These images, taken by the High Resolution Stereo Camera (HRSC) on board ESA’s Mars Express spacecraft, show a patch of water ice sitting on the floor of an unnamed crater near the Martian north pole.
This white patch is present all year round, as the temperature and pressure conditions do not favour the sublimation of water ice.
It cannot be frozen carbon dioxide since carbon dioxide ice had already disappeared from the north polar cap at the time the image was taken (late summer in the Martian northern hemisphere).
Source: ESA – Water ice in crater at Martian north pole
Ericson Report 