A two-component volatile atmosphere for Pluto. I - The bulk hydrodynamic escape regime

Abstract

We have formulated the seasonal behavior of Pluto's atmosphere and surface deposit for two volatile components which have largely disparate saturation vapor pressures in their pure states; i.e., CH4 and N2. Simplifying approximations are introduced in order to model the seasonal variation resulting from the variable insolation of a two-component solid solution in equilibrium with its gases. Neglect of the seasonal variation altogether leads to analytic expressions for the CH4 atmospheric mixing ratio and mole fraction of the surface volatile ice in terms of the CH4 mole fraction of Pluto's primordial volatile reservoir. We assume that Pluto's atmosphere is hydrodynamically escaping in bulk and investigate the compositional regime over which bulk escape is valid. We find that bulk escape occurs for CH4 mole fractions of Pluto's volatile reservoir (Xc0) less than 0.07. Greater CH4 abundance leads to diffusive separation during the escape of both species and an atmospheric mixing ratio of ∼Xc0/15. If Xc0 is in the range 0.02-0.10, we find the surprising result that Pluto's atmosphere remains largely intact at aphelion rather than virtually freezing out as it does for Xc0 > 0.3 or <0.001, or for an atmosphere with only a single volatile gas. An upper limit to the CH4 mixing ratio is ∼0.07 if N2 is the second gas. On the other hand, CH4 is the dominant surface constituent of the volatile deposit provided Xc0 > 10-4. The composition at the surface of the volatile deposit or exposed reservoir is generally different from the rest of the solid, because it is controlled by the atmosphere. Pluto's seasonal variation is sufficiently sensitive to the composition of Pluto's volatile reservoir for post-perihelion ground-based spectrophotometry to provide important information on the CH4 mole fraction of this reservoir.