N.sub.2 escape rates from Pluto's atmosphere

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.08.021 Byline: Darrell F. Strobel Keywords: Pluto; Pluto; atmosphere; Aeronomy; Atmospheres; dynamics; Atmospheres; structure Abstract: Hydrodynamic escape of N.sub.2 molecules from Pluto's atmosphere is calculated under the assumption of a high density, slow outflow expansion driven by solar EUV heating by N.sub.2 absorption, near-IR and UV heating by CH.sub.4 absorption, and CO cooling by rotational line emission as a function of solar activity. At 30 AU, the N.sub.2 escape rate varies from (4-6.4)x10.sup.26 moleculess.sup.-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 N.sub.2 escape rate range is (1.8-6.7)x10.sup.27 moleculess.sup.-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 CH.sub.4 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.sub.0[approximately equal to]81 K, and N.sub.2 escape rate [approximately equal to]2.2x10.sup.27 moleculess.sup.-1, and peak thermospheric temperature [approximately equal to]103 K at 1890 km, based on expected solar medium conditions. Author Affiliation: Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218-2687, USA Article History: Received 21 March 2007; Revised 20 July 2007