Gullies seen to form on Mars

What created the gullies on Mars is still a mystery

By Dr David Whitehouse BBC News Online science editor

The formation of gullies has been seen for the first time on Mars.

Mars is not a cold Earth. It is a different world

Dr Nick Hoffman

According to University of Melbourne geologist Dr Nick Hoffman, gullies near the Red Planet's south pole form as the seasonal ice cap retreats in the Martian spring.

"In itself the observation of active flows is a dramatic discovery since no movement has yet been seen on Mars, except for some dry dust avalanches," he told BBC News Online.

But contrary to the common idea that such features were carved by liquid water, Dr Hoffman says the flow is most likely gaseous carbon dioxide which, if true, would deal a blow to the prospects for life on Mars.

'To concrete'

Writing in the latest edition of the journal Astrobiology, Dr Hoffman presents evidence for the first in-action flow events on Mars and argues that liquid water is not to blame, in fact no liquid is.

"In the Martian Spring," he says, "carbon dioxide frost and snow at temperatures of minus 130 degrees Celsius fill the valleys, yet the flow events are occurring.

Nasa researchers believe water can explain the grooves in the sides of craters

"But at such low temperatures everything turns to concrete so whatever is moving cannot be a liquid," he told BBC News Online.

"Nothing based on water can flow at these temperatures, so the culprit must be avalanches of gaseous carbon dioxide and rocky debris."

This is because carbon dioxide does not melt on Mars: it boils directly from the solid - a process called sublimation.

"The boiling dry ice acts like a armada of miniature hovercraft carrying a shower of sand, dust, and tumbling rocks down the slope, carving out the gullies as it goes," Dr Hoffman says.

Outer planets

Life would find it more difficult to take hold on Mars without supplies of liquid water.

"Without liquid water there cannot be life and despite recent reports of more and more ice on the Red Planet, Nasa has yet to find liquid water," he says.

Although many US space agency scientists are doubtful about Dr Hoffman's observations, the geologist says that at a meeting of the American Geophysical Union held last month, they struggled to find arguments against the evidence he presented.

To Dr Hoffman the possibility that the gullies are formed by gas flows emphasises how different Mars is from the Earth.

Geologically it may have more things in common with some of the ice-crusted moons of the outer planets than our own world.

"Mars is not a cold Earth. It is a different world, it works in another way. It is more like a warmer version of Europa, a moon of Jupiter, than our Earth," he says.