Abstract The detection of CH 4 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 CH 4 can be prevented by the presence of a heavy gas mixed with the CH 4 . The resulting slow escape (“Jeans escape”) of CH 4 can be accomodated by sublimation of the surface CH 4 frost so that an atmosphere exists in the steady state. A heavier gas must exist, otherwise the CH 4 frost would have sublimated away long ago because of solar heat and rapid blowoff of gaseous CH 4 . Pluto is currently near perihelion where the CH 4 component of the atmosphere may be 500 times denser than at apehelion. Significant seasonal changes in the atmospheric bulk are therefore possible. Currently, the CH 4 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.
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