The CH ₄ cycles on Pluto over seasonal and astronomical timescales

Pluto's surface is covered in numerous CH ₄ ice deposits, that vary in texture and brightness, as revealed by the New Horizons spacecraft as it flew by Pluto in July 2015. These observations suggest that CH ₄ on Pluto has a complex history, involving reservoirs of different composition, thickness and stability controlled by volatile processes occurring on different timescales. In order to interpret these observations, we use a Pluto volatile transport model able to simulate the cycles of N ₂ and CH ₄ ices over millions of years. By assuming fixed solid mixing ratios, we explore how changes in surface albedos, emissivities and thermal inertias impact volatile transport. This work is therefore a direct and natural continuation of the work by Bertrand et al. (2018), which only explored the N ₂ cycles. Results show that bright CH ₄ deposits can create cold traps for N ₂ ice outside Sputnik Planitia, leading to a strong coupling between the N ₂ and CH ₄ cycles. Depending on the assumed albedo for CH ₄ ice, the model predicts CH ₄ ice accumulation (1) at the same equatorial latitudes where the Bladed Terrain Deposits are observed, supporting the idea that these CH ₄ -rich deposits are massive and perennial, or (2) at mid-latitudes (25°− 70°), forming a thick mantle which is consistent with New Horizons observations. In our simulations, both CH ₄ ice reservoirs are not in an equilibrium state and either one can dominate the other over long timescales, depending on the assumptions made for the CH ₄ albedo. This suggests that long-term volatile transport exists between the observed reservoirs. The model also reproduces the formation of N ₂ deposits at mid-latitudes and in the equatorial depressions surrounding the Bladed Terrain Deposits, as observed by New Horizons. At the poles, only seasonal CH ₄ and N ₂ deposits are obtained in Pluto's current orbital configuration. Finally, we show that Pluto's atmosphere always contained, over the last astronomical cycles, enough gaseous CH ₄ to absorb most of the incoming Lyman-α flux.