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

TL;DR

Pluto's slope-induced insolation could drive localized frost of $N_2$ and $CH_4$, but its influence on the global volatile cycle is uncertain. The study implements a sub-grid slope parameterization within the Pluto Volatile Transport Model to resolve insolation, surface/subsurface temperatures, and condensation/sublimation of $N_2$ and $CH_4$ on slope surfaces, then compares outputs to New Horizons observations. Results reproduce $CH_4$ frost on North-facing slopes as seasonal features and predict perennial $CH_4$ frost on South-facing slopes, while $N_2$ frost forms on slopes but slope microclimates exert only a minor effect on global cycles. In the Cthulhu region, frost is expected to disappear from slopes and flats by ~2030, with southern latitudes gradually brightening; overall, the local microclimates explain some frost localization but large-scale climate controls dominate Pluto's volatile budget, and the approach provides a pathway to extend such analyses to other airless bodies.

Abstract

Context: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, which accounts for the specific solar irradiation and resulting surface and subsurface temperatures on sloped terrains. This parameterization also allows the condensation and sublimation of volatiles (either N2 or CH4) on slopes, including the effect of large-scale transport of these species, which is key to determining the amount of frost that forms or disappears. 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.

Figures (9)

• Figure 1: Annual mean solar irradiance (at the surface) for several slope angles $\theta$ and azimuths $\psi$ ($\psi$ = 0° corresponding to a slope oriented Northward) at a latitude of 70° N (black), 30° N (green), 15° N (red), and 0° (blue). The correlation between the annual mean solar irradiance and the projected slope $\mu$ for each latitude is given by the coefficient of determination R$^2$.
• Figure 2: Schema of the sub-grid scale slope parameterization. Each coarse mesh of the global Pluto Volatile Transport Model/Pluto Planetary Climate Model (left) is decomposed into North-South facing sub-grid slopes (defined by characteristic slopes $\mu_i$) (right) or flat terrain. Each of these sub-grid surfaces is associated with its cover fraction $\delta_i$, which represents the percentage of the mesh grid occupied by a given slope $\mu_i$ within the observed local topography (middle). These sub-grid terrains have their own microclimate (insolation, surface and sub-surface temperatures, condensation/sublimation of frosts, etc.), and the interactions between the atmosphere and surface are made through averaged values over the mesh. On the right, bluish surfaces mean colder slopes than flat surfaces (and warmer for reddish surfaces).
• Figure 3: A) Normalized distribution of projected slopes $\mu$ — where $\mu$ = 0° corresponds to flat and East-West-facing slopes — on Pluto (solid grey histogram), Mars Lange2023, and Triton Schenk2021. B) Cover fraction $\delta_i$ derived from panel A), obtained by rebinning the histogram into the $\mu_i$ bins defined in the main text, each with a width of $\pm$ 5°.
• Figure 4: CH$_4$ integrated LEISA band depth maps from Schmitt2017 and Gabasova2021, overlaid with LORRI albedo map Hofgartner2023. The white patches in the Cthulhu region, which are not colored, are considered to be CH$_4$ frost (see the main text for explanation).
• Figure 5: Normalized count of A) the slope angle, B) the slope azimuth, C) the projected slope $\mu$ where CH$_4$ frost is detected for all locations on Figure \ref{['fig:mapfrost']} (grey histogram) and only the Cthulhu region ($\pm$ 10° latitudes, blue histogram).