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 soluti on of N-2 and CH4 is described. Laboratory experiments show that rapid sublimation of a dilute (0.2% mole fraction) solid solution of CH4 in alpha-N-2 produces a residue of nearly pure CH4 grains. The CH4 grain s begin to form very quickly, and most of the CH4 originally in solid solution with the N-2 is taken up by the grainy residue rather than su blimating. If the same is true for the much slower sublimation rates o n Pluto, patches of nearly pure CH4 ice grains will be built up on sub -seasonal timescales. Such CH4 patches will be in contact with Pluto's predominantly N-2 atmosphere. Further sublimation of these patches wi ll be controlled by molecular and turbulent diffusion, as will be the condensation of CH4 from the atmosphere in other areas. It is shown th at the balance between diffusive sublimation and condensation can easi ly produce 1% mixing ratios of CH4 in the atmosphere, generally consis tent with requirements for explaining Pluto's 100 K upper-atmospheric temperature and producing a steep positive temperature gradient in the 2-3 mu b region. The same mechanism can explain Triton's less elevate d atmospheric CH4 mixing ratio. Copyright (C) 1996 Elsevier Science Lt d