Surface Martienne photographie par la sonde MRO (Mars Reconnaissance Orbiter) - Hirise. Strange Days on Dunes in Russell Crater. There are very large dunes sitting in Russell crater. The dune slip faces here are very odd—covered by long narrow channels. Every late winter through early spring they exhibit strange behavior: dark spots form, concentrated along the channels, and dark streaks grow downhill from the dune crest. When the frost is gone, we see that new small channels have formed. The simplest model for this is that the dynamic sublimation of carbon dioxide frost triggers sand flows on steep slopes, but exactly how this works is not fully understood.