Pluto's Atmosphere above 1500 km DPS Meeting, Madison, October 1998 Session 49. Then the measured CH 4 abundance corresponds to a mixing ratio of 0.006 which is close to that required by the thermal balance at 1200 to 1400 km.
Molecular diffusion prevents depletion of methane by photolysis.
Despite diffusive separation, CH 4 is a minor species in the upper atmosphere reaching a mixing ratio of 0.1 at 2400 km. 12 cm -3 at 1500 km from the stellar occultation data. The UV absorption by CH 4 near 1600 km heats the atmosphere to 103 and 121 K at 1800 km at solar minimum and maximum, respectively. Profiles of gas temperature, density, and velocity have been calculated between 1500 and 4000 km for various solar activity. The atmosphere becomes collisionless above 3000 to 4000 km which makes possible a close flyby of future spacecraft.