Detection of C3 in Titan with VLT-ESPRESSO

ABSTRACT Titan is regarded as a Solar system natural laboratory for studying atmospheric photochemistry and abiotic production of organic molecules on cold small exoplanets. Since the Cassini-Huygens mission ended, telescope observations enabled new detections of increasingly complex carbon-based molecules at infrared and sub-mm wavelengths, while the optical regime has been largely overlooked. Following a recent tentative detection of the 4050 Å absorption band of C$_3$ in Titan – a photochemical precursor to aromatic chemistry – in archived optical VLT-UVES spectra (R$\approx$ 60 000), this work presents a 8$\sigma$ detection of the C$_3$ 4050 Å absorption band in Titan from dedicated ultra-high-resolution VLT-ESPRESSO observations of Titan (R$\approx$ 190 000, highest spectral resolution observations of Titan in optical wavelengths ever). VLT-ESPRESSO spectrum is compared to a model spectrum of Titan, for varying C$_3$ abundances; a $\chi ^2$ curve is drawn to assess the agreement of non-solar spectral features with C$_3$ absorption when varying C$_3$ abundance; and a Bayesian Markov chain Monte Carlo (MCMC) fit between model and observed spectra is performed. $\chi ^2$ curve analysis yields an 8$\sigma$ C$_3$ detection, consistent with a C$_3$ column density of $N = 1.5 \times 10^{13}$ cm$^{-2}$, whereas the MCMC fit retrieved a C$_3$ column density on Titan’s atmosphere of $N = (1.47 \pm 0.30) \times 10^{13}$ cm$^{-2}$ at 5$\sigma$, the same order of magnitude of predicted C$_3$ abundances by photochemical models, reaching ppm levels on Titan’s mesosphere. This work showcases the usefulness of instruments and techniques originally dedicated to exoplanet research when applied to Solar system targets and science cases.