Temperature lapse rate and methane in Titan's troposphere

Christopher P. McKay, Shuleen Chau Martin, Caitlin A. Griffith, Richard M. Keller

Research output: Contribution to journalArticle

39 Scopus citations

Abstract

We have reanalyzed the Voyager radio occultation data for Titan, examining two alternative approaches to methane condensation. In one approach, methane condensation is facilitated by the presence of nitrogen because nitrogen lowers the condensation level of a methane/nitrogen mixture. The resulting enhancement in methane condensation lowers the upper limit on surface relative humidity of methane obtained from the Voyager occultation data from 0.7 to 0.6. We conclude that in this case the surface relative humidity of methane lies between 0.08 and 0.6, with values close to 0.6 indicated. In the other approach, methane is allowed to become supersaturated and reaches 1.4 times saturation in the troposphere. In this case, surface humidities up to 100% are allowed by the Voyager occultation data, and thus the upper limit must be set by other considerations. We conclude that if supersaturation is included, then the surface relative humidity of methane can be any value greater than 0.08 - unless a deep ocean is present, in which case the surface relative humidity is limited to less than 0.85. Again, values close to 0.6 are indicated. Overall, the tropospheric lapse rate on Titan appears to be determined by radiative equilibrium. The lapse rate is everywhere stable against dry convection, but is unstable to moist convection. This finding is consistent with a supersaturated atmosphere in which condensation - and hence moist convection - is inhibited.

Original languageEnglish (US)
Pages (from-to)498-505
Number of pages8
JournalIcarus
Volume129
Issue number2
DOIs
StatePublished - Oct 1 1997
Externally publishedYes

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ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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