Polar Ice on Mercury: Insight into Delivery Timing and Sources from Crater and Poisson Analyses

The poles of Mercury show evidence for large expanses of water ice covering an estimated 2500 m² [1]. Earth-based radar observations suggest the ice is 95 wt. % pure water ice [2], and regolith gardening models suggest that the ice must have been delivered in the last 50 Myr in order to maintain this purity [3]. Images [4] and reflectance measurements [5] acquired by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft showed that these deposits are spatially homogeneous within permanently shadowed regions (PSRs).

Using images acquired by the Wide-Angle Camera during MESSENGER's low-altitude campaign [6], we identify small craters in the PSRs that are correlated with high reflectance, suggestive of excavated material. If these small craters superpose the ice deposit, then they can be used to date the ice surface. We estimate the absolute ages of individual ice deposits using crater-counting statistics [7] and Poisson timing analyses [8], and find that the majority of ice was likely delivered very recently during the Kuiperian period ( 100 Ma). Here we discuss these results and implications for ice-delivery mechanisms on Mercury.

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