12C/13C and 16O/18O isotopic ratios in Titan’s stratospheric CO2

Three oxygen-bearing molecules have been detected in Titan's atmosphere. Carbon monoxide (CO) is the most abundant one with
a volume mixing ratio (VMR) of ~50 ppm (e.g. Serigano et al. 2016). Because of its very long chemical lifetime (several millions years) and as it does not condense in Titan's atmosphere, it is well homogenized by atmospheric dynamics. The second most abundant oxygen-bearing molecule is carbon dioxide (CO₂) with a VMR of ~15 ppb. It shows a local minimum near 0.1 mbar (~ 250 km altitude) at almost all latitudes (Mathé et al. 2020). Such vertical VMR profile is currently not reproduced by photochemical models. The less abundant oxygen molecule detected in Titan's middle atmosphere is H₂O (Cottini et al. 2012) with a VMR smaller than 0.5 ppb at 230 km with no evidence of latitudinal variations.
There is currently no consensus regarding the source of oxygen compounds in Titan's atmosphere. The most recent and complete photochemical models, which couple oxygen, nitrogen and hydrocarbon ion and neutral chemistry, incorporate different external sources. An external flux of only OH can lead to the formation of all three oxygenated molecules in the model of Vuitton et al. (2019), while Dobrijevic et al. (2014) explored a possible internal (outgassing of primordial CO from surface) or external sources of O (from Enceladus plumes) and OH/H₂O (from micrometeorites). Both type of models roughly reproduce the observed VMR of CO, CO₂ and H₂O.
As studied by Loison et al. (2017), one way to constrain the source of oxygen in Titan's atmosphere is to model measurements of isotopic ratios in oxygenated molecules.
Our objectives here are to determine the most accurate ¹²C/¹³C and ¹⁶O/¹⁸O isotopic ratios in CO₂ using the entire CIRS dataset acquired between 2004 and 2017 and to investigate potential latitudinal and seasonal variations, which could be linked to potential fractionation processes. We have analyzed 41 limb observations acquired between 2004 and 2017 at all latitudes. Although our work is still in progress, we derive preliminary values of CO₂/¹³CO₂ = 50 ± 3 and CO₂/C¹⁶O¹⁸O = 107 ± 6 combining all the 41 datasets. The inferred isotopic ratios are smaller in CO₂ than in CO and other carbon-bearing species, suggesting that some fractionation processes occur. We will also discuss the search for latitudinal and seasonal variations. 


References :
- Cottini et al. 2012. Icarus 220, 855.
- Dobrijevic et al. 2014. Icarus 228, 324.
- Loison et al. 2017. Icarus 291, 17.
- Serigano et al. 2016. ApJ 821, L8.
- Mathé et al. 2020. Icarus 344, 113547.
- Vuitton et al. 2019. Icarus 324, 120.