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New Horizons observations suggest that CH4 on Pluto has a complex history,involving reservoirs of different composition, thickness and stabilitycontrolled by volatile processes occurring on different timescales. In order tointerpret these observations, we use a Pluto volatile transport model able tosimulate the cycles of N2 and CH4 ices over millions of years. By assumingfixed solid mixing ratios, we explore how changes in surface albedos,emissivities and thermal inertias impact volatile transport. This work istherefore a direct and natural continuation of the work by Bertrand et al.(2018), which only explored the N2 cycles. Results show that bright CH4deposits can create cold traps for N2 ice outside Sputnik Planitia, leading toa strong coupling between the N2 and CH4 cycles. Depending on the assumedalbedo for CH4 ice, the model predicts CH4 ice accumulation (1) at the sameequatorial latitudes where the Bladed Terrain Deposits are observed, supportingthe idea that these CH4-rich deposits are massive and perennial, or (2) atmid-latitudes (25{\deg}N-70{\deg}N), forming a thick mantle which is consistentwith New Horizons observations. In our simulations, both CH4 ice reservoirs arenot in an equilibrium state and either one can dominate the other over longtimescales, depending on the assumptions made for the CH4 albedo. This suggeststhat long-term volatile transport exists between the observed reservoirs. Themodel also reproduces the formation of N2 deposits at mid-latitudes and in theequatorial depressions surrounding the Bladed Terrain, as observed by NewHorizons. At the poles, only seasonal CH4 and N2 deposits are obtained inPluto's current orbital configuration. Finally, we show that Pluto's atmospherealways contained, over the last astronomical cycles, enough gaseous CH4 toabsorb most of the incoming Lyman-flux.

The CH4 cycles on Pluto over seasonal and astronomical timescales