Hydrodynamic flow of N₂ from Pluto

A new method of analytic solution for the equation of hydrodynamic flow from an atmosphere has been developed. This method conserves a term neglected in the equation for the McNutt [1989] approximation and significantly improves uncertainty of the solution. Structure of Pluto's upper atmosphere near perihelion was calculated by this method at various solar activity and at various methane mixing ratios. Both the UV absorption by CH 4 at 1000–1400 Å and the EUV absorption by N 2 heat the upper atmosphere with the global‐mean rates of 1.4 × 10 −3 and 2.9 × 10 −4 ergs cm −2 s −1 , respectively, at mean solar activity. Hydrodynamic outflow of N 2 from Pluto at perihelion is equal to (2.0–2.6)×10 27 s −1 at mean solar activity and varies by a factor of 3–4 from solar minimum to solar maximum. The exobase lies below the sonic level. Pluto's atmosphere does not completely fit both the conditions of static and hydrodynamically escaping atmospheres and is an intermediate case of a slow hydrodynamic escape. The total loss for the age of the solar system corresponds to an ice layer that is 500 m thick and 5 × 10 −4 of Pluto's mass. Pluto's atmosphere is restricted to 3000–4500 km, which makes possible a close flyby of future spacecraft.