Widespread pseudo-perennial water ice patches at high northern latitudes on Mars

The distribution and stability state of Martian water ice deposits are of great interest for understanding recent climate history. Perennial surface ice deposits are rare, but in many regions ice occurs in the subsurface. We observe that summertime meter-scale bright patches are widespread on the plains around the North Polar Layered Deposits. These patches can persist long after disappearance of most seasonal frost but are variable both spatially and year-to-year. These are interpreted as persistent water frost under conditions very near those for perennial surface ice stability. Near this stability point, summer sublimation amounts should be very sensitive to small differences in albedo and thermal inertia, allowing significant variation in the survival time of the patches. Conditions favorable for such pseudo-perennial surface ice are also favorable for current or recent subsurface accumulation. This suggests that the north polar region may be a depositional sink for unstable ice inferred to be receding elsewhere on the planet.