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The BBC's Ania Lichtarowicz
"These gullies were more likely to be formed by liquid carbon dioxide"
 real 28k

Monday, 2 April, 2001, 15:25 GMT 16:25 UK
Doubts over water on Mars
Nasa
Carved by liquid water or by liquid carbon dioxide?
By BBC News Online science editor Dr David Whitehouse

Liquid carbon dioxide and not water may be responsible for the remarkable gullies pictured on the surface of Mars, according to University of Arizona (UA) scientists.

Last year, astronomers made the dramatic announcement that they had found evidence that water ran on the Martian surface in the recent geological past, and possibly even today.

But looking at the same images of the Red Planet, taken by the Mars Global Surveyor spacecraft, Donald Musselwhite and colleagues publish an alternative hypothesis in the 1 April issue of Geophysical Research Letters.

If it is liquid carbon dioxide rather than water that has been flowing on Mars, it would be a severe blow to the chances of finding primitive live on the planet.

Suspended flow

Space scientists have identified small channels on slopes facing away from mid-day sunlight, with most channels occurring at high latitudes, near Mars' south pole.

Nasa
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.

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See also:

12 Mar 01 | Sci/Tech
Volcanoes on Mars 'may be active'
23 Jun 00 | Sci/Tech
Water may flow on Mars
05 Dec 00 | Sci/Tech
Red Planet's wet and warm past
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