Modeling the formation of N2 and CH4 frost on Pluto’s slopes

Context. The equatorial region of Cthulhu, revealed by New Horizons, appears generally dark and largely devoid of volatiles due to its low-albedo surface. However, localized bright patches, interpreted as CH4 frost, are observed on crater rims and slopes. Aims. Previous studies suggested that these frosts could result from the peculiar insolation driven by the geometry of these slopes, but this has never been quantitatively tested. We aim to investigate the origin, stability, and potential role in Pluto’s volatile cycle of these localized frost deposits. Methods. We implemented a new sub-grid-scale slope parameterization in the Pluto Volatile Transport Model to account for solar irradiation and surface temperatures on sloped terrains, and to simulate the condensation/sublimation of volatiles together with the large-scale transport of N2 and CH4, which directly controls the stability of frost deposits. Results. Our simulations reproduce the observed CH4 frost on North-facing slopes as seasonal deposits currently sublimating, predict perennial CH4 frost on South-facing slopes, and show that slope microclimates should not alter global volatile cycles. Conclusions. Seasonal and perennial N2 and CH4 frosts can form across Pluto’s slopes, even in its darkest and warmest regions, due to the locally reduced sunlight received on inclined terrain. Nevertheless, despite Pluto’s abundance of sloped surfaces, slope microclimates appear to have only a minor effect on the planet’s global volatile cycles.