Abstract Permanently shadowed regions near Mercury's poles are thought to harbor significant deposits of water ice, the origin of which remains to be conclusively determined. One leading hypothesis is that most of the water observed today may have been delivered by a relatively recent, volatile‐rich impact comparable in scale and age to that which formed Hokusai, a prominent, rayed, northern‐hemisphere crater 97 km in diameter. In this work, we model the transport, loss and deposition of water on Mercury in two different scenarios. We first perform a baseline simulation to update older estimates of the efficiency of exospheric transport of water on Mercury, and find that in the optically thin limit, ∼3% of gravitationally bound water released at perihelion from a point source in Hokusai crater (57.7°N, 16.8°E) migrates to polar cold traps, primarily at the nearest pole. In contrast, we find that a volatile‐rich, Hokusai‐scale impact gives rise to a transient, optically thick atmosphere in which lower photolysis rates due to atmospheric self‐shielding allow for ∼31% of gravitationally bound water to be cold‐trapped and more evenly distributed between north and south polar regions. Cold‐trap deposition is largely completed within one solar day, and the total mass of water deposited is within the range estimated to be present at Mercury's poles today. However, simulation results also suggest that in order to emplace deposits that are sufficiently thick to be radar bright, a larger, slower impact than the 17 km diameter, 30 km/s scenario modeled here may be necessary.
Prem et al. (Fri,) studied this question.
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