The news of the suc­cessful landing of the NASA space­craft “Phoe­nix” on Mars on 25 May 2008 went around the world. Now, images have been published that show sur­face struc­tures that resem­ble frost pat­ter­ned ground which is com­mon in the Arc­tic. The stone rings shown in the NASA images are not per­ma­frost struc­tures: their deve­lo­p­ment does not requi­re per­ma­frost, but fre­quent and strong free­zing and tha­wing. But more important­ly, they requi­re water/ice for their for­ma­ti­on. 
The NASA images.

Source: NASA