CO₂ on Titan

Abstract

A sharp stratospheric emission feature at 667 cm−1 in the Voyager infrared spectra of Titan is associated with the ν2 Q branch of CO2. A coupling of photochemical and radiative transfer theory yields an average mole fraction above the 110 mbar level of , 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 1.1×10−4; 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.

CO on Titan: More Evidence for a Well-Mixed Vertical Profile

“…Until recently, however, only two oxygen-bearing species had been detected on Titan: CO 2 (observed by Voyager 1; Samuelson et al 1983) and CO (observed from Earth; Lutz et al 1983).…”

“…To sustain the carbon dioxide abundance a source of oxygen is needed, and it is generally assumed to be supplied in water from bombardment of the upper atmosphere by icy grains. In this model vaporized water is quickly photolyzed to produce OH, and OH reacts with hydrocarbon radicals such as CH 3 to produce CO. CO in turn reacts with OH to produce CO 2 (Samuelson et al 1983, Yung et al 1984, Toublanc et al 1995, Lara et al 1996. While CO 2 has a short lifetime (order 10 3 -10 4 years), the photochemical lifetime of CO in the atmosphere of Titan is estimated to be very long (∼ 10 9 years; Yung et al 1984, Chassefière andCabane 1991).…”

The evolution of infalling sulfur species in Titan’s atmosphere

“…Three oxygen-bearing species (CO, CO 2 , and H 2 O) have been unambiguously identified in the atmosphere of Titan (Lutz et al 1983;Samuelson et al 1983;Coustenis et al 1998), but the origin of these compounds is a matter of debate. In particular, the source of CO (primordial/external) is still uncertain (Hörst et al 2008).…”

Between heaven and Earth: the exploration of Titan

“…Trace amounts of several photochemically produced hydrocarbons and nitriles are present in the upper atmosphere (e.g. Raulin, Wilson and Atreya 6 and Waite et al 7 ), where 50 ppb of CO 2 is created by reactions between CO and infalling H 2 O (Samuelson et al 8 ). Thus, we find a world literally frozen in time, where we can study chemical and physical processes that may have been important during our planet's earliest history.…”