Because the refractivity of a gas is a function
We have reanalyzed the Voyager radio occultation data for Titan, examining two alternative approaches to methane conof density and composition, self consistent inversions of densation. In one approach, methane condensation is facilitated the density, temperature, and pressure profiles of Titan's by the presence of nitrogen because nitrogen lowers the condenatmosphere can be derived from the occultation data. For sation level of a methane/nitrogen mixture. The resulting ena pure N 2 atmosphere, the inversion is unique and implies hancement in methane condensation lowers the upper limit on surface temperatures of 94.0 and 93.9 K and surface pressurface relative humidity of methane obtained from the Voysures of 1.495 and 1.498 bar for ingress and egress, respecager occultation data from 0.7 to 0.6. We conclude that in this tively (Lindal et al. 1983). However, when CH 4 is included case the surface relative humidity of methane lies between 0.08 there is no longer a unique solution and the inversion and 0.6, with values close to 0.6 indicated. In the other approach, methane is allowed to become supersaturated and depends on the surface humidity and vertical profile of reaches 1.4 times saturation in the troposphere. In this case, CH 4 (Flasar 1983, Lellouch et al. 1989).
surface humidities up to 100% are allowed by the Voyager Previous analyses have assumed that the CH 4 concentraoccultation data, and thus the upper limit must be set by other tion is limited by its saturation vapor pressure. In this paper considerations. We conclude that if supersaturation is included, we reconsider this assumption in two ways: first by allowing then the surface relative humidity of methane can be any value for the effects of N 2 on CH 4 condensation, and second by greater than 0.08-unless a deep ocean is present, in which allowing for CH 4 supersaturation. We then derive new case the surface relative humidity is limited to less than 0.85. limits for the range of possible temperature solutions and Again, values close to 0.6 are indicated. Overall, the tropospheric lapse rate on Titan appears to be determined by radiaassociated concentrations of CH 4 . We also consider the tive equilibrium. The lapse rate is everywhere stable against implications of the presence of moist convection.
dry convection, but is unstable to moist convection. This finding Following the work of Lindal et al. (1983), Flasar ( 1983) is consistent with a supersaturated atmosphere in which conwas the first to consider the effect of CH 4 on the inversion densation-and hence moist convection-is inhibited. Β© 1997 of the Voyager radio occultation data. Based on the as-
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sumption that CH 4 is limited by its saturation value, they found that the lapse rates are unstable to dry convection for surface humidities of CH 4 larger than Θ0.7.