The veinous texture seen here is part of the south polar region on Mars, imaged by the HiRISE hi-resolution camera on the Mars Reconnaissance Orbiter. Receding carbon dioxide ice, melting during the ongoing spring months, creating the polygonal shapes separated by sinuous ridges. This is known as “spider” terrain.

The dark marks are caused by subsurface geysers. Gases built up by sublimating dry ice, heated by the sun, break through the frozen ground and scatter dark dust into the thin Martian polar air, staining the surface in whatever direction the winds were blowing at the time.

Of course, this hasn’t been proven (yet) but is the most accepted theory to date.

Image credit: NASA/JPL/University of Arizona