CO2 on Titan

A sharp stratospheric emission feature at 667/cm in the Voyager infrared spectra of Titan is associated with the nu2 Q branch of CO2. A coupling of photochemical and radiative-transfer theory yields an average mole fraction above the 110 mbar level of (1.5 + 1.5 or - 0.8) x 10 to the -9th, 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 0.00011; the thoeretical 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.