Valley networks branching across the Martian surface leave little doubt that water once flowed on the Red Planet. But where that ancient water came from — whether it bubbled up from underground or fell as rain or snow — is still debated by scientists. A new study by researchers at Brown University puts a new check mark in the precipitation column.
The researchers, including Jim Head, professor of geological sciences, started by identifying four locations where valley networks were found along tall mountain ridges or raised crater rims. To establish the direction of the prevailing winds at each location, the researchers used a newly developed general circulation model (GCM) for Mars. 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, given the prevailing winds from the GCM, 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,” “We were able to confirm that in a pretty solid way,” said Kat Scanlon at Brown,
The atmospheric parameters used in the GCM are based on a new comprehensive general circulation model that predicts a cold climate, so the precipitation modeled in this study 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, Scanlon and Head say.
“The next step is to do some snowmelt modeling,” Scanlon said. “The question is how fast can you melt a giant snowbank. Do you need rain? Is it even possible to get enough discharge [to carve the valleys] with just the snowmelt?”
The work was supported by NASA and included co-authors Jean-Baptiste Madeleine and François Forget of the Institut Pierre Simon Laplace, Université Paris, and Robin Wordsworth of the University of Chicago.
The Daily Galaxy via Brown University