The density and thermal structure of Pluto’s atmosphere and associated escape processes and rates

Abstract The original Strobel et al. (Strobel, D.F., Zhu, X., Summers, M.E., Stevens, M.E. [1996]. Icarus 120, 266–289) model for Pluto’s stratospheric density and thermal structure is augmented to include a radial momentum equation with radial velocity associated with atmospheric escape of N2 and in the energy equation to also include the solar far ultraviolet and extreme ultraviolet (FUV–EUV) heating in the upper atmosphere and adiabatic cooling due to hydrodynamic expansion. The inclusion of radial velocity introduces important negative feedback processes such as increased solar heating leading to enhanced escape rate and higher radial velocity with stronger adiabatic cooling in the upper atmosphere accompanied by reduced temperature. The coupled set of equations for mass, momentum, and energy are solved subject to two types of upper boundary conditions that represent two different descriptions of atmospheric escape: Jeans escape and hydrodynamic escape. For the former which is physically correct, an enhanced Jeans escape rate is prescribed at the exobase and parameterized according to the direct simulation Monte Carlo kinetic model results. For the latter, the atmosphere is assumed to remain a fluid to infinity with the escape rate determined by the temperature and density at the transonic point subject to vanishing temperature and pressure at infinity. For Pluto, the two escape descriptions approach the same limit when the exobase coincides with the transonic level and merge to a common escape rate ∼1028 N2 s−1 under elevated energy input. For Pluto’s current atmosphere, the hydrodynamic approach underestimates the escape rate by about 13%. In all cases, the escape rate is limited by the solar FUV–EUV power input. Specific results for the New Horizons Pluto flyby July 2015 are escape rate ∼3.5 × 1027 N2 s−1, exobase at 8r0 ∼ 9600 km, with Jeans λ ∼ 5 for a reference Pluto atmosphere model. With Pluto’s highly elliptic orbit and variable solar activity affecting its atmosphere, Pluto’s escape rates’ range is (1–10) × 1027 N2 s−1, exobase radius is bounded by ∼(5–13)r0, and at the exobase Pluto is locked in the enhanced Jeans regime with λ ∼ (6–4). Finally, a systematic review of previous approximate hydrodynamic escape models is presented to compare the constraints which determine the escape rate in each model.