The northern dry ice and water ice caps of Mars, in the winter.

Scientists using data from images taken by Mars Reconnaissance Orbiter (MRO) have found [pdf] they can tell if Martian sand dunes are coated with either water ice (H2O) or dry ice (CO2), depending on the features seen on the dunes. From the paper’s abstract:

We found that dune ice with both a high surface brightness and sublimation spots or cracks corresponded to CO2 ice, whereas dunes that exhibited increased surface brightness without sublimation spots or cracks corresponded to H2O ice. These interpretations are corroborated by prior seasonal ice cap mapping results, surface temperature observations, and the position of spectrally derived seasonal ice cap edges, despite orders-of-magnitude differences in data set resolution.

This data will help scientists better map the existence of near surface water ice on Mars. To me however the biggest take-away from this paper is its figure 3, shown to the right, annotated to post here. It looks down at the north pole, and shows the farthest extent of the caps during the winter, both water ice and the overlying winter mantle of dry ice that sublimates away every summer. The water ice cap also shrinks considerably in summer, but based on this data, during the fall and early winter months the layer of snow reaches latitudes as low as 50 degrees.

This is more evidence that future Martian colonists will not lack for water. More important, this winter water ice will require almost no processing. Catch it as it condenses on the ground, likely as distilled as rain water.