A study of very high resolution visible spectra of Titan - Caracterization of 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), shedding light on a poorly explored part of Titan’s spectrum – containing understudied visible methane absorption bands – with high-resolution ground-based spectroscopy. Here we also present a new, Doppler-based line detection method for backscattered planetary atmosphere spectra: By comparing VLT-UVES spectra of Titan obtained in distinct observation nights (with distinct radial velocities between Titan and the Sun) it was possible to automatically distinguish absorption lines originated on Titan’s atmosphere from backscattered solar absorption lines. This allowed to retrieve an empirical, low Temperature (T < 200K), high resolution (R = 100.000) line list of methane absorption on Titan, between 525nm and 618nm, for which no similar line lists are yet available (5), identifying and characterising more than 90 new high energy CH₄ lines. Interestingly, these newly detected individual absorption lines explain previous low-resolution and low-temperature (T < 200K) profiles of visible methane absorption bands (6).

Furthermore, we searched for the predicted, but previously undetected carbon trimer molecule, C₃ (7) (8), on the atmosphere of Titan, at its 405.1 nm band, by comparing VLT-UVES Titan spectra with a line-by-line (9) model spectrum of Titan’s atmosphere with C₃. This understudied transient photochemical molecule is thought to play a key role on the photochemical pathways leading to the formation of aromatic organic molecules such as benzene in Titan (10). C₃ abundance on Titan’s upper atmosphere could therefore be a proxy for the photochemical reaction rates that govern the production of aromatic compounds on Titan.  Our results suggest the presence of C₃ at the upper atmosphere of Titan, with an estimated column density of ((1.0 ± 0.6) ×10¹³ cm⁻²), a value 5x lower than the C₃ column density predicted by previous photochemical models of Titan’s high atmosphere (7)(8). 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 (11). 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 (12).

Figure 1: Line detection with the Doppler Method for Spectral line identification at a section of the 5430 Å CH₄ band. By comparing distinct nightly VLT-UVES spectra of Titan (coloured spectra), corresponding to distinct radial velocities between Titan and the Sun, it is possible to distinguish between Doppler-shifted backscattered solar absorption lines (also present in at the normalized solar spectrum, in black) and “fixed” absorption lines originating from Titan’s rest frame.

Figure 2: Normalised Spectrum of Titan (black) compared with synthetic normalised spectra of Titan for several C₃ column densities, N(C₃), sweeping from 0 to 2,0 × 10¹³ cm⁻², with steps of 4,0 × 10¹² cm⁻². Vertical grey lines correspond to C₃ line centres and respective uncertainty (9). Red ellipses evidence three possible detections of absorption lines in Titan spectrum that are not explained by backscattered of solar radiation alone — but instead match the expected wavelengths of C₃ absorption lines.

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