According to a new study published online in the Geophysical Research Letters, some water-carved Martian valleys appear to have been caused by runoff from precipitation, likely meltwater from snow.
Water-carved valley on Mars. Researchers from U.S. and France have found that some of these valleys appear to have been formed by orographic precipitation – rain or snow that falls when moist air is pushed upward by mountain ridges or raised crater rims. The findings suggest that at least some of the water on ancient Mars fell from the atmosphere, rather than bubbling up from the subsurface (NASA)
The study finds that valleys at four different locations on Mars appear to have been caused by snow or rain that falls when moist prevailing winds are pushed upward by mountain ridges. The new findings are the most detailed evidence yet of an orographic effect on ancient Mars and could shed new light on the planet’s early climate and atmosphere.
Study lead author Kathleen Scanlon from Brown University in Providence, RI, and her colleagues from the University of Chicago and the Institut Pierre Simon Laplace in Paris, are well-acquainted with orographic effects. They thought such effects might have been at play on early Mars and that the valley networks might be an indicator. “That’s what immediately came to mind in trying to figure out if these valleys on Mars are precipitation related,” Scanlon said.
The scientists started by identifying four locations where valley networks were found along tall mountain ridges or raised crater rims.
They used a newly developed general circulation model (GCM) for Mars to establish the direction of the prevailing winds at each location: Terra Cimmeria, Margaritifer Sinus, Terra Sabea and Thaumasia Planum. 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 model, 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,” Scanlon said.
“The atmospheric parameters used in the model 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 her colleagues said.
______
Bibliographic information: Scanlon KE et al. Orographic precipitation in valley network headwaters: Constraints on the ancient Martian atmosphere. Geophysical Research Letters, published online before print; doi: 10.1002/grl.50687
