Tracks seen on Mars' dunes may have been formed by dry ice Several types of downhill flow features have been observed on Mars. This image from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter is an example of a type called "linear gullies." Unlike gullies caused by water-lubricated flows on Earth and possibly on Mars, they don't have aprons of debris at the downhill end of the channel. Credit: NASA/JPL-Caltech/Univ. of Arizona PASADENA, Calif., June 12 (UPI) -- Marks on Martian dunes may be tracks from hunks of dry ice that may have slid down the dunes similarly to a hovercraft, U.S. space researchers said. Researchers said this scenario may explain gullies seen on Mars' sand dunes through NASA's Mars Reconnaissance Orbiter, the government space organization said Tuesday in a release. The grooves, called linear gullies, have a relatively constant width with raised banks along the sides, NASA said. Unlike water-formed gullies on Earth -- and possibly on Mars -- they do not have debris at the downhill end. NASA researchers said many have pits at the downhill end. "In debris flows, you have water carrying sediment downhill, and the material eroded from the top is carried to the bottom and deposited as a fan-shaped apron," said Serina Diniega, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and lead author of a report published online by the journal Icarus. In the linear gullies, you're not transporting material. You're carving out a groove, pushing material to the sides. Images from MRO's High Resolution Imaging Science Experiment camera show sand dunes with linear gullies covered by carbon-dioxide frost during Mars' winter. By comparing before-and-after images from different seasons, researchers said they determined the grooves are formed during early spring. Researchers theorize the pits could result from blocks of dry ice morphing into carbon dioxide gas after they have stopped traveling, NASA said. "Linear gullies don't look like gullies on Earth or other gullies on Mars, and this process wouldn't happen on Earth," Diniega said. You don't get blocks of dry ice on Earth unless you go buy them.