Mars has grooves in its rusty crust that appear to have been created by the motion of blocks of dry ice dragged along the surface. These features, known as linear gullies, were observed by the Mars Reconnaissance Orbiter (MRO).
The markings range in length from around 1,000 feet to a mile-and-a-half long. They are a few yards across on average, with banks of martian sand pushed up on each side.
The gullies form in areas where the ground is covered by frozen carbon dioxide during the winter months. The south pole on Mars can form a layer of dry ice over three feet thick in the winter. They appear to be created when blocks of frozen CO2, buoyed on a cushion of carbon dioxide gas, slide down sand dunes. As the sheets of frozen CO2 slide across the sand, they part the grains in the surface into lines. When they complete their journey, they create a pit, rather than the fan shape seen from water flows, like the Mississippi Delta. These pits are thought to be formed as the dry ice at the bottom of the hill sublimates (melts directly from a solid state to a gas). The process on Mars leaves a lot less debris at the bottom of such a path than is usually seen on Earth.
"Linear gullies don't look like gullies on Earth or other gullies on Mars, and this process wouldn't happen on Earth. We don't get blocks of dry ice on Earth unless you go buy them," Serina Diniega, a planetary scientist at Jet Propulsion Laboratory, said. Diniega is lead author of the paper announcing the results.
Researchers wanted to test whether blocks of dry ice could form tracks like those observed by the orbiter. So, they purchased several blocks of the frozen gas, and slid them down sand dunes in California and Utah. Sure enough, the blocks tested created marks similar enough to those on Mars to account for the phenomenon. The scientists even observed the dry ice in the tests creating a bed of CO2 gas on which to slide, like those hypothesized for Mars.
Images from MRO were taken with that craft's High Resolution Imaging Science Experiment (HiRISE) camera. The observatory also spotted several bright spots along the gullies, thought to be pieces of dry ice which broke off the sliding block of frozen gas.
"I have always dreamed of going to Mars. Now I dream of snowboarding down a Martian sand dune on a block of dry ice," Diniega said.
Observations of the Martian gullies were detailed in the journal Icarus.
