The Relative Cold-trapping Likelihood of Water on Airless Rocky Bodies

Abstract Many planetoids are airless, but despite the lack of a collisional atmosphere, these bodies can experience volatile migration toward the polar regions via the exosphere. However, there is a lack of parametrical studies that focus on cold-trapping as a general planetary process, as opposed to within the context of specific bodies. Therefore, we run a Monte Carlo simulation to determine the water cold-trapping likelihood for planetoids of varying size, bulk density, and insolation. As expected, small planets lose most of their water via thermal escape, while large planets favour photolysis. For a given bulk density and solar irradiation, we find the optimal planetary size where cold-trapping is most likely. Surprisingly, the Moon has the ideal size and instellation parameters to maximally cold-trap water in our model. We also find that the ideal size for cold-trapping changes very little across a wide range of cold-trap area coverage, planetary rotation, and photodissociation rate relative to bolometric flux.