Water ice features on Mars. The ice is protected from the Martian atmosphere by thin layers of dust which prevents, or significantly slows down, the loss of ice through sublimation. The depressions are caused by some loss of ice and the surface polygonal patterns are indicators of shallow subsurface water ice as seen in cold regions on Earth, such as Alaska, northern Canada, and Siberia. Image obtained by the High Resolution Imaging Experiment (HiRISE) instrument on the Mars Reconnaissance Orbiter (MRO) which is in orbit around Mars.