N2 escape rates from Plutoʹs atmosphere

Hydrodynamic escape of N2 molecules from Plutoʹs atmosphere is calculated under the assumption of a high density, slow outflow expansion driven by solar EUV heating by N2 absorption, near-IR and UV heating by CH4 absorption, and CO cooling by rotational line emission as a function of solar activity. At 30 AU, the N2 escape rate varies from ( 4 − 6.4 ) × 10 26 molecules s −1 in the absence of heating, but driven by an upward thermal heat conduction flux from the stratosphere, for lower boundary temperatures varying from 70–100 K. With solar heating varying from solar minimum to solar maximum conditions and a calculated lower boundary temperature, 88.2 K, the N2 escape rate range is ( 1.8 − 6.7 ) × 10 27 molecules s −1 , respectively. LTE rotational line emission by CO reduces the net solar heat input by at most 35% and plays a minor role in lowering the calculated escape rates, but ensures that the lower boundary temperature can be calculated by radiative equilibrium with near-IR CH4 heating. While an upward thermal conduction heat flux at the lower boundary plays a fundamental role in the absence of heating, with solar heating it is downward at solar minimum, and is, at most, 13% of the integrated net heating rate over the range of solar activity. For the arrival of the New Horizons spacecraft at Pluto in July 2015, predictions are lower boundary temperature, T 0 ∼ 81 K , and N2 escape rate ∼ 2.2 × 10 27 molecules s −1 , and peak thermospheric temperature ∼ 103 K at 1890 km, based on expected solar medium conditions.