On the vertical thermal structure of Pluto's atmosphere

Abstract

A radiative-conductive model for the vertical thermal structure of Pluto's atmosphere is developed with a non-LTE treatment of solar heating in the CH4 3.3 μm and 2.3 μm bands, non-LTE radiative exchange and cooling in the CH4 7.6 μm band, and LTE cooling by CO rotational line emission. The model includes the effects of opacity and vibrational energy transfer in the CH4 molecule. Partial thermalization of absorbed solar radiation in the CH4 3.3 and 2.3 μm bands by rapid vibrational energy transfer from the stretch modes to the bending modes generates high altitude heating at sub-microbar pressures. Heating in the 2.3 μm bands exceeds heating in 3.3 μm bands by approximately a factor of 6 and occurs predominantly at microbar pressures to generate steep temperature gradients ∼10-20 K km-1 for p > 2 μbar when the surface or tropopause pressure is ∼3 μbar and the CH4 mixing ratio is a constant 3%. This calculated structure may account for the "knee" in the stellar occultation lightcurve. The vertical temperature structure in the first 100 km above the surface is similar for atmospheres with Ar, CO, and N2 individually as the major constituent. If a steep temperature gradient ∼20 K km-1 is required near the surface or above the tropopause, then the preferred major constituent is Ar with 3% CH4 mixing ratio to attain a calculated ratio of T/ script M (= 3.5 K amu-1) in agreement with inferred values from stellar occultation data. However, pure Ar and N2 ices at the same temperature yield an Ar vapor pressure of only ∼0.04 times the N2 vapor pressure. Alternative scenarios are discussed that may yield acceptable fits with N2 as the dominant constituent. One possibility is a 3 μbar N2 atmosphere with 0.3% CH4 that has 106 K isothermal region (T/ script M = 3.8 K amu-1) and ∼8 K km-1 surface/tropopause temperature gradient. Another possibility would be a higher surface pressure ∼10 μbar with a scattering haze for p > 2 μbar. Our model with appropriate adjustments in the CH4 density profile to Triton's inferred profile yields a temperature profile consistent with the UVS solar occultation data (Krasnopolsky, V. A., B. R. Sandel, and F. Herbert 1992. J. Geophys. Res. 98, 3065-3078.) and ground-based stellar occultation data (Elliot, J. L., E. W. Dunham, and C. B. Olkin 1993. Bull. Am. Astron. SOC. 25, 1106.). © 1996 Academic Press, Inc.