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V. A. Krasnopolsky (CUA/NASA GSFC), D. P. Cruikshank (NASA Ames)
We assume that the observed bottom of Pluto's atmosphere at 1200 km is a tropopause, and a wet adiabatic troposphere extends down to the surface at rs =1164 km where ps = 26 \bar{\mu} and Ts = 38.6 K. This troposphere agrees the stellar occultation data with the observed ice temperature of 40±2 K. Then the measured CH4 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. The escape of CH4 is diffusion limited at 2.3x1026 s-1. Despite diffusive separation, CH4 is a minor species in the upper atmosphere reaching a mixing ratio of 0.1 at 2400 km. We adopt T = 92 K and [N2] = 1.4x1012 cm-3 at 1500 km from the stellar occultation data. The UV absorption by CH4 near 1600 km heats the atmosphere to 103 and 121 K at 1800 km at solar minimum and maximum, respectively. We developed an improved method for analytic solution of the equations for hydrodynamically escaping atmosphere which results in the N2 escape flux of (1.5 to 3)x1027 s-1 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.