SAN FRANCISCO, CALIFORNIA—After the Mars Reconnaissance Orbiter began beaming back close-up images of the Red Planet, researchers spotted peculiar features along the slopes of dunes: long, sharply defined grooves (pictured) that seem to appear and disappear seasonally. They look like trails left behind by tumbling boulders, but rocks never appear in the sunken pits at the trail ends. Researchers initially took these gullies as signs of flowing liquid water, but a new model suggests they’re the result of sand-surfing dry ice. During the martian winter, carbon dioxide ice freezes over parts of the planet’s surface and sublimates back into a gas during the spring thaw. But according to the model presented here today at a meeting of the American Geophysical Union, chunks of warming dry ice may also break off from the crests of dunes and skid down slopes. This is no ordinary tumble—according to the model, the bases of the chunks are continually sublimating, resulting in a hovercraftlike motion that gouges the dune while propelling the ice down slopes. Solid ice that survives to the bottom settles into a pit before dissipating back into the atmosphere. Experiments dropping dry ice and water ice onto dunes in Utah show that dry ice is up to the task, at least on Earth. While researchers continue to hone their model, they hope to eventually spot sand-surfing ice in action on Mars.