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Modeling the Deposition of Mercury’s Polar Water Ice

Presentation #116.05 in the session Mercury (Poster + Lightning Talk)

Published onOct 23, 2023
Modeling the Deposition of Mercury’s Polar Water Ice

Permanently shadowed regions near the poles of Mercury are thought to contain substantial amounts of water ice, and potentially other volatiles. Several lines of evidence (such as the high radar reflectivity of the polar deposits, and the presence of overlying dark, perhaps organic, material in some regions) suggest that volatile-rich asteroid or comet impacts may have played a significant role in delivering water to the innermost planet. Reconstructing the geological history of Mercury’s polar deposits involves multiple lines of investigation; here, we focus on understanding the transport and deposition of water following a volatile-rich impact. In particular, we consider an impact of the scale of that which formed Hokusai (a large, relatively young, northern-hemisphere crater) and perform numerical simulations of the transport of impact-generated water vapor to polar cold traps, using a Direct Simulation Monte Carlo (DSMC) rarefied gas dynamics code. Assuming representative impact parameters, we find that the fallback of gravitationally bound water leads to near-surface gas densities that are many orders of magnitude greater than typical exospheric densities on Mercury, such that the migration of water vapor is driven by pressure gradients rather than collisionless, ballistic hops. Short-term simulations indicate that water begins to accumulate in the nearest cold traps within minutes after impact, and that the impact-generated vapor cloud is optically thick to solar ultraviolet radiation, protecting water molecules from photolysis and temporarily increasing the efficiency of transport to cold traps during the initial hours after impact. Long-term simulations (currently in progress) are required to determine the total fraction of impact-delivered water that ultimately accumulates in the polar regions over the course of the long solar day on Mercury. Other questions of interest include how long it takes for the impact-generated vapor to become collisionless and optically thin, and the resulting distribution of water between the north and south polar regions. We will present the results of long-term simulations, and discuss broader implications for the origins of Mercury’s polar water ice deposits.

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