Gullies are cut away from sunlight

It has been suggested that water just beneath the surface, warmed perhaps by geothermal activity, periodically bursts out and runs down the slopes and cuts the channels.

If channels are forming today, liquid water may exist in some regions of Mars barely 500 metres beneath the surface.

However, UA scientists think that carbon dioxide (CO2) and not water may be responsible for the gullies. They suggest several reasons why CO2 is a better candidate.

One reason is that most gullies are found in the southern highlands, the oldest and coldest part of the planet, a place where liquid water is least likely to be stable.

Suspended flow

But according to the researchers, the most convincing argument is that gullies always start about 100 m below the top of a cliff or crater. At that depth, the pressure of the rock overhead is just enough for liquid CO2 to be stable, if the temperature is low enough.

"If you have water cutting these gullies, you should see that everywhere, not just at these specific locations. And where is the water coming from? There is not much of it in the Martian atmosphere or on the surface," said Dr Musselwhite.

"What's coming out is liquid CO2 that suddenly vaporises," he maintains.

"As it comes out, it expands very quickly, cools, and actually produces CO2 snow. The snow is suspended in CO2 gas that hasn't solidified yet.

"Together with rock debris, it forms slurry. Geologists call it a 'suspended flow'. Suspended flow acts like a liquid. It doesn't take very much liquid each time to add to gully formation.

"In wintertime, the cliff surface gets so cold that its temperature falls below the freezing point of CO2, which at low pressure goes directly to solid," said Dr Musselwhite.

The researchers believe that as the cold wave moves from the surface, the rock pore space is completely filled in. When spring comes, dry ice warms up and expands. Since all the pore space is filled, pressure builds until the ice turns to liquid.