| Auteur(s) supplémentaire(s) | Nancy L. Chabot, Colin D. Hamill, Michael K. Barker, Erwan Mazarico, Matthew A. Siegler, Jose M. Martinez Camacho, Stefano Bertone, Ariel N. Deutsch |
| Institution(s) supplémentaire(s) | JHU APL, Purdue University, NASA Goddard Space Flight Center, Planetary Science Institute, Southern Methodist University, University of Maryland Baltimore County, NASA Ames Research Center |
Abstract | Earth-based radar observations by Goldstone and Arecibo Observatory revealed radar-bright features in Mercury's polar regions that have been interpreted as evidence for water-ice deposits. Following these observations, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft collected a myriad of evidence in its time orbiting Mercury that confirmed the hypothesis that the radar-bright deposits consist primarily of water-ice. MESSENGER data were then used to identify Permanently Shadowed Regions (PSRs) in Mercury’s north polar region and model their thermal environments. Such models indicated that the radar-bright regions correspond to extensive PSRs in the northernmost craters on Mercury, which have thermal environments conducive to the presence of exposed water-ice at the surface. However, detailed illumination and thermal studies of these northernmost craters have been impeded by the limited topographic data acquired by MESSENGER’s Mercury Laser Altimeter (MLA) within 5° of the north pole.
In this study, we constructed local high-resolution (125 m/pixel) digital elevation models (DEMs) for four of the largest northernmost craters, Kandinsky (60 km), Chesterton (37 km), Tolkien (50 km), and Tryggvadottir (31 km), using MLA data in conjunction with the shape-from-shading techniques based on Mercury Dual Imaging System (MDIS) images collected by MESSENGER. These DEMs were then leveraged to create high-resolution illumination and thermal models. The illumination models mapped out accurate PSRs for each crater and informed how scattered light reflecting off the topography could be responsible for brightness variations observed in MDIS images (rather than variations in volatile composition). Our high-resolution thermal models then indicated the maximum and average surface temperatures over a Mercury solar day, from which we inferred the depth at which various volatiles would be stable.
Using these new high-resolution models, we investigated where the models predict that ice or volatile organic compounds are stable at the surface in these craters. Previous work concluded that coronene is one appropriate volatile to model the low-reflectance surfaces observed within many of Mercury’s PSRs that have thermal conditions too warm for the presence of water-ice at the surface. The predictions of our thermal models were then compared against the MLA and MDIS data to search for evidence of ice or low-reflectance volatiles at the surface. Preliminary MLA results agree with previous findings that these craters have a surface reflectance that is much brighter than the average reflectance of Mercury, suggesting the presence of water-ice at the surface. Preliminary comparisons to the limited MDIS images that successfully reveal the surface features within the PSRs are so far inconclusive to verify the presence of high-reflectance surfaces due to water-ice or low-reflectance surfaces due to complex organic compounds. |
