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Tropospheric Lapse Rate and Methane on TitanWe have reanalyzed the Voyager radio occultation data for Titan with two alternative approaches to methane condensation. In one approach, methane condensation is enhanced by the presence of nitrogen. In the other approach, methane condensation does not occur. As pointed out by Thompson, nitrogen lowers the condensation level for a methane/nitrogen Mixture and we find that the upper limit on surface relative humidity of methane obtained from the Voyager occultation data is lowered from 0.7 to 0.6. However, 140% supersaturation of methane in the troposphere, suggested by Courtin et al., allows all surface humidities to be consistent with the Voyager occultation data and the upper limit is set by other considerations. We conclude that if supersaturation is not included then the surface relative humidity of methane is between 0.08 and 0.6, with values close to 0.6 indicated. If supersaturation is included then the surface relative humidity of methane is between 0.08 and 0.85, again, with values close to 0.6 indicated. The tropospheric lapse rate on Titan appears to be determined by radiative equilibrium. It is everywhere stable against dry convection but is unstable to moist convection. This is consistent with a supersaturated atmosphere in which condensation - and hence moist convection - is inhibited. The absence of dry convection in the troposphere of Titan can be explained by a simple grey model which shows that the radiative profile of any gas for which the ratio of the gas constant to the specific heat at constant pressure is greater than 0.25 never becomes unstable to dry convection.
Document ID
20020038896
Acquisition Source
Ames Research Center
Document Type
Conference Paper
Authors
McKay, C. P.
(NASA Ames Research Center Moffett Field, CA United States)
Martin, S. Chau
(NASA Ames Research Center Moffett Field, CA United States)
Griffith, C. A.
(NAU United States)
Keller, R. M.
(NASA Ames Research Center Moffett Field, CA United States)
Cuzzi, Jeffery N.
Date Acquired
August 20, 2013
Publication Date
January 1, 1995
Subject Category
Astronomy
Meeting Information
Meeting: DPS Meeting
Location: Kona, HI
Country: United States
Start Date: October 8, 1995
End Date: October 13, 1995
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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