Wind-sculpted
Martian landscapes raise questions for scientists about the Red Planet's
atmosphere and terrain.
Sand dunes
are among the "bedforms" or wind-deposited landforms that appear in
new images from NASA's
Mars Reconnaissance Orbiter (MRO). However, scientists remain unsure as to
whether winds on present-day Mars are strong enough to create such geological
features.
"We're
seeing what look like smaller sand bedforms on the tops of larger dunes, and,
when we zoom in more, a third set of bedforms topping those," said Nathan Bridges, NASA's Jet Propulsion Laboratory
in Pasadena, Calif. "On Earth, small bedforms can form and change on time
scales as short as a day."
Other bedforms
on Mars take the shape of smaller and more linear ripples, in which sand is
mixed with coarser particles.
New details
emerged about sediments deposited by winds on the downwind side of rocks. Such
"windtails" show which way the most current winds have blown, Bridges
said. Only rovers
and landers have seen such features before, as opposed to an orbiter.
With the University of Arizona's High Resolution Imaging Science Experiment camera (HiRISE),
MRO sees features as small as 20 inches from 155 to 196 miles above the Martian
surface. Researchers can now use HiRISE images to infer wind directions over
the entire planet.
Scientists
also previously discovered miles-long, wind-scoured ridges called "yardangs"
with the first Mars orbiter, Mariner 9, in the early 1970s. New HiRISE images
reveal surface texture and fine-scale features that are giving insight into how
yardangs form.
"HiRISE
is showing us just how interesting layers in yardangs are," Bridges said.
"For example, we see one layer that appears to have rocks in it. You can
actually see rocks in the layer, and if you look downslope, you can see rocks
that we think have eroded out from that rocky layer above."
New images
show that some layers in the yardangs are made of softer materials that have
been modified by wind, he added. The soft material could be volcanic ash
deposits, or the dried-up remnants of what once were mixtures of ice and dust,
or something else.
"The
fact that we see layers that appear to be rocky and layers that are obviously
soft says that the process that formed yardangs is no simple process but a
complicated sequence of processes," Bridges added.
Some
researchers have begun comparing HiRISE images with those taken by NASA's Mars
Exploration Rover, in order to identify previously mysterious features such as
dark streaks surrounding Victoria
Crater. Others continue to find surprises while reexamining features once
considered common and uninteresting.
"HiRISE
keeps showing interesting things about terrains that I expected to be
uninteresting," said Alfred McEwen of the University of Arizona
Lunar and Planetary
Laboratory, HiRISE principal investigator. "I was surprised by the
diversity of morphology of the thick dust mantles. Instead of a uniform blanket
of smooth dust, there are often intricate patterns due to the action of the
wind and perhaps light cementation from atmospheric volatiles."
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