Ground-based monitoring of atmospheric species on Titan and a search for new nitriles with IRTF/TEXES

The atmosphere of Titan is known to be a laboratory of complex organic chemistry. (Coustenis, 2021) From the Voyager missions, and later the Cassini-Huygens mission, several hydrocarbons and nitriles have been detected and their seasonal variations have been monitored during a period of one Titan season (30 years). Other minor species have been detected from the ground mainly in the millimeter range or using space-borne observatories like ISO. These results have been included in photochemical models that have also predicted the presence of other minor species, among which some have infrared transitions in the 5-25-µm spectral range where propane (C₃H₈) and allene (CH₂CCH₂) have already been detected. We have started an observing program using the TEXES thermal infrared imaging spectrometer at the Infrared Telescope Facility (Mauna Kea Observatory) to monitor the infrared signatures of hydrogen cyanide (HCN) and cyanoacetylene (HC₃N), along with acetylene (C₂H₂ and C₂HD). In addition, we have been searching for cyanopropyne (C₄H₃N) and isobutyronitrile (C₄H₇N) in the 20-micron region. High-resolution spectra of Titan with TEXES were recorded before where Lombardo et al. (2019) measured HNC (hydrogen isocyanide) in Titan’s lower stratosphere (1 ppb around 100 km), which is the first time HNC has been measured at these altitudes.  In September 2022 we obtained spectra of Titan in the following spectral ranges: (1) 498-500 cm^-1 (C₂HD, HC₃N, search for C₄H₃N); (2) 537-540 cm^-1 (C₂HD, search for C₄H₇N); (3) 744-749 cm^-1 (C₂H₂, HCN); (4) 1244-1250 cm^-1 (CH4). Observations are presently being processed. In 2023, laboratory spectra of cyanopropyne and isobutyronitrile have been recorded at Sorbonne Université in the 495-505 cm^-1 and 510-570 cm^-1 spectral ranges, respectively, with a spectral resolution of 0.01 cm^-1 and 0.056 cm^-1 (Coustenis et al., 2023). Cross sections have been derived for these two molecules and upper limits will be derived for these two molecules in the atmosphere of Titan. TEXES data will also be used for a study of the variations of HCN and HC₃N since the end of the Cassini mission, and for a retrieval of D/H from C₂HD/C₂H₂.

References

• Coustenis, A., 2021. “The Atmosphere of Titan”. In Read, P. (Ed.), Oxford Research Encyclopedia of Planetary Science. Oxford University Press. doi:https://doi.org/10.1093/acrefore/9780190647926.013.120
• Lombardo, N.A., Nixon, C.A., Greathouse, T.K., Bézard, B., Jolly, A., Vinatier, S., Teanby, N.A.A, Richter, M.J., Irwin, P.J.G., Coustenis, A., Flasar, F.M., 2019. Detection of propadiene on Titan. Astroph. J. Lett. 881, Issue 2, article id. L33, 6 pp.
• Coustenis, A., Nixon, C. A., Encrenaz, Th., Lavvas, P., 2023. Titan’s chemical composition from Cassini and ground-based measurements. IUGG 2023, Berlin, Germany, 11-20 July.