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Dynamic wind action discovered on Mars.

Mars has an ethereal, tenuous atmosphere at less than one percent of the surface pressure of Earth, so scientists working on the University of Arizona's High Resolution Imaging Experiment, or HiRISE, are challenged to explain the complex, wind-sculpted landforms they now are seeing in unprecedented detail.

The HiRISE camera on NASA's Mars Reconnaissance Orbiter, the most powerful camera to circle another planet, can see 20-inch-diameter features while flying at about 7,500 mph between 155 and 196 miles above the Martian surface. HiRISE co-investigator Nathan Bridges of the Jet Propulsion Laboratory in Pasadena, Calif., notes, "On Earth, small bedforms can form and change on time scales as short as a day."

There are two types of "bedforms" or wind-deposited landforms: They can be sand dunes, which typically are larger and have distinct shapes, or they can be ripples, which is sand mixed with coarser, millimeter-sized particles. Ripples typically are smaller, more linear structures. HiRISE also displays detail in sediments deposited by winds on the lee side of rocks. Such rock "windtails" are indicative of which way the most current winds have blown. Such features have been seen before, but only by rovers and landers, never an orbiting camera. Researchers now can use HiRISE images to infer wind directions over the entire planet. Scientists 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 scientists insight on how yardangs form.

"HiRISE is showing us just how interesting layers in yardangs are," Bridges observes. "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."

HiRISE makes clear that some layers in the yardangs are made of softer materials that have been modified by wind, he adds. 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.

HiRISE images also show that what covers the slopes of the high Martian volcanoes definitely are dunes or ripples that appear to have an organized "reticulate" structure possibly formed by winds blowing from multiple directions. "On Earth, winds blowing from many different directions form what are called 'star dunes,' and these look somewhat like those," Bridges explains.

"The reticulate surface looks like a network of connected wind-blown dunes and ripples. The fact that the air pressure near the volcano tops is so low and the material is dust challenges us to understand what these features are. Perhaps the dust is clumping together and making sand-sized material, but how this stuff can be blown around in this low pressure is at the edge of our understanding of aeolian physics. Possibly the bedforms on the volcanoes formed un der a different Martian climate in the past, when atmospheric density was greater, but I'm not sure that's the case because you can see evidence that a lot of the mantle appears to be fairly recent."
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Title Annotation:Astronomy; University of Arizona's High Resolution Imaging Experiment (HiRISE)
Publication:USA Today (Magazine)
Geographic Code:1USA
Date:Aug 1, 2008
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