Scientists have been surprised to discover that Martian sand dunes long thought to be static are in fact changing quite fast.
Dune fields covering an area the size of Texas near Mars' north polar cap seem to be moving, sometimes suddenly and sometimes gradually, the HiRISE camera on NASA's Mars Reconnaissance Orbiter (MRO) has shown.
Scientists had considered the dunes fairly static, shaped long ago when winds on the planet's surface were much stronger than today, says HiRISE deputy principal investigator Candice Hansen. "The numbers and magnitude of the changes have been really surprising," she says.
She believes the changes are caused by the seasonal coming and going of carbon-dioxide ice and stronger-than-expected gusts of wind.
A seasonal layer of frozen carbon dioxide blankets the region in winter and changes directly back to gaseous form in the spring.
"This gas flow destabilizes the sand on Mars' sand dunes, causing sand avalanches and creating new alcoves, gullies and sand aprons on Martian dunes," she says.
"The level of erosion in just one Mars year was really astonishing. In some places hundreds of cubic yards of sand have avalanched down the face of the dunes."
Especially surprising was the discovery that scars of past sand avalanches could be partially erased in just one Mars year. Models of Mars' atmosphere predict that wind speeds simply aren't adequate to lift sand grains, and data from Mars landers such as Phoenix show high winds are a rare occurrence.
"Perhaps polar weather is more conducive to high wind speeds," Hansen says.
"There's lots of current activity in areas covered by seasonal carbon-dioxide frost, a process we don't see on Earth," says HiRISE principal investigator Alfred McEwen.
"It's important to understand the current effects of this unfamiliar process so we don't falsely associate them with different conditions in the past."