Description of how freeze/thaw cycles form polygons.
http://www.crrel.usace.army.mil/permafrosttunnel/1g3g_Wedge_Formation.htm
The first close-up color image of the northern arctic region on Mars was taken by the Mars Phoenix Lander about two hours after its arrival on the Red Planet May 25. Visible is polygonal structure, which may indicate regions where water ice lies just below the surface. Similar polygons are found in Earth’s polar regions, where they typically form from cycles of freezing and thawing.
http://www.sciencenews.org/view/generic/id/32580/title/Touchdown%21_Phoenix_lands_on_Mars