Search for cyanopropyne and isobutyronitrile in Titan with TEXES

Titan’s organic chemistry has been partly revealed from Cassini-Huygens and recent ground-based observations so far, but the full degree of its complexity is not yet fully understood (e.g. Coustenis, 2021; Nixon, 2024). Several hydrocarbons and nitriles have already been detected in the atmosphere and their seasonal variations studied in particular by the CIRS instrument aboard Cassini. Other minor species have been detected from the ground mainly in the millimeter range or space-borne observatories like ISO (Coustenis et al., 1998). These results have been included in photochemically models (Lavvas et al. 2008, and this work) that have also predicted the presence of other minor species, among which some have infrared transitions in the 5-25-micron spectral range, like cyanopropyne (CH3C3N) and isobutyronitrile (i-C3H7CN).

Jacquemart et al. (2025) have derived absorption cross-sections at room temperature for these two non-cyclic organic molecules from laboratory spectra recorded 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 and have proposed them for the 2024 update of the HITRAN database. In this group, we have started an observing campaign 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 C4H3N and C4H7N in the 20-micron region. High resolution spectra of Titan have been obtained in September 2022 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). As a first application, we used the retrieved spectra in a radiative transfer code to simulate observations of Titan’s stratosphere acquired using the Texas Echelon Cross Echelle Spectrograph (TEXES at the Infrared Telescope Facility (IRTF, Mauna Kea Observatory). We discuss preliminary results and perspectives, among which estimated upper limits of 3×10-9 for CH3C3N and 3×10-7 for isobutyronitrile in Titan’s stratosphere.

In the future, we plan to use the TEXES instrument in conjunction with other larger telescopes in order to optimize the search range and to acquire detection or upper limits for some of these new molecules.

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

- Nixon, C. A., 2024. The Composition and Chemistry of Titan’s Atmosphere. ACS Earth and Space Chemistry 2024 8 (3), 406-456. DOI: 10.1021/acsearthspacechem.2c00041

- Coustenis, A., Salama, A., Lellouch, E., Encrenaz, Th., Bjoraker, G., Samuelson, R. E., de Graauw, Th., Feuchtgruber, H., Kessler, M. F., 1998. Evidence for water vapor in Titan’s atmosphere from ISO/SWS data. Astron. Astrophys. 336, L85-L89.

- Lavvas, P., Coustenis, A., Vardavas, I. M., 2008. Coupling photochemistry with haze formation in Titan's atmosphere. Part I: Model description. Plan. Space Sci. 56, 27-66.

- Jacquemart, D., et al. 2025. Near- and mid-infrared spectroscopy of isobutyronitrile and cyanopropyne: absorption cross-sections for quantitative detection in astrophysical objects. JQSRT, submitted.