Eek, spiders! All right, so it's not actually little green arachnids we're talking about here, but they are definitely spidery features. Called araneiform terrain, these clusters of radially-branching cracks in Mars' south polar surface are the result of the progressing spring season, when warmer temperatures thaw subsurface CO2 ice.
As dry ice below the surface warms it can sublimate rapidly and burst through the frozen ground above, creating long cracks. If the material below is dark it can be carried upwards by the escaping gas, staining the surface.
Each dark splotch is around 100 meters wide.
This image was acquired by the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter on September 26, from a distance of 262 km (163.8 miles). See the full-size scan here, and check out more recent HiRISE images in the November PDS release here.
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