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CO2 gets Martian sand dunes moving
Sand dunes surrounding the North Pole of Mars are on the move, powered by carbon dioxide, a finding that has surprised planetary scientists.

Until now, it was though the dunes of the vast northern plains were thought to be frozen in time and cemented in place by water ice.

Now a report in the journal Science claims high resolution imaging by NASA's Mars Reconnaissance Obiter (MRO) spacecraft shows these dunes are dynamic and moving.

Dr Candice Hansen from the Planetary Science Institute in Tucson, Arizona, says knowing how Mars is changing helps our understanding of basic planetary processes.

According to Hansen, scientists thought the dunes were fairly static, shaped long ago when winds on the planet's surface were much stronger than today.

"The before and after pictures show a surprising number of changes in the higher latitude dunes," says Hansen.

She says the changes are caused by the periodic deposition and sublimation of carbon dioxide.

"A seasonal layer of frozen carbon dioxide blankets the region in winter, changing directly back to gas in spring, causing the sand to avalanche down in dark streaks, forming new alcoves and debris aprons on about 40 per cent of dunes."

Hansen says the other surprising discovery was the impact of the Martian wind.

"Scars of past avalanches were covered over in a single year by winds strong enough to move sand. That's a surprise because most of our climate models didn't predict strong winds and our experience with the landers at lower latitudes tells us winds are rare."

She says the winds indicate there's a lot going on weather-wise near the poles.
Different processes in the south

Hansen says the processes occurring in the Southern Hemisphere are different to those in the north.

"Things are different because there's less sand, most of which is confined to craters", she says. "Water ice solidifies the core of dunes and dry ice provides a seasonal cover."

"And the planet's highly elliptical orbit makes southern winters longer and colder, so seasonal ice covers the dunes for longer, allowing the grains to get bigger and more compacted."

Despite these differences, Hansen was surprised water ice has so little impact in the Northern Hemisphere.

"It's far less than originally thought and not enough to cement the dunes in place," she says.

Hansen plans to continue studying the dunes during several Martian years to see if there are any differences during that time.

"There appears to be a difference in the weather in a subsequent winter after a dust storm, so we want a longer time history to puzzle out those differences."

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