Billions of years ago, flowing water etched valleys into the surface of Mars; these scars are readily visible, but the origin of the water that carved them has been mysterious. It could have been groundwater percolating up from below the surface, or it could have been runoff from rain or snow. A new study led by Brown University graduate student Kat Scanlon has provided support for the precipitation hypothesis.
Scanlon’s work has revealed that valleys at four different spots on the Martian surface were probably carved by runoff from orographic precipitation. Orographic precipitation occurs when moisture-laden winds meet a mountain range and are forced upwards. Lacking sufficient kinetic energy to crest the mountains’ summits, the winds release their moisture as precipitation on one side of the mountains but not the other.
Having studied orographic precipitation on the big island of Hawaii, Scanlon decided to test whether the Martian valleys might be the results of a similar phenomenon. “That’s what immediately came to mind in trying to figure out if these valleys on Mars are precipitation related,” she said in a press release.
Scanlon and her colleagues began by identifying four places on Mars where networks of valleys are adjacent to tall mountain ranges or crater rims. The team called upon a recently developed general circulation model for Mars, which simulates air movement using the inferred gas composition of the early Martian atmosphere, to determine the direction of prevailing winds at each location.
Using these parameters, the team employed an orographic precipitation model to ascertain the most likely locations where precipitation would fall in each study area. This simulation demonstrated that precipitation was most intense at the heads of the densest valley systems.
Given the cold climate of early Mars, the precipitation would likely have fallen as snow, which would have melted during periodic warmer conditions and flowed down the slopes to shape the valleys seen today.
Scanlon plans to continue her research on ancient Martian precipitation. “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 [to carve the valleys] with just the snowmelt?”
In 2008, NASA’s Phoenix Mars Lander detected falling snow from Martian clouds, which vaporized before it reached the ground.
The new research by Scanlon and colleagues has been accepted for publication in the journal Geophysical Research Letters and was published online in June.