Characterizing the exchange of water between the Martian atmosphere and the (sub)surface is a major challenge for understanding the mechanisms that regulate the water cycle. Here we present a new data set of water ice detected on the Martian surface with the Thermal Emission Imaging System (THEMIS). The detection is based on the correlation between bright blue-white patterns in visible images and a temperature measured in the infrared that is too warm to be associated with CO 2 ice and interpreted instead as water ice. Using this method, we detect ice down to 21.4°S, 48.4°N, on pole-facing slopes at mid-latitudes, and on any surface orientation poleward of 45°latitude. Water ice observed with THEMIS is most likely seasonal rather than diurnal. Our data set is consistent with near-infrared frost detections and predictions by the Mars Planetary Climate Model. Water frost average temperature is 170 K, and the maximum temperature measured is 243 K, lower than the water ice melting point. Melting of pure water ice on the surface is unlikely due to cooling by latent heat during its sublimation. However, 243 THEMIS images show frosts that are hot enough to form brines if salts are present on the surface. The water vapor pressure at the surface, calculated from the ice temperature, indicates a dry atmosphere in early spring, during the recession of the CO 2 ice cap. The large amount of water vapor released by the sublimation of warm frost cannot stabilize subsurface ice at mid-latitudes.

Plain Language Summary During spring, parts of the Martian surface at mid and high latitudes are covered by a thin, bright layer of frost. Some of these ice deposits are made of CO 2 ice, but some frosts have a temperature that is too warm to be CO 2 frosts and are thus constituted of water ice that forms in winter and sublimes in spring. We conducted a spatial and temporal mapping of these ice deposits using the camera THEMIS onboard the Mars Odyssey orbiter to better characterize the exchange of volatiles between the Martian atmosphere and the surface. Water ice is preferentially detected on pole-facing slopes rather than flat surfaces below 45°latitude and all types of surfaces at higher latitudes. In late spring, water ice cannot melt despite solar heating because it cools down with the release of latent heat. On the other hand, these ice deposits are warm enough to form brines if salt crystals are present at the surface. When the frost sublimes, it serves as a source of water vapor that can diffuse into the ground and recondense to stabilize the permafrost below the surface. However, this effect is not sufficient to stabilize subsurface ice at low latitudes.