Non-LTE models of Titan's upper atmosphere

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Abstract

Models for the thermal structure of Titan's upper atmosphere, between 0.1 mbar and 10-2 nbar are presented. The calculations include non-LTE heating/cooling in the rotation-vibration bands of CH4, C2H2, and C2H6, absorption of solar IR radiation in the near-IR bands of CH4 and subsequent cascading to the ν4 band of CH4, absorption of solar EUV and UV radiation, thermal conduction and cooling by HCN rotational lines. Unlike earlier models, the calculated exospheric temperature agrees well with observations, because of the importance of HCN cooling. The calculations predict a well-developed mesopause with a temperature of 135-140 K at an altitude of approximately 600 km and pressure of ∼0.1 μbar. The mesopause is at a higher pressure than predicted by earlier calculations because non-LTE radiative transfer in the rotation-vibration bands of CH4, C2H2, and C2H6 is treated in an accurate manner. The accuracy of the LTE approximation for source functions and heating rates is discussed. It found that C2H6 acts as a heat source near the mesopause by absorbing radiation from the warm stratosphere; consequently, the temperature at the mesopause depends sensitively on the C2H6 abundance. Because of the strong coupling between photochemistry and thermal structure the agreement between calculated and observed temperatures lend support to the photochemical model of Yung et al.

Original languageEnglish (US)
Pages (from-to)380-400
Number of pages21
JournalAstrophysical Journal
Volume383
Issue number1
StatePublished - Dec 10 1991

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mesopause
Titan
upper atmosphere
thermal structure
cooling
vibration
temperature
heating
local thermodynamic equilibrium
photochemistry
thermal radiation
radiation
heat source
heat sources
stratosphere
photochemical reactions
radiative transfer
solar radiation
conduction
approximation

Keywords

  • Planets: satellites
  • Planets: Saturn

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

Non-LTE models of Titan's upper atmosphere. / Yelle, Roger.

In: Astrophysical Journal, Vol. 383, No. 1, 10.12.1991, p. 380-400.

Research output: Contribution to journalArticle

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AB - Models for the thermal structure of Titan's upper atmosphere, between 0.1 mbar and 10-2 nbar are presented. The calculations include non-LTE heating/cooling in the rotation-vibration bands of CH4, C2H2, and C2H6, absorption of solar IR radiation in the near-IR bands of CH4 and subsequent cascading to the ν4 band of CH4, absorption of solar EUV and UV radiation, thermal conduction and cooling by HCN rotational lines. Unlike earlier models, the calculated exospheric temperature agrees well with observations, because of the importance of HCN cooling. The calculations predict a well-developed mesopause with a temperature of 135-140 K at an altitude of approximately 600 km and pressure of ∼0.1 μbar. The mesopause is at a higher pressure than predicted by earlier calculations because non-LTE radiative transfer in the rotation-vibration bands of CH4, C2H2, and C2H6 is treated in an accurate manner. The accuracy of the LTE approximation for source functions and heating rates is discussed. It found that C2H6 acts as a heat source near the mesopause by absorbing radiation from the warm stratosphere; consequently, the temperature at the mesopause depends sensitively on the C2H6 abundance. Because of the strong coupling between photochemistry and thermal structure the agreement between calculated and observed temperatures lend support to the photochemical model of Yung et al.

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