The atmosphere of Titan as seen by VLT-ESPRESSO: the first search for minor compounds with Ultra High-Resolution visible spectra on Titan’s atmosphere

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). A recent work by our team (4) has shed light on Titan’s visible High-Resolution Spectrum - a poorly explored part of Titan’s spectrum, which may nonetheless contain relevant absorption features that could enable a more complete understanding of this moon’s rich atmospheric chemistry (4). On these archived high-resolution VLT-UVES spectra (R < 100 000), it was possible to identify 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 a higher spectral resolution were still required to obtain more conclusive answers regarding the presence of C₃ in Titan’s atmosphere, and in order to complete the search for previously uncharacterized CH₄ absorption features across the entirety of the visible spectrum (for which dedicated HR linelists are still currently unavailable (5)). 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 for the first time, as the observations of Titan with the highest spectral resolution conducted so far.

This unprecedented spectral resolution at a considerably high signal-to-noise ratio (SNR > 300) 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 (4) 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 400 nm to 780 nm, for which no similar theoretical line lists are yet available (5) and significantly complementing our previous work which was limited to the 520 nm to 620nm range (4). On this new work we identify and characterise hundreds of new high energy CH₄ lines, retrieving a number of new CH₄ absorption features one order of magnitude higher than the empirical linelist obtained by VLT-UVES presented in our previous work (4). Interestingly, these newly detected individual absorption lines explain previous low-resolution and low-temperature (T < 200K) profiles of visible methane absorption bands (6).

Beyond the CH₄ visible band, VLT-ESPRESSO observations' increased spectral resolution and spectral coverage enable the search for other minor chemical compounds on Titan’s atmosphere – namely C₃, for which 2 updated linelists have recently been published (7)(8). Here we present the application to this new dataset of Titan’s visible spectrum at an unprecedented spectral resolution this more recent and complete C₃ linelist, which, following a preliminary analysis, appears to point to a more study detection of C₃ compared to (4). This is since 7 matching features to C₃ lines are found in this analysis, with a line depth consistent with the presence of C₃ at the upper atmosphere of Titan, with a column density of 10¹³ cm⁻². We also present the search for C₂ spectral features on this high-resolution visible spectrum of Titan (9).

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 (10). 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 (11).

Figure 1: Line detection with the Doppler Method for Spectral line identification at a section of the 6200 Å CH₄ band. Here we compare one nightly VLT-UVES spectra of Titan (in red), with the new VLT-ESPRESSO spectrum of Titan (in blue) and the Kurucz solar spectrum (in black). The many non-solar spectral line on both Titan spectra, originating from a visible CH₄, absorption band, illustrate the increased detectability of fainter spectral features in the higher-resolution VLT-ESPRESSO spectrum.

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