Dynamic wind action discovered on Mars.

• Position: Astronomy - University of Arizona's High Resolution Imaging Experiment (HiRISE

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...