Constraints on bulk composition, seasonal variation, and global dynamics of Pluto's atmosphere

Abstract

Cosmic abundance, vapor pressure, and molecular weight considerations restrict the likely gas candidates for Pluto's atmosphere to Ne, N 2, CO, O 2, and Ar, in addition to the already detected CH 4. The vapor pressures and cosmic abundances of these gases indicate that all except Ne should be saturated in Pluto's atmosphere. The vapor pressure of Ne is so high that the existence of solid or liquid Ne on Pluto's surface is very unlikely; cosmic abundance arguments imply that Ne cannot attain saturation in Pluto's atmosphere. At both perihelion, N 2 should dominate the saturated gases. CO 2 should have the next highest mixing ratio, followed by O 2 and Ar. CH 4 should have the smallest mixing ratio. Because vapor pressures of these gases vary with temperature at diverse rates, the bulk and constituent mixing ratios of Pluto's atmosphere should vary with season. Between perihelion and aphelion, the column abundance of CH 4 may change by a factor of 260 while that of N 2 changes by only a factor of 52. The potential seasonal variation of Pluto's atmosphere was investigated by considering the behavior of these gases when individually mixed with CH 4. The effects of diurnal and latitudinal variation of insolation and eclipses on the atmosphere also were investigated. Seasonal effects are shown to dominate. It was shown that the atmospheric bulk may not be a minimum near aphelion but rather at intermediate distances from the Sun during summer/winter inadequate ice deposits may allow the atmosphere to collapse by freezing out over winter latitudes. If the atmosphere does not collapse, its weight is sufficient to keep it distributed uniformly around Pluto's surface. In this case, the atmosphere tends to regulate the surface temperature to a seasonally dependent value which is uniform over the globe. Finally, the likely global circulation regimes for each model atmosphere as a function of temperature were investigated and it was concluded that if CH 4, O 2, or CO dominates the atmosphere, Pluto will exhibit cyclic variations between an axially symmetric circulation system at perihelion and a baroclinic wave regime at aphelion. However, if N 2 dominates, as is likely, the wave regime should hold continuously. If the atmosphere collapses to a thin halo during summer/winter seasons, only a weak, symmetric circulation should occur.