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.”