Pluto's atmospheric bulk near perihelion

The detection of CH₄ frost on Pluto's surface implies a significant atmosphere for Pluto. Although Pluto's mass is small, about 7% of Triton's mass, the rapid escape (“blowoff”) of gaseous CH₄ can be prevented by the presence of a heavy gas mixed with the CH₄. The resulting slow escape (“Jeans escape”) of CH₄ can be accomodated by sublimation of the surface CH₄ frost so that an atmosphere exists in the steady state. A heavier gas must exist, otherwise the CH₄ frost would have sublimated away long ago because of solar heat and rapid blowoff of gaseous CH₄. Pluto is currently near perihelion where the CH₄ component of the atmosphere may be 500 times denser than at apehelion. Significant seasonal changes in the atmospheric bulk are therefore possible. Currently, the CH₄ column abundance on Pluto's sunlit hemisphere is on the order of 1 m-Amagat. The abundance of the heavier gas should be at least an order of magnitude greater but is uncertain.