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 Rovers, 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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