Transport of water in a Transient, Impact-Generated Atmosphere on Mercury

Mercury's polar cold traps host water ice deposits that are likely populated with impact-delivered water via Mercury's exosphere. However, Mercury's near-sun location experiences an extremely high photodestruction rate that rapidly destroys water with a timescale of only 3.5 hours. Here we use the PLANET DSMC code to investigate the fate of water from a single 1 km radius comet impact striking Mercury's North Pole (30 km/s at angle of 60). We find that the evolving plume separates into four distinct phases: 1) an early plume phase in which ballistic escape and photodestruction reach their peaks, 2) a reentry phase in which water falling back toward the surface forms a self-shielded shock-topped atmosphere that migrates across the surface and ballistic loss ceases, 3) a quasi-steady phase in which a self-shielding dawn atmospheric enhancement (DAE) forms and drives, a tenuous migration of exospheric water to the cold traps with a longitudinal dependence, and finally 4) a late phase in which self-shielding ends and photodestruction dominates, effectively ending substantial water migration. In this work, we quantify the fates of the arriving water molecules, and describe some of the more important features of this highly unsteady, evolving three-dimensional atmosphere. We find that 23% of the initial water is photodestroyed, 65% of the water ballistically escapes the system (of which, 79% photodissociates prior to reaching the Hill radius), and 14% ends up in Mercury's cold traps, which is significantly more than the 5% that migrates to the Moon's cold traps during an equivalent impact.