Some things are uniquely Martian. And dry ice hovercraft may be one of them.
For 10 years, scientists have been trying to understand what causes “linear gullies” on Mars—long, thin canals carved into sandy slopes and crater walls. Flowing water doesn’t quite work as an explanation. Water should carry rocks and dirt downstream, dumping it in a fan-shaped apron at the bottom. But these gullies don’t have debris deposits. They abruptly stop.
New gullies appear—and old ones grow—every spring as the winter ice thaws. And some researchers noticed that the sinuous, leveed ditches looked like trails carved by blocks sliding down the hills. That led astronomers to consider an intriguing possibility for the gullies’ origin: chunks of dry ice—frozen carbon dioxide—plowing through the sand while riding cushions of gas. But the idea had never been tested.
Researchers at NASA’s Jet Propulsion Laboratory (JPL) spent a couple of days in Coral Pink Sand Dunes State Park in Utah to see if ice, on its own, could carve out a furrow on the shallow slopes. Water ice—the same stuff you put in your lemonade—didn’t do much. Mostly, it just melted. The dry ice, however, came alive. “The dry ice behaved dramatically different,” says JPL systems engineer Serina Diniega, lead author on a paper in Icarus. “The water ice blocks didn’t move at all, but the dry ice just took off!” After a small nudge, the dry ice glided down the dune, blowing away sand as it did. When the block stopped, the sand beneath it writhed as jets of carbon dioxide escaped from under the ice. “We were surprised,” she adds.
Dry ice doesn’t melt; that’s what makes it “dry.” Instead, it changes from a solid to a gas via sublimation. And that’s the secret ingredient. The gas collects in a very thin, flat bubble under the block. Like a hovercraft, the gas cushion makes the ice highly mobile. Something similar happens when you flick water onto a hot pan: the bubbles dance around on miniature cushions of steam.
Frozen CO2 is prevalent on Mars. During the Martian winter the polar caps grow as carbon dioxide condenses out of the atmosphere. Ice sheets form a couple of meters thick. In the spring the ice breaks up. Blocks jutting out from overhangs and crater lips, weakened by warming temperatures, can break off and land on the shallow slopes below. The fall serves the same purpose as the nudge from the researchers—it gets the blocks going. From here, sublimation takes over and the ice rides its way to the bottom of the hill, carving out a groove as it goes.