The Transformation of the Lunar Exosphere by a Comet Impact

Abstract:

Several observations suggest that water and other volatiles delivered by past comet impacts may have migrated to permanently shadowed regions near the poles of the Moon and Mercury, where they may still remain cold-trapped. Here, we compare the nature of gas transport and loss processes in a collisionless exosphere to the situation after a lunar comet impact. The sheer quantity of volatiles delivered by a comet can transform the way in which gas transport and deposition occur; we model this using the Direct Simulation Monte Carlo method. It is observed that post-impact gas transport occurs primarily through low-altitude winds that sweep over the lunar day-side, as opposed to migration of molecules through ballistic hops. The resultant deposition patterns are markedly non-uniform, with preferential redistribution of water antipodal to the point of impact, and short-term variations in simulated volatile abundance between different cold traps, suggesting that a non-uniform delivery mechanism could contribute towards the non-uniform distribution of lunar polar volatiles observed in remote sensing datasets. Due to the amount of vapor that remains gravitationally bound to the Moon, the transient, impact-generated atmosphere is initially sufficiently dense that lower layers are shielded from photodestruction, prolonging the lifetime of water molecules and allowing greater amounts of water to reach cold traps. Other physical processes, such as radiative heat transfer and photochemical reactions, also operate differently when the exosphere becomes collisionally and optically thick. The longevity of the transient atmosphere and the significance of the phenomena mentioned above depend on specific impact parameters, as well as the gravitational and solar environment of the body in question - for instance, Mercury could retain a thicker atmosphere, but sees much more intense ultraviolet radiation than the Moon. However, the characteristic structure of the atmosphere and the nature of volatile transport, loss and sequestration should be similar whenever a nominally airless body holds a significant quantity of vapor gravitationally bound after an impact, thus temporarily transforming its exosphere, with implications for the abundance and distribution of volatiles today.