Gullies on Martian sand dunes may reveal tracks of dry ice sleds

Washington: NASA scientists have indicated that hunks of frozen carbon dioxide-dry ice-may glide down some Martian sand dunes on cushions of gas similar to miniature hovercraft, plowing furrows as they go.

Researchers deduced this process could explain one enigmatic class of gullies seen on Martian sand dunes by examining images from NASA`s Mars Reconnaissance Orbiter (MRO) and performing experiments on sand dunes in Utah and California. The hillside grooves on Mars, called linear gullies, show relatively constant width-up to a few yards or meters across-with raised banks or levees along the sides. Unlike gullies caused by waterflows on Earth and possibly on Mars, they do not have aprons of debris at the downhill end of the gully. Instead, many have pits at the downhill end. Images from MRO`s High Resolution Imaging Science Experiment (HiRISE) camera show sand dunes with linear gullies covered by carbon dioxide frost during the Martian winter. The location of the linear gullies is on dunes that spend the Martian winter covered by carbon dioxide frost. The grooves are formed during early spring, researchers determined by comparing before-and-after images from different seasons. Some images have even caught bright objects in the gullies. Scientists theorize the bright objects are pieces of dry ice that have broken away from points higher on the slope. According to the new hypothesis, the pits could result from the blocks of dry ice completely sublimating away into carbon-dioxide gas after they have stopped traveling. "Linear gullies don`t look like gullies on Earth or other gullies on Mars, and this process wouldn`t happen on Earth," said Serina Diniega, a planetary scientist at NASA`s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and lead author of the study. Study co-author Candice Hansen, of the Planetary Science Institute in Tucson, Ariz., suspected a role for dry ice in forming linear gullies, so she bought some slabs of dry ice at a supermarket and slid them down sand dunes. That day and in several later experiments, gaseous carbon dioxide from the thawing ice maintained a lubricating layer under the slab and also pushed sand aside into small levees as the slabs glided down even low-angle slopes. The outdoor tests did not simulate Martian temperature and pressure, but calculations indicate the dry ice would act similarly in early Martian spring where the linear gullies form. Although water ice, too, can sublimate directly to gas under some Martian conditions, it would stay frozen at the temperatures at which these gullies form, the researchers calculate. Hansen also noted the process could be unique to the linear gullies described on Martian sand dunes. The finding was published online by the journal Icarus. ANI