The atmospheres of Titan and the Galilean satellites

The stability of various atmospheres on Titan and the Galilean satellites is examined relative to escape. One method consists of the comparison of exospheric temperature and a blow-off temperature defined for the various constituents. Another method consists of the evaluation of outflow based on a polytropic model. Hydrogen, methane, nitrogen, and neon atmospheres are examined. It is found that the Jovian satellites require heavier gases if they are to have any but very tenuous atmospheres, since all constituents examined are unstable. Nitrogen is stable on Titan, but methane and neon are marginally stable. Hydrogen is extremely unstable as a major constituent. As a minor constituent it is rapidly lost from Titan's thermosphere and must be replenished by outgassing or dissociation of a hydrogen compound, if the presently observed hydrogen content on Titan is not a transient occurrence. The rate of loss may be limited by diffusion, but could be augmented by vertical convection.