EXPLORING THE MORPHOLOGY OF SIMPLE CRATERS THAT HOST POLAR DEPOSITS ON MERCURY: IMPLICATIONS FOR THE SOURCE AND STABILITY OF WATER ICE
Pre-MESSENGER thermal models suggested that long-lived water ice would not be stable in idealized (a ratio of depth d to diameter D of 0.2), bowl-shaped craters <10 km in diameter located more than 2° of latitude from the poles, even if covered by regolith. Yet MESSENGER results show that radar-bright deposits collocate with numerous small craters that are located substantially farther than 2° from the poles. The presence of polar deposits within these small craters could indicate that the water ice on Mercury was relatively recently emplaced; however, if these host craters are shallower than assumed for earlier calculations, thermal models indicate interior thermal environment could potentially be more favorable to the retention of water ice on geologic timescales. Determining the geometry of small polar craters that host polar deposits offers the potential for special insight into the source, nature, emplacement timeframe, and migration of the radar-bright material at Mercury’s polar regions. With data from MDIS and MLA, we characterized the shapes of all craters with diameters between five and ten km in diameter within a region south of Prokofiev crater (75°–85°N, 30°–90°E). Of the 91 craters examined, 77 (or 85%) host radar-bright material. Preliminary results suggest that these craters are shallower (average d/D of 0.12) than assumed for earlier thermal models, and that there is no substantial difference between the shapes of craters that do and do not host radar-bright deposits.