On the Vertical Thermal Structure of Pluto's Atmosphere

Details On the Vertical Thermal Structure of Pluto's Atmosphere On the Vertical Thermal Structure of Pluto's Atmosphere

Apr 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996icar..120..266s&link_type=abstract

Icarus, Volume 120, Issue 2, pp. 266-289.

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41

Scientific paper

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 CH_4 3.3 μm and 2.3 μm bands, non-LTE radiative exchange and cooling in the CH_4 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 CH_4 molecule. Partial thermalization of absorbed solar radiation in the CH_4 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 mubar when the surface or tropopause pressure is ~3 mubar and the CH_4 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 N_2 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% CH_4 mixing ratio to attain a calculated ratio of T_solarM (= 3.5 K amu^-1) in agreement with inferred values from stellar occultation data. However, pure Ar and N_2 ices at the same temperature yield an Ar vapor pressure of only ~0.04 times the N_2 vapor pressure. Alternative scenarios are discussed that may yield acceptable fits with N_2 as the dominant constituent. One possibility is a 3 mubar N_2 atmosphere with 0.3% CH_4 that has 106 K isothermal region (T_solarM = 3.8 K amu^-1) and ~8 K km^-1 surface/tropopause temperature gradient. Another possibility would be a higher surface pressure ~10 mubar with a scattering haze for p > 2 mubar. Our model with appropriate adjustments in the CH_4 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.).

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