Interplay between morphology and thermal conditions within the Permanent Shadowed Regions, North pole of Mercury.

Introduction: The polar regions of Mercury host Permanent Shadowed Regions (PSRs), which are constantly in shadow (due to the planet’s small obliquity, [1]), and can thus reach very low temperatures [2]. These conditions might have facilitated the long-term survival of water ice, as suggested in the 90s after the discovery of radar bright material within the PSRs by Earth-based observations [3]. This hypothesis was later confirmed through the comparison with the signal from icy satellites and Mars’s polar cap [4], hydrogen concentration on PSR measured from MESSENGER mission [5], and thermal models [6]. The morphological analysis of such craters in [7] highlights some interesting features within them that may be attributed to water ice. In this work, we show the results obtained by applying a shape-based thermophysical model [8] to 1) characterize the thermal environment; 2) predict the evolution of the analysed craters [7]; and 3) understand if the PSRs thermal conditions could affect the occurrence and evolution of specific landforms.

Method: Two different methodologies are used to explore the thermal behaviour of craters located in the north pole of Mercury. We start our evaluation from [7], focusing in specifying morphologies within Fuller crater (82.63°N, 42.65°W, diameter 25.9 km) and their potential thermal implications. Secondly, we performed a thermal analysis in order to evaluate the possible implication of the temperature on these morphologies. To investigate the thermal environment of the analyzed craters we applied a shape-based thermophysical model [8]. It employs ray tracing techniques and treats the Sun as a disk, reflecting the proximity of Mercury to the Sun and the resulting extensive coverage of the Sun in Mercury’s sky. By considering the Sun as a disk, this makes shadow and penumbra modeling more thorough and complete in terms of heat flux calculation. Indeed, the actual morphology of a crater strongly affects the final amount of insolation received from a geometry element.

Morphological analysis: The first type of morphologies concerns the fractures present in the PSRs within Fuller crater. These have been classified as “Landforms of Uncertain origin” by [7] because their morphogenesis cannot be unambiguously defined at the available resolution. One hypothesis explaining the formation of these landforms is that the subsurface ice could act as permafrost. For instance, on Earth, permafrost forms when ground is frozen for more than two years [9]. Previous studies (e.g., [6,11]) highlighted that craters with PSRs have a characteristic lag deposit on their floor, defined as dark carbon rich-material, representing the leftover of sublimated ice. This layer could act as a permafrost active layer, which flows and produces cracks on the surface, when assuming that the freeze cycle in penumbra areas is affected by heat transfer from the nearby permanently illuminated area [12]. Alternatively, we cannot exclude the possibility that fractures could instead form from the cooling of impact melt material and/or volcanic infilling. In fact, many of those kinds of fractures have been observed on Mercury [13].

A second characteristic feature is landslides, which can partially cover crater’s floor. Its peculiarity relies on being formed after crater formation and is referred as “rockslide [14]. Its occurrence can be explained by the thermal weathering rate, which may result in varying volumes of regolith that are subject to failure. The effectiveness of weathering may be reduced in the areas of the Hermean polar craters that are permanently shaded, causing the decrease of the probability of later landslides.

The third feature is the “Rough unit” [7] presents in the Fuller floor, placed just next to the central peak, rough and with a dark grey tone. It appears completely different from the smooth floor and the landslide material, thus it become really intriguing to investigate its origin.

Preliminary observations: The plot in Fig. 1 shows the max temperatures measured in 173 days, reached by different portion of the crater (the colored curves are the different profiles). From the graph, it is possible to notice:

• The part of the floor with fractures experiences a maximum temperature T of 200 K;
• The blue unit (Rough unit) reached the lowest T (around 150 K). This is intriguing because it corresponds to the area where the presence of radar-bright material has been observed. Therefore, it could represent a morphology very similar to terrestrial debris-covered glaciers;
• The surface affected by landslide undergoes significant temperature fluctuations, ranging from 300 K to 550 K within a distance of less than 10 km.

Conclusions and future works: We focused our attention on craters with PSRs in the north pole of Mercury. In particular, based on [7], we selected specific morphologies (e.g. fractures, landslides), whose origin and evolution could be connected to the specific temperature conditions of the hermean poles. The next step involves applying the thermal model by [8] to these specific structures, in order to assess the potential influence of thermal conditions on the evolution of those craters and the ice deposits within them.

Acknowledgements: The study has been supported by the Italian Space Agency (ASI-INAF agreement no. 2020-17-HH.0).

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