Would you like ice with that?

Blocks of frozen carbon dioxide sliding down Martian sand dunes, rather than flowing liquid water, may have formed some unique long, narrow grooves seen on the surface of Mars.

The Martian linear gullies, typically with a uniform width and a length from a few hundred meters to 2.5 kilometers, were studied using images from NASA’s Mars Reconnaissance Orbiter. TCD Geography’s  Dr. Mary Bourke is co-author of a new paper, “A new dry hypothesis for the formation of Martian linear gullies,” that appears in Icarus.

“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 and lead author of the paper. “We don’t get blocks of dry ice on Earth unless you go buy them.”

Linear gullies show relatively constant width – up to a few meters across – with raised banks or levees along the sides. Unlike gullies caused by water-lubricated flows on Earth, and possibly on Mars, they don’t have aprons of debris at the downhill end of the gully. Instead, many have pits at the downhill end. The pits could result from the blocks of dry ice completely sublimating away into carbon dioxide gas after they stopped moving, according to the new dry ice hypothesis.

The Martian sand dunes featuring linear gullies are covered by carbon-dioxide frost in winter. Before and after images from the High Resolution Imaging Science Experiment (HiRISE) camera aboard the Mars Reconnaissance Orbiter have determined the gullies are formed in early spring. Some HiRISE images have shown bright objects in the gullies that could be pieces of dry ice that have broken away from higher on the slope.

“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,” co-author Hansen said. “Just because this dry ice hypothesis looks like a good explanation for one type doesn’t mean it applies to others.”

This image is a portion of the HiRISE exposure catalogued as PSP_001440_1255 taken on Nov. 16, 2006, at 54.25 degrees south latitude, 12.92 degrees east longitude.
This image is a portion of the HiRISE exposure catalogued as PSP_001440_1255 taken on Nov. 16, 2006, at 54.25 degrees south latitude, 12.92 degrees east longitude.

Hansen has studied other effects of seasonal carbon-dioxide ice on Mars, such as 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. She suspected a role for dry ice in forming linear gullies, so she purchased some slabs of dry ice at a supermarket and tested sliding them down sand dunes. She found that gaseous carbon dioxide from the thawing ice provided a lubricating layer under the slab and also pushed sand aside into small levees as the slabs glided down even low-angle slopes. Of course this was an experiment under earthconditions, not Martian.

Co-author McElwaine contributed theoretical calculations to show that this process would indeed work on Mars. Heat from the Martian surface causes sublimation of the CO2 ice with the resulting gas levitating the block. This levitation mechanism is similar to the Leidenfrost effect where a drop of water on a hot plate will hover on a cushion of steam generated by heat from the plate boiling the water. Once the block of ice is supported by gas it can slide down gentle slopes on this lubrication layer. This works similarly on Earth and Mars despite differences in gravity, temperature and air pressure.

“Mars Reconnaissance Orbiter 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.”

Posted by: Dr. Mary Bourke, Geomorphologist, TCD.

BBC News Science and Environment

The Guardian Science


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