A high-resolution optical spectrum of methane (CH4) for exoplanet atmospheric science from VLT-ESPRESSO observations of Titan’s atmosphere

Exoplanet atmosphere characterization heavily relies on spectroscopic data (as molecular cross-sections or high-resolution line-by-line lists). Despite efforts to obtain comprehensive molecular spectral libraries for exoplanet atmospheres, large gaps remain, particularly for larger molecules and higher frequencies at high spectral resolution (1, 2). One such key example is the methane (CH₄) visible spectrum. CH₄, the simplest hydrocarbon, is a crucial species for exoplanet atmosphere characterization and a possible biosignature in telluric planets (3), but whose complexity has prevented the production of high-resolution linelists at optical wavelengths (4, 5). This is due to the complexity of methane’s visible spectrum (making it computationally impossible to extract its visible linelists through quantum ab-initio calculations) and due to the relatively weak absorption strength of its optical absorption bands (requiring very long pathlengths which are not attainable in experimental set-ups on Earth). This lack of spectroscopic data hampers the exploration of CH₄ atmospheric features at the optical wavelength regime, which will become increasingly relevant as upcoming instruments such as ELT-ANDES or VLT-RISTRETTO will start probing the optical reflection spectra of the atmospheres of close, temperate telluric exoplanets, in search of possible biosignatures (6, 7).

To address this spectroscopic data limitation, we aimed to use Titan as a natural spectroscopy laboratory, using its visible methane absorption bands to extract an empirical high-resolution spectrum of CH₄ optical absorption (8, 9). For this, we observed Titan’s visible spectrum – dominated by CH₄ absorption – at the highest spectral resolution ever with VLT-ESPRESSO (10). From this spectrum we extract a list of all present spectral lines and compare it with a stellar calibration spectrum obtained in the same night with the same instrument (to remove contaminating telluric lines) and with a solar high-resolution spectrum (to remove contaminating solar lines from Titan’s reflection spectrum). Following this analysis we are left with a set of spectral lines originating in absorption occurring on Titan’s atmosphere that is attributable to CH₄ absorption.

Thus, we present here for the first time RRS-2026, an empirical, low-temperature high-resolution (R ∼ 190000) linelist of CH₄ in optical wavelengths, with several thousands of previously unidentified lines. We employ this retrieved CH₄ linelist to model spectroscopic products, such as templates suitable for high-resolution cross-correlation spectroscopy (HRCCS) studies - a first for CH₄ in optical wavelengths. This HRCCS technique is a powerful tool to search for minor chemical compounds in exoplanet spectra, requiring accurate, high-resolution spectral linelists of the chemical species of interest in the observed wavelength range (11).

Our extracted linelist enabled us to perform the first HRCCS detection of CH4 in visible High-Resolution observations of Titan and Jupiter – which demonstrates the applicability of our CH₄ linelist to a diverse set of planetary atmospheres (12). We also use the new retrieved methane linelist to explain past measurements of CH₄ visible bands done in lower spectral resolutions (9, 13). This work sets the stage for the search for CH₄ in exoplanet atmospheres through visible HRCCS using our empirical CH₄ linelist, opening a new window to probe for this crucial chemical species in exoplanet atmospheres. This work also further showcases how Solar System observations provide useful products for exoplanet atmospheres research.

 

References:

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