Mysterious dark sand dunes around Mars' northern polar cap are shifting with the seasons, as carbon dioxide gas changes form and sparks landscape-altering avalanches, said a study published on Thursday.
Images from NASA's Mars Reconnaissance Orbiter, which has been collecting data from the red planet for five years, have shown the unexpected shifts, said the research in the journal Science.
Noticeable changes can occur within the course of one year on Mars, which at 680 days is almost double the length of an Earth year, researchers said.
One major cause is the frozen carbon dioxide, or dry ice, that coats the area in the winter and changes to a gaseous form by the spring.
"This gas flow destabilises the sand on Mars' sand dunes, causing sand avalanches and creating new alcoves, gullies and sand aprons on Martian dunes," said lead author Candice Hansen of the Planetary Science Institute in Arizona.
"The level of erosion in just one Mars year was really astonishing. In some places hundreds of cubic yards (meters) of sand have avalanched down the face of the dunes," she said.
Another cause is "stronger-than-expected gusts of wind," said the study, which describes wind ripples in the dunes that appear to have been formed by gusts that come from multiple directions.
"There's lots of current activity in areas covered by seasonal carbon-dioxide frost, a process we don't see on Earth," said study co-author Alfred McEwen.
Mars's northern cap measures around 1,000 kilometers (600 miles) across, with layers of ice and dust stacked up to three kilometers (two miles) deep.