H2O grain size and the amount of dust in Mars' residual North polar cap

In Mars' north polar cap the probable composition of material residual from the annual condensation cycle is a mixture of fine dust and H₂O grains of comparable size and abundance. However, metamorphism of such material will gradually lower its albedo by increasing the size of the H₂O grains only. If the cap is undergoing net annual sublimation (as inferred from water vapor observations), late summer observations should be of old ice with H₂O grain sizes of 100 μm or more. Ice of this granularity containing 30% fine dust has a reflectivity similar to that of dust alone; the observed albedo and computed ice grain size imply dust concentrations of 1 part per 1000 or less. The brightness of the icy areas conflicts with what would be expected for a residual cap deposited by an annual cycle similar to that observed by Viking and aged for thousands of years. The residual cap surface cannot be “old dirty” ice. It could be old, coarse, and clean; or it could be young, fine, and dirty. This brings into question both the source of the late summer water vapor and the formation rate of laminated terrain.