A model for the overabundance of methane in the atmospheres of Pluto and Triton

A model for producing atmospheric CH4 mixing ratios larger than would be expected from simple vapor pressure equilibrium over a solid solution of N2 and CH4 is described. Laboratory experiments show that rapid sublimation of a dilute (0.2% mole fraction) solid solution of CH4 in α-N2 produces a residue of nearly pure CH4 grains. The CH4 grains begin to form very quickly and most of the CH4 originally in solid solution with the N2 is taken up by the grainy residue rather than sublimating. If the same is true for the much slower sublimation rates on Pluto, patches of nearly pure CH4 ice grains will be built up on subseasonal timescales. Such CH4 patches will be in contact with Plutoʹs predommantly N2 atmosphere. Further sublimation of these patches will be controlled by molecular and turbulent diffusion, as will be the condensation of CH4 from the atmosphere in other areas. It is shown that the balance between diffusive sublimation and condensation can easily produce 1% mixing ratios of CH4 in the atmosphere, generally consistent with requirements for explaining Plutoʹs 100 K upper-atmospheric temperature and producing a steep positive temperature gradient in the 2–3 μb region. The same mechanism can explain Tritonʹs less elevated atmospheric CH4 mixing ratio.