Detection of C3 in Titan with VLT-ESPRESSO

Abstract

Titan is regarded as a natural laboratory in the Solar System for studying atmospheric photochemistry and the abiotic production of organic molecules on cold small exoplanets. Since the end of the Cassini-Huygens mission, telescope observations have enabled new detections of increasingly complex carbon-based molecules at infrared and sub-millimetre wavelengths, while the optical regime has been largely overlooked. Following a recent tentative detection of the 405 nm absorption band of C3 in Titan in archived optical VLT UVES spectra at resolving power R = 60000, this work reports an eight sigma detection of the C3 405 nm absorption band in Titan using dedicated ultra high resolution VLT ESPRESSO observations at R = 190000, the highest spectral resolution optical observations of Titan to date. The VLT ESPRESSO spectrum is compared to model spectra of Titan with varying C3 abundances. A chi squared analysis is used to assess the agreement between non solar spectral features and C3 absorption as the C3 abundance is varied, and a Bayesian Markov Chain Monte Carlo fit between model and observed spectra is performed. The chi squared analysis yields an eight sigma detection of C3, consistent with a C3 column density of approximately 1.5E13 cm-2, while the MCMC fit retrieves a C3 column density of 1.47E13 cm-2 at five sigma. These values are consistent with the order of magnitude predicted by photochemical models, which reach parts per million levels in the Titan mesosphere. This work demonstrates the usefulness of instruments and techniques originally developed for exoplanet research when applied to Solar System targets.

Figures (16)

• Figure 1: VLT-ESPRESSO normalized spectrum of Titan (black datapoints) at the spectral region of interest for the $\tilde{A}^1 \Pi_u - \tilde{X}^1\Sigma^+_g$ 000-000 band of C$_3$ compared to an array of spectral models of Titan with varying column densities of C$_3$ ($N$). Spectral models based on the Fan2024 C$_3$ linelist. Telluric transmission obtained from Planetary Spectrum Generator, shifted upwards by 1% for clarity Villanueva2018. The yellow triangles mark the identified spectral features we associate to C$_3$ absorption features, described in table \ref{['tab:linelist_c3']}.
• Figure 2: $\Delta\chi^2$ curve as a function of column density of C$_3$, $N(C_3)$, following the approach of Nixon2020. This plot shows an increase in the quality of the spectral as the column density approaches $N(C_3) = 1.5 \times 10^{13}$ cm$^{-2}$, suggesting a detection of C$_3$ in Titan.
• Figure 3: Posterior plots from the MCMC fit to the VLT-ESPRESSO spectrum of Titan. Fit for the column density of C$_3$, ($N$) and for the Temperature ($T$). Retrieved values showcased with 1$\sigma$ errorbars. Contour plots on the 2D histogram correspond to the (0.5, 1, 1.5, 2)-sigma confidence regions for a 2D Gaussian distribution. Histogram y axis correspond to the retrieved probability distribution, unitless.
• Figure 4: Posterior plots from the MCMC fit to the VLT-ESPRESSO spectrum of Titan. Fit for the column density of C$_3$, ($N$) for a fixed Temperature value of $T = 200$K. Retrieved value showcased with 1$\sigma$ errorbar. Histogram y axis correspond to the retrieved probability distribution, unitless.
• Figure 5: VLT-ESPRESSO normalized spectrum of Titan (black datapoints) at the spectral region of interest for the $\tilde{A}^1 \Pi_u - \tilde{X}^1\Sigma^+_g$ 000-000 band of C$_3$ compared to the backscattered solar spectrum (a proxy for Titan's spectrum with no C$_3$ absorption, in red) and to the best-fit model obtained from the MCMC likelihood fit, associated with the retrieved values for $N(C_3)$ and $T$, in blue. The shaded blue area corresponds to the best fit model uncertainty based on the retrieved $N(C_3)$ and $T$ values with 3$\sigma$ errorbars. The yellow triangles mark the identified spectral features we associate to C$_3$ absorption features, described in table \ref{['tab:linelist_c3']}.