CO2 on Titan

A sharp stratospheric emission feature at 667 cm -• in the Voyager infrared spectra of Titan is associated with the •'2 Q branch of CO2. A coupling of photochemical and radiative transfer theory yields an average mole fraction above the 110 mbar level of fco2 1.5 + 2.5 -9 = -o.8X 10 , with most of the uncertainty being due to imprecise knowledge of the vertical distribution. CO2 is found to be in a steady state, with its abundance being regulated principally by the ---72 K cold trap near the tropopause and secondarily by the rate at which water-bearing meteoritic material enters the top of the atmosphere. An influx of water about 0.4 times that at the top of the terrestrial atmosphere is consistent with a combination of the observed CO2 abundance and a steady state CO mole fraction of 1.1 x 10-4; the theoretical value for CO is close to the value observed by Lutz et al. (1983), although there are large margins for error in both numbers. If steady state conditions for CO prevail, little information is available regarding the evolution of Titan's atmosphere.