Temperature lapse rate and methane in Titan's troposphere

Area of Science:

• Planetary Science
• Atmospheric Science
• Astrobiology

Background:

• Titan's atmosphere exhibits complex methane condensation processes.
• Previous analyses of Voyager radio occultation data provided constraints on Titan's surface relative humidity.

Purpose of the Study:

• To reanalyze Voyager radio occultation data for Titan.
• To investigate two distinct models of methane condensation and their impact on surface relative humidity.
• To determine the atmospheric lapse rate and its stability.

Main Methods:

• Reanalysis of existing Voyager radio occultation data.
• Modeling methane condensation with and without nitrogen facilitation.
• Modeling methane supersaturation in the troposphere.
• Analysis of atmospheric stability against dry and moist convection.

Main Results:

• Methane condensation facilitated by nitrogen lowers the upper limit of surface relative humidity to 0.6.
• If methane supersaturation occurs, surface relative humidity can reach 100%, unless a deep ocean is present (limit < 0.85).
• Titan's tropospheric lapse rate is likely determined by radiative equilibrium and is stable to dry convection but unstable to moist convection.

Conclusions:

• The surface relative humidity of methane on Titan is likely between 0.08 and 0.6, with values near 0.6 indicated.
• Supersaturation allows for higher surface relative humidities, but other factors may impose limits.
• The atmospheric conditions are consistent with a supersaturated atmosphere where condensation inhibits moist convection.