June 12, 2013
Martian Dunes Show Signs Of Sledding Dry Ice
[ Watch the Video: Dry Ice Moves on Mars ]
April Flowers for redOrbit.com - Your Universe Online
The research team suggests that this process could explain an enigmatic class of gullies seen on Martian sand dunes, coming to this conclusion by examining images from NASA's Mars Reconnaissance Orbiter (MRO) and performing experiments on sand dunes in Utah and California.
"I have always dreamed of going to Mars," said Serina Diniega, a planetary scientist at NASA's Jet Propulsion Laboratory (JPL). "Now I dream of snowboarding down a Martian sand dune on a block of dry ice."
These strange grooves in the Martian sand dunes are called linear gullies. They show relatively constant width - up to a few yards, or meters, across - with raised banks or levees along the sides. Linear gullies differ from water carved gullies on Earth and possibly on Mars in that they do not have aprons of debris at the downhill end of the gully. Instead, many have pits at the downhill end.
"In debris flows, you have water carrying sediment downhill, and the material eroded from the top is carried to the bottom and deposited as a fan-shaped apron," said Diniega. "In the linear gullies, you're not transporting material. You're carving out a groove, pushing material to the sides."
The scientists analyzed images from MRO´s High Resolution Imaging Science Experiment (HiRISE) camera showing sand dunes with linear gullies covered by carbon-dioxide frost during the Martian winter. The linear gullies are located on dunes that spend the Martian winter covered by carbon-dioxide frost. Comparison of before and after images from different seasons allowed the team to determine that the gullies are formed during early spring. Some of the HiRISE images even show bright objects in the gullies.
Those bright objects are pieces of dry ice that have broken away from points higher on the slope, according to the team´s hypothesis. They also suggest that 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 Diniega. "You don't get blocks of dry ice on Earth unless you go buy them."
Buying them is exactly what Candice Hansen, of the Planetary Science Institute in Tucson, Ariz., did. Previously, Hansen has studied other effects of seasonal carbon-dioxide on Mars. Such effects include spider-shaped features that result from explosive release of carbon-dioxide gas trapped beneath a sheet of dry ice as the underside of the sheet thaws in spring. Because she suspected the role of dry ice in forming the linear gullies, she bought slabs of dry ice at a supermarket and slid them down sand dunes.
During that experiment and several that followed, Hansen observed that 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.
Although Hansen´s experiments did not simulate Martian temperature and pressure, calculations did indicate the dry ice would act similarly in early Martian spring where the linear gullies form. Water ice can also sublimate directly to gas under some Martian conditions. However, it would stay frozen at the temperatures at which these gullies form, the researchers calculate.
"MRO is showing that Mars is a very active planet," Hansen said. "Some of the processes we see on Mars are like processes on Earth, but this one is in the category of uniquely Martian."
According to Hansen, the process could be unique to the linear gullies described on Martian sand dunes.
"There are a variety of different types of features on Mars that sometimes get lumped together as 'gullies,' but they are formed by different processes," she said. "Just because this dry-ice hypothesis looks like a good explanation for one type doesn't mean it applies to others."
The findings of this study have been published in the journal Icarus.