The study finds that water-carved valleys at four different
locations on Mars appear to have been caused by runoff from
orographic precipitation -- 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.
A paper describing the work has been accepted by Geophysical
Research Letters and published online in June.
Kat Scanlon, a geological sciences graduate student at Brown, led
the research and is well-acquainted with the orographic effect. She
did graduate work in meteorology in Hawaii, which is home to a
quintessential orographic pattern. Moist tropical winds from the
east are pushed upward when they hit the mountains of Hawaii's big
island. The winds lack the kinetic energy to reach the mountain
summit, so they dump their moisture on the eastern side of the
island, making parts of it a tropical jungle. The western side, in
contrast, is nearly a desert because it sits in a rain shadow cast
by the mountain peak.
Additional modeling might determine how fast Martian snow could
have melted and whether snowmelt alone could have carved the
valleys.Scanlon thought similar orographic patterns 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," she said.
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," Scanlon said. "We were
able to confirm that in a pretty solid way."
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," 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?"
With the knowledge from this study that precipitation was
important in carving the valleys, the answers to those additional
questions could provide important insight into the climate on Mars
billions of years ago.