Abstract EANA2025-25

Titan, Saturn’s largest moon, is a unique natural laboratory for the study of atmospheric evolution and photochemistry akin to the primitive Earth with a wide array of complex molecules discovered through infrared and sub-mm spectroscopy. A recent work by our team (Rianço-Silva et al, 2024) shed light on Titan’s visible High-Resolution Spectrum - a poorly explored part of Titan’s spectrum, which nonetheless contains relevant absorption features allowing a more complete understanding of its rich atmospheric chemistry. Using high-resolution VLT-UVES spectra (R < 100 000), it was possible to map tens of previously unidentified CH4 visible high-resolution features and obtain the first tentative detection of C3 on Titan through its 405 nm “comet”-band.

Despite these encouraging results, further observations covering the entirety of Titan’s visible spectrum at higher spectral resolution were still required to confirm the presence of C3 in Titan’s atmosphere, and to complete the mapping of previously uncharacterized CH4 absorption features across the entire visible spectrum (for which dedicated HR linelists are still currently unavailable). We performed these observations with VLT-ESPRESSO at its Ultra High-Resolution (R = 190 000) mode in December 2024 and we present their results here, as the observations of Titan with the highest spectral resolution conducted so far.

This unprecedented spectral resolution at a considerably high SNR allows for a complete survey of non-solar absorption features on Titan’s visible spectrum, enabling the extraction of a more comprehensive methane visible High-Resolution linelist, from 400 to 780 nm. Using our original Doppler-based line detection method for backscattered planetary atmosphere spectra, we retrieve an updated empirical, low Temperature (T < 200K), ultra-high-resolution (R = 190.000) line list of methane absorption on Titan, from 400nm to 780nm, for which no similar theoretical line lists are yet available, significantly complementing our previous work, limited to the 520nm to 620nm range. Here we identify and characterise hundreds of new high energy CH4 lines, retrieving one order of magnitude more new CH4 absorption features than the empirical linelist obtained from VLT-UVES data. Interestingly, these newly detected individual absorption lines explain previous low-resolution and low-temperature (T < 200K) profiles of visible methane absorption bands.

VLT-ESPRESSO observations increased spectral resolution and spectral coverage also enable the search for other minor chemical compounds on Titan’s atmosphere – namely C3. Here we present the detection of 8 matching absorption features to C3 lines are which found in this analysis, leading to a sturdier detection of C3 on Titan’s upper atmosphere – the first detection of this carbon-based photochemical product in any planetary atmosphere. This is an important detection since C3 has been described as a possible precursor to the photochemical production of benzene in Titan's atmosphere.

This study of Titan's atmosphere with ultra-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. 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.