Dry ice, not liquid water formed gullies on Mars' surface

NASA has revealed that the gullies on Mars' surface are primarily formed by the seasonal freezing of carbon dioxide, not liquid water.

NASA's Mars Reconnaissance Orbiter (MRO)'s High Resolution Imaging Science Experiment (HiRISE) camera was used to examine gullies at 356 sites on Mars, beginning in 2006.

Thirty-eight of the sites showed active gully formation, such as new channel segments and increased deposits at the downhill end of some gullies.

Lead author Colin Dundas of the U.S. Geological Survey's Astrogeology Science Center in Flagstaff, Arizona, said that they were able to get many more observations, and as we started to see more activity and pin down the timing of gully formation and change, we saw that the activity occurs in winter.

The first reports of formative gullies on Mars in 2000 suggested the presence of liquid water on the Red Planet, the eroding action of which forms gullies here on Earth. Mars has water vapor and plenty of frozen water, but the presence of liquid water on the neighboring planet, a necessity for all known life, has not been confirmed.

Although the findings about gullies point to processes that do not involve liquid water, possible action by liquid water on Mars has been reported in the past year in other findings from the HiRISE team. Those observations were of a smaller type of surface-flow feature.'

The study was published online in the journal Icarus.