Image copyright Nasa/JPL-Caltech/U Arizona/Science Photo Library Image caption The study looked at gullies like these in Russell Crater, shown with winter CO2 frost accumulating

Gullies seen on the surface of Mars could be formed by trapped CO2 gas causing sand to flow downhill, a new modelling study suggests.

In the Martian winter, a layer of CO2 frost - dry ice - forms on the surface in many regions of the red planet.

Writing in Nature Geoscience, French researchers now suggest that as it defrosts, this icy layer traps gaseous CO2, causing a build-up of pressure.

They say this gas, rather than liquid water, can explain the Martian gullies.

These small valleys, cut into hillsides and dunes in many areas near the planet's equator, had been something of a puzzle.

Some researchers had suggested they were evidence for a wet period in Mars's ancient past, but many of the formations are relatively young, in geological terms, and others are still forming today.

This means something other than water must be responsible, because the surface of Mars in these locations is far too cold to support liquid water.

Sublime and slide

Proposed explanations have stretched as far as "ice hovercraft" - blocks of solid CO2 tobogganing down the dunes.

But Cedric Pilorget and Francois Forget, from Université Paris-Sud and Sorbonne Universités respectively, have a new idea.

As the CO2 frost starts to sublime into gas at the end of winter, they suggest, some of that gas can be trapped under as much as 30cm of dry ice.

They modelled how this process might unfold on a huge dune in the Russell crater, where brand new gullies have been glimpsed by Nasa satellites.

That build-up of pressure, Drs Pilorget and Forget found, can produce a "gas-lubricated debris flow" that cascades down the slope, leaving a gully in its wake.

"Such processes may involve large amounts of material and do not require any liquid water," they write.

Image copyright Science Photo Library Image caption The gullies have been studied in great detail thanks to satellite images like this one, from Newton Crater

This is important, because the gullies concerned can be tens of metres wide - unlike the smaller "recurring slope lineae" which scientists proposed in September might be caused by small amounts of very salty water.

Furthermore, the team's modelling work predicts that the best conditions for these gas-powered landslides occur in the very same places where these gullies have been observed.

In a comment article for Nature Geoscience, Colin Dundas from the US Geological Survey said the French team's model was "a good match to many observations of gully activity", but added that there are probably multiple processes at work.

"The story emerging over the past decade of monitoring the Martian surface is that Mars, like Earth, is a dynamic world with many active processes," Dr Dundas writes.

"It would be surprising if only a single process was involved in gully formation."