A study of very high resolution visible spectra of Titan: Line characterisation in visible CH4 bands and the search for C3

The atmosphere of Titan is a unique natural laboratory for the study of atmospheric evolution and photochemistry akin to that of the primitive Earth (1), with a wide array of complex molecules discovered through infrared and sub-mm spectroscopy (2) (3). Here, we present the results of the exploration of original, ground-based, very high-resolution visible spectra of Titan, obtained with VLT-UVES (4). We have developed a new, Doppler-based line detection method which allowed to retrieve an empirical, high resolution (R = 100.000) line list of methane between 525 nm and 618 nm, for which no similar line lists are yet available (5), identifying and characterising more than 90 new high energy CH4 absorption lines at low temperature (T = 150 K). Furthermore, we searched for the predicted, but previously undetected carbon trimer molecule, C3, (6) (7), on the atmosphere of Titan, at its 405.1 nm band, by comparing VLT-UVES Titan spectra with a line-by-line (8) model spectrum of Titans atmosphere with C3. Our results are consistent with the presence of C3 at the upper atmosphere of Titan, with a column density of 1013 cm-2. This study of Titan's atmosphere with very high-resolution visible spectroscopy presents a unique opportunity to observe a planetary target with a CH4-rich atmosphere, from which CH4 optical proprieties can be studied (9). It also showcases the use of a close planetary target to test new methods for chemical retrieval of minor atmospheric compounds, in preparation for upcoming studies of cold terrestrial exoplanet atmospheres (10). References: (1) Hrst S., 2017; J. Geophys. Res. Planets, doi:10.1002/2016JE005240; (2) Nixon C., et al, 2020; The Astronomical Journal, doi:10.3847/1538-603881/abb679; (3) Lombardo N., et al, 2019, The Astrophysical Journal Letters, 2019, doi:10.3847/2041- 658213/ab3860; (4) Riano-Silva R., et al, 2023, submitted to Planetary and Space Sciences (under peer-review). (5) Hargreaves R., et al, 2020; The Astrophysical Journal Supplement Series, doi:10.3847/1538-4365/ab7a1a; (6) Hrbad E., et al, 2013; Astronomy Astrophysics, doi:10.1051/0004-6361/201220686; (7) Dobrijevic M., et al, 2016; Icarus, doi.org/10.1016/- j.icarus.2015.12.045; (8) Schmidt M., et al, 2014; MNRAS, doi.org/10.1093/mnras/stu641; (9) Thompson M., et al, 2022; PNAS, doi.org/10.1073/pnas.2117933119; (10) Tinetti G., et al, 2018; Experimental Astronomy, doi:10.1007/s10686-018-9598-x