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, with a wide array of complex molecules discovered through infrared and sub-mm spectroscopy. Here, we explore high resolution visible spectra of Titan (obtained with VLT-UVES) and retrieve an empirical high resolution list of methane absorption features at high resolution, ($R=100.000$) between 5250 Å and 6180 Å, for which no linelists are yet available. Furthermore, we search for the predicted, but previously undetected carbon trimer, C${}_3$, on the atmosphere of Titan, at its 4051 Å band. Our results are consistent with the presence of C${}_3$ at the upper atmosphere of Titan, with a column density of $10^{13}$ cm⁻². This study of Titan’s atmosphere with high-resolution visible spectroscopy presents a unique opportunity to observe a planetary atmosphere where CH${}_4$ is the main visible molecular absorber, from which CH${}_4$ optical proprieties can be studied. 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.