Isotopic ratios in Titan’s HCN and HC3N derived from Cassini/CIRS observations

Molecular nitrogen (N₂) and methane (CH₄) are the two major gas of Titan’s atmosphere. Their dissociation in the upper atmosphere by photons and photo-electrons leads to a wealth of chemical reactions forming more complex molecules like nitriles and hydrocarbons, which subsequently combine to form Titan’s photochemical haze.

Isotopic ratios measured in N₂ and CH₄ are of particular interest to constrain the origin and evolution of Titan’s atmosphere. While the same isotopic ratios measured in photochemical species bring constraints on fractionation processes occurring through their formation and/or loss.      

We focus on the determination on the ¹⁴N/¹⁵N and the ¹²C/¹³C isotopic ratios in HCN and the ¹²C/¹³C ratio in HC₃N by analyzing their thermal emission acquired by the Cassini Composite Infrared Spectrometer (CIRS) from 2004 to 2017 (from the northern winter to the northern summer).  We used the entire CIRS dataset acquired with a limb-geometry viewing at the highest spectral resolution (0.5 cm^-1). This allows us to search for potential variations of these isotopic ratios with latitude or with season, which could help to identify potential fractionation processes. Our analysis incorporates the temperature and minor species volume mixing ratio profiles inferred previously by Mathé et al. (2020) from the same limb dataset. We will present our results regarding the isotopic ratios in HCN for all latitudes, while we will present the ¹²C/¹³C ratio in HC₃N only at high latitudes, as this nitrile is not detected at mid- and low-latitudes.

References:
- Mathé et al., 2020. Seasonal changes in the middle atmosphere of Titan from Cassini/CIRS observations: Temperature and trace species abundance profiles from 2004 to 2017. Icarus 344,  id. 113547.