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On Mars, Shifting Sand Dunes Surprise Scientists

A sand dune on Mars changes over the course of two seasons. (Image courtesy Science/AAAS).

A Texas-sized field of sand dunes around Mars’s north pole — long thought to be a static, unchanging remnant of the planet’s more active geological past — is actually still shifting with the wind and seasons, according to studies of new NASA images.

Scientists have known about the dunes for decades, but had long thought that unlike sand dunes on Earth, these dunes remained frozen and still.

“Because we had all these years of data where no one saw any changes, people developed theories — the dunes are cemented by ice, maybe they’re crusted over — theories for why they were not changing,” Candice Hansen, of the Planetary Science Institute in Tucson, Ariz., told Wired Science. “In fact, they were probably changing all along, and we just didn’t have instruments that were good enough to see it.”

Hansen and her colleagues analyzed two Martian years (about four Earth years) worth of new images of the dunes, from the HiRise camera on NASA’s Mars Reconnaissance Orbiter.

In a paper published Thursday in the journal Science, they explain that the shifting dunes could be explained by seasonal changes. Each winter, carbon dioxide in the atmosphere freezes atop the dunes, creating a layer of thin dry ice. In the spring, the dry ice evaporates into carbon dioxide again, and the resulting gas bubbles could trigger sand avalanches.

Other images suggest that some smaller changes could be driven by wind, surprising scientists who believed that the wind on Mars wouldn’t be strong enough to lift sand particles.

“We’ve got to look at Mars as a place that has active geology in today’s climate, not just sometime in the past,” Hansen told Space.com.

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