bubbled up from under the ground, while others claim it fell as rain or snow.
Researchers at Brown University believe that some of the water-carved valleys were created by runoff from a geological phenomenon known as orographic precipitation -- snow or rain that falls when moist prevailing winds are pushed upward by mountain ridges, Xinhua reported.
The researchers wrote in a paper in the journal Geophysical Research Letters that they identified four Martian locations where valley networks were found along tall mountain ridges or raised crater rims.
They then create a theoretical model to assess the direction of the prevailing winds at each location.
The model simulates air movement based on the gas composition scientists think was present in the early Mars atmosphere.
Next, the team used a model of orographic precipitation to determine where precipitation would be likely to fall in each of the study areas.
Their simulations showed that precipitation would have been heaviest at the heads of the densest valley networks.
'Their drainage density varies in the way you would expect from the complex response of precipitation to topography,' said Kat Scanlon, a geological sciences graduate student at Brown University who led the study.
'We were able to confirm that in a pretty solid way.'
According to the researchers, the atmospheric parameters used in the study are based on models that predict a cold climate, so the precipitation modeled was snow.
But this snow could have been melted by episodic warming conditions to form the valley networks, and indeed some precipitation could have been rain during this period, they said.
'The next step is to do some snowmelt modeling,' Scanlon said.
'The question is how fast can you melt a giant snow bank. Do you need rain? Is it even possible to get enough discharge to carve the valleys with just the snowmelt?