| Auteur(s) supplémentaire(s) | Cremonese Gabriele, Pajola Maurizio, Lucchetti Alice, Cambianica Pamela, Simioni Emanuele, Martellato Elena, Massironi Matteo |
| Institution(s) supplémentaire(s) | Center of Studies and Activities for Space "G. Colombo" (CISAS), Padova; 2 INAF, Padova Astronomical Observatory, Padova, Italy; 3 Department of Geological Sciences, University of Padova, Padova, Italy |
Abstract | The Earth-based radar observations performed by both Goldstone and the Very Large Array in 1991 revealed high radar backscattered areas in the Mercury’s North polar terrains (Butler B. et al., 1993) which correspond to the permanently shadowed regions (PSRs) of impact craters. These radar-bright materials have been interpreted as water ice deposits due to their similarity with similar features observed on Martian polar ice caps. This interpretation is also supported by thermal models developed by Paige (1992), showing that the Hermean polar environment is capable of hosting stable water-ice deposits at geologic timescales within the near-surface layers. Furthermore, multiple datasets from the MESSENGER mission measured high hydrogen concentrations in these regions, providing additional support to the water ice nature of the radar-bright materials (Chabot N. et al,2018a)
The analysis of such craters can thus provide new insights into both the behavior of water ice deposits and the surface evolution. For this reason, we have investigated fourteen craters located at latitude over 80° N with diameters ranging from 17 to 45 km. We have firstly performed a high detailed geomorphological map of each impact crater using the highest resolution MDIS (Mercury Dual Imaging System) images. In particular we identified different areas that have the same characteristics of tone, texture and structure as Geological Units. In addition, we mapped the landforms associated with gravitational and impact morphogenesis. This analysis allowed to highlight the two following morphological trends: (i) eight craters characterized by the typical complex morphology, with central peaks and flat floors and landslides extending along the inner walls, dominated by terraces and trenches, and (ii) six craters lacking both terraced walls and central peaks, but with smoothed walls and flat floors. Successively, we focused on the crater’s retention age to derive the chronostratigraphic relationship between the studied impact structures. We performed crater counting with the purpose of dating both the ejecta and floor of each crater. The floor was subdivided into two areas, one corresponding to the bright radar deposits and the other without these deposits. A preliminary correlation between age and radar-bright material extent highlights that the oldest craters have more than 30% of the floor occupied by such deposit. Furthermore, the preliminary results outlined a general trend of rejuvenation of the crater floor with respect to the ejecta and, in particular, the part of the floor occupied by the radar appears to be younger. Such inhomogeneity of the floor age respect to the ejecta age could be related to morphological factors, such as resurfacing caused by landslides from the crater or thermal and illumination factors that could have led to a different deposition and persistence of water ice and, thus, different floor responses.
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