Ice Tracks on Mars
This image taken by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter shows many linear gullies etched into the hillside on the surface of Mars. It was first thought that these gullies were formed by water in the ancient history of the planet, but unlike gullies caused by water flows on Earth there is no apron of debris at the bottom end of the channels. New research being conducted upon them has shown that they may be caused by chunks of dry ice (solid carbon dioxide) which floated down the hill on a layer of sublimating gas from the dry ice. The hypothesis is also supported by the fact that the gullies are commonly found in the areas of Mars where the ground is covered in carbon dioxide frost during the Martian winters.