Gullies on Mars were probably not created by liquid water but by the seasonal freezing and thawing of carbon dioxide, according to an analysis of high-resolution images.
Many scientists have argued that flowing water — a prerequisite for life — carved the gullies (pictured) that are widespread across Mars. Colin Dundas of the US Geological Survey in Flagstaff, Arizona, and his team studied 98 gully sites in Mars's northern hemisphere and 258 in the southern half, using data from NASA's Mars Reconnaissance Orbiter.
Looking at the same areas each year, the researchers saw the gullies growing and changing shape at the same time as carbon dioxide frost appeared and disappeared. Liquid water is not required in this process, the authors say.
Icarus http://doi.org/trx (2014)