A Piece of Mars: Martian spiders, or araneiforms, are geological structures found at high latitudes on Mars. The dark splotch with branching arms in this 0.48×0.27 km (0.3×0.17 mi) scene is a good example. They form in the springtime, when bright frost still covers a darker sandy soil, but some sunlight filters through the frost to warm the underlying surface. Sublimation of gas (under the frost but just above the soil) creates enough pressure that little explosions occur like dry geysers, punching through the frost and blowing up sand that then falls back to the surface as a dark splotch. If the wind is blowing when this happens, then the dark splotch is carried a ways downwind, but that hasn’t happened in this case. (HiRISE ESP_048189_0985, NASA/JPL/Univ. of Arizona)

from Welcome to Cosmic Diary bit.ly/2h7ndx5