LAUSR

Abstract Volatile-rich impacts on nominally airless solar system bodies such as the Moon can give rise to collisional, transient atmospheres, traces of which may be preserved at cold, permanently shadowed regions near the lunar poles. Understanding the physical mechanisms that influence gas dynamics and volatile transport in an impact-generated atmosphere is critical to interpreting the lunar polar volatile record. Here, we investigate the influence of radiative heat transfer on the structure of an impact-generated atmosphere, an issue that previous work has not addressed in detail. We develop a Monte Carlo radiative transfer model for the attenuation and absorption of thermal radiation by a three-dimensional, evolving, non-uniform, rarefied water vapor atmosphere, accounting for spontaneous emission at the molecular level, as well as radiation from the Sun and the lunar surface. This radiative transfer model is coupled to Direct Simulation Monte Carlo (DSMC) simulations of an impact-generated lunar atmosphere. We find that the trapping of heat by atmospheric water vapor causes characteristic atmospheric shock structures to become more diffuse than if the gas were modeled as transparent to radiation. Trapping of thermal radiation also allows infalling vapor to retain more energy, increasing the efficacy of transport of water vapor to the lunar night side by day side winds, and reducing the overall rate of volatile loss in the short-term. Cold trap deposition patterns are also affected. Accounting for radiative heat transfer also has consequences for temperature-sensitive physical and chemical processes in a collisional atmosphere – for instance, dense regions of the modeled impact-generated atmosphere remain sufficiently warm that condensation is unlikely.