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.
On Mars these gullies can sometimes stretch for two kilometers. This led the team to Sand Hollow State Park, also in Utah, where the dunes are larger than the ones at Coral Pink. “We wanted to see how far the dry ice would go,” Diniega explains. In one run the ice block cleared the tallest dune in the park: 180 meters.
Whereas the experiments are not conclusive, the similarities between the tracks formed in Utah and those seen on Mars are remarkable. Furthermore, images from the HiRISE (High-Resolution Imaging Science Experiment) camera on the Mars Reconnaissance Orbiter show blocks sitting in some of the gullies. In later photos the blocks have disappeared—strong evidence that they are made of ice. “That was the smoking gun for me that this is really happening,” says Alfred McEwen, principle investigator for HiRISE and a co-author of the paper. McEwen wrote about other Martian puzzles in “Mars in Motion,” in the May 2013 issue of Scientific American.
Nicolas Mangold, one of the scientists who initially noticed linear gullies on Mars in 2002, is more cautious. Although agreeing that the results are fun and an interesting new look at gully formation, he is not completely convinced that ice is the cause. Mangold worries that we do not yet know whether there is enough CO2 on the surface to form large enough blocks. And the occasionally wavy shapes might be difficult to explain with ice.
But Diniega is not done with her investigations. Researchers know that new gullies form each spring. But they don’t know how quickly. Diniega wants to use HiRISE to acquire images more frequently to see if she can get a better handle on the timescales. Thermal data from THEMIS (Thermal Emission Imaging System) on the Mars Odyssey orbiter, now in its 12th year of exploration, will also reveal the CO2 temperature, which tells researchers how quickly the ice can sublimate and, in turn, whether it can generate enough force to slide down the hills.
Ultimately, Diniega would like to repeat the experiments in a more Martian-like environment. Utah is not Mars, of course. The Beehive State has higher temperatures, pressure and gravity. A mathematical model developed by the science team indicates that CO2 ice can sublimate and wander under Martian conditions. But getting data in such an environment will help tighten the argument. Repeating the experiment in controlled environment facilities where Martian pressure and temperature can be reproduced will be helpful. Simulating lower Martian gravity, however, will have to wait.
Fortunately, expensive equipment isn’t necessary to start thinking about alien environmental processes. “One of the fantastic things about this idea is that it’s so simple,” Diniega says. In fact, she worried that it was too simple: visit supermarket, buy ice, push ice down hill. Certainly someone must have tried that before. No one had. She sees this as a great opportunity for going beyond the lab and getting kids interested in science. The message for students is that science is not about fancy gadgets and complex algorithms, it’s about curiosity. “There is lots of simple science to be done,” she adds, “if you have a creative idea.”