Identification of Likely Methane Absorption Features in the Optical Spectra of Titan

Abstract

The optical spectra of Titan reveal a rich set of absorption features, most of which are likely associated with methane (CH$_4$). Methane is a key molecule in planetary and exoplanetary atmospheres, yet a comprehensive high-resolution linelist at optical wavelengths remains incomplete. This study identified and characterized potential CH$_4$ absorption features in high-resolution optical spectra of Titan, providing essential data for linelist development and improving CH$_4$ detection and characterization. We analyzed Titan spectra from the ESPRESSO spectrograph (R $\approx$ 190,000), identifying intrinsic features and measuring their relative strengths. A conservative detection approach was employed, slightly overestimating solar and telluric contributions to distinguish them from Titan's intrinsic features. To assess the impact of spectral resolution, we compared the ESPRESSO data with Titan UVES data (R $\approx$ 110,000). We identified 6,195 absorption features in the ESPRESSO spectra potentially associated with CH$_4$, of which 5,436 are newly reported. ESPRESSO detected twice as many features as UVES in overlapping regions, highlighting the advantage of higher-resolution data. Most detected lines remained unresolved, so our reported features are primarily blended absorption structures. We estimated the detection limit for feature identification to correspond to a CH$_4$ absorption coefficient of approximately 0.02 km-am$^{-1}$. Comparison of our results with a previous analysis of Titan UVES spectra and with experimental CH$_4$ data at a similar temperature showed good agreement, while some discrepancies were observed when compared with data acquired at a different temperature. We provide a comprehensive list of Titan absorption features with key reliability metrics, along with Titan's intrinsic spectra, to support future studies.