Modeling the formation of N_2 and CH_4 frost on the Pluto slopes

The equatorial region of Cthulhu as revealed by New Horizons appears to be generally dark and largely devoid of volatiles because its surface albedo is low. Localized bright patches, however, which are interpreted as CH_4 frost, are observed on crater rims and slopes. Previous studies suggested that these frosts might result from the peculiar insolation driven by the geometry of these slopes, but this has never been tested quantitatively. We investigated the origin, stability, and potential role of these localized frost deposits in the volatile cycle of Pluto. We implemented a new subgrid-scale slope parameterization in the volatile transport model for Pluto, which accounts for the specific solar irradiation and the resulting surface and subsurface temperatures on sloped terrains. This parameterization also allows the condensation and sublimation of volatiles (either N_2 or CH_4) on slopes, including the effect of large-scale transport of these species. This is key to determining the amount of frost that forms or disappears. Our simulations reproduce the observed CH_4 frost on north-facing slopes as seasonal deposits that currently sublimate, predict perennial CH_4 frost on south-facing slopes, and show that the slope microclimates are not expected to alter global volatile cycles. Seasonal and perennial N_2 and CH_4 frosts can form on the Pluto slopes, even in its darkest and warmest regions, because the locally sunlight received on inclined terrain is reduced. Despite the abundance of sloped surfaces on Pluto, the slope microclimates still only appear to have a minor effect on the global volatile cycles of the planet.