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Last Updated:  Thursday, 20 February, 2003, 10:47 GMT
Martian gullies 'carved by water'
Gullies on Mars (AP)
The gullies are at mid latitudes
The gullies on Mars that have long puzzled scientists could have been carved by melting snow.

Previous explanations have included water bubbling up from underground springs or frozen carbon dioxide.

The latest theory, by a US geologist, depends on the slight "wobble" of the planet.

Over the course of hundreds of thousands of years, Mars slowly tilts on its axis, changing by more than 20 degrees.

The idea is that one of the poles becomes tilted toward the Sun and is heated more than usual.

According to Philip Christensen, a Professor at Arizona State University at Tempe, this creates water vapour that falls as snow closer to the equator.

If the snow is later melted by the Sun as the axis of the planet shifts, it could cause trickles of water capable of making the sort of gullies seen on Mars.

Liquid water

The research is based on images from two unmanned Mars spacecraft: Nasa's Mars Odyssey and Mars Global Surveyor.

Dr Christensen, principal investigator for Odyssey's camera system, said: "Snow on Mars is most likely to accumulate on the pole-facing slopes, the coldest areas.

"It accumulates and drapes the landscape in these areas during one climate period, and then it melts during a warmer one.

"Melting begins first in the most exposed area right at the crest of the ridge. This explains why gullies start so high up."

The gullies were discovered in 2000 by Mars Global Surveyor, suggesting liquid water was present near the surface of the planet no more than a few thousand years ago.

It led to calls to send robots to the Red Planet to search for definitive evidence.

Nasa is sending two rovers to Mars later this year to gather data. A landing craft delivered by the European Space Agency will also dig for water.

The latest research is published in the journal Nature.

Ankle-deep on Mars
16 Feb 03 |  Denver 2003
Gullies seen to form on Mars
07 Jan 03 |  Science/Nature
Martian water gone in a flash
06 Dec 02 |  Science/Nature
Mars 'once warm and wet'
14 Feb 03 |  Science/Nature

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