Permanent Shallow-Induced Relief Stabilization in Mercury’s North Pole: A History on the Origin of the Largest Cluster of Ice-filled Craters in the Innermost Solar System

Abstract:

Previous investigations show that Mercury’s North Pole contains permanently shadowed craters with large interior ice deposits [e.g., 1,2]. The distribution of these craters is unusual in that they form a tightly packed cluster. These closely-spaced craters are surrounded by broad plains with highly degraded craters and relatively few superposed craters. This plains unit also lacks the larger crater population present in the adjoining highlands.

We found evidence that the circum-polar plains and adjoining cratered regions formed in a crustal layer originally composed of volatile-rich materials that are not able to retain impact-induced relief. The apparent higher degree of crater obliteration in the plains areas is consistent with reduced topography due to significant sublimation from solar heating on steep sun-facing slopes within the cratered landscapes. In this hypothesis, the magnitude of volatile removal increases with the total yearly duration of solar illumination, explaining the transition from the cratered terrains to their enclosed circum-polar plains.

We propose that the permanent shadows in the polar craters could effectively hindered volatile-losses from the crater interior walls. If this hypothesis is correct, the proposed “thermal-armoring” would effectively stabilized rim-forming volatiles, at the pole, hence producing the north polar cluster of well defined, permanently shadowed craters. In the surrounding volatile plains at slightly lower circumpolar latitudes, where permanent shadows are rarer or absent, crater rims and walls would have collapsed due to sublimation.

We are currently developing thermal models to bound the types of volatile materials (and lag materials left after their sublimation) that could comprise the north polar region of Mercury and to assess whether potentially habitable temperature ranges might exist within the subsurface of the peripheries of the permanently shadowed craters. This thermal and compositional information, when integrated with knowledge of possible vapor diffusion from the craters’ interior water ice deposits, could provide further constraints on the distribution possible habitable environments on planets closet to their star.

[1] Paige, D.A. et al. 1992. Science 258, 643 – 646.

[2] Lawrence, D.J. et al. 2013. Science 338, 292 -296.