EXPLORING THE MORPHOLOGY OF SIMPLE CRATERS THAT HOST POLAR DEPOSITS ON MERCURY: IMPLICATIONS FOR THE SOURCE AND STABILITY OF WATER ICE

More than 20 years ago, Earth-based radar images of Mercury’s polar regions revealed bright features, which led to the hypothesis that water ice is present at the poles. Comparison with Mariner 10 images indicated that these deposits are located within high-latitude impact craters, and thermal models indicated that such features should contain areas that are permanently shadowed. Images from the MESSENGER Mercury Dual Imaging System (MDIS) indicate that all radar-bright deposits within craters > 10 km in diameter collocate with regions of permanent shadow. Observations by the MESSENGER Neutron Spectrometer, Mercury Laser Altimeter (MLA), and thermal models incorporating MLA topography provide independent lines of evidence in support of the hypothesis that Mercury’s polar deposits consist primarily 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.