Titan

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

By Keith Cowing
Status Report
astro-ph.IM
September 9, 2025
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Identification of Likely Methane Absorption Features in the Optical Spectra of Titan
An illustration of the adjusted telluric models for the solar and Titan spectra, where a 10% reduction from the best-fit model is applied to the solar spectrum and a 10% increase from the best-fit model is applied to the Titan spectrum, as described in Section 2.2. ‘o’ and ‘x’ markers indicate uncontaminated telluric lines used to estimate these adjustments. ‘o’ markers represent lines with expected strength, while ‘x’ markers denote lines with anomalous strength. The center of each ‘o’ or ‘x’ corresponds to the peak of the adjusted telluric model. The four regions shown were selected to clearly demonstrate both marker scenarios and to span a broad wavelength range relevant to the study. — astro-ph.IM

The optical spectra of Titan reveal a rich set of absorption features, most of which are likely associated with methane (CH4).

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 CH4 absorption features in high-resolution optical spectra of Titan, providing essential data for linelist development and improving CH4 detection and characterization. We analyzed Titan spectra from the ESPRESSO spectrograph (R ≈ 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 ≈ 110,000).

We identified 6,195 absorption features in the ESPRESSO spectra potentially associated with CH4, 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 CH4 absorption coefficient of approximately 0.02 km-am−1. Comparison of our results with a previous analysis of Titan UVES spectra and with experimental CH4 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.

Sirinrat Sithajan, Lalita Kaewbiang, Hugh R. A. Jones, Pakakaew Rittipruk, Sukanya Meethong

Comments: Accepted for publication in AJ
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2509.06496 [astro-ph.IM] (or arXiv:2509.06496v1 [astro-ph.IM] for this version)
https://doi.org/10.48550/arXiv.2509.06496
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Submission history
From: Sirinrat Sithajan
[v1] Mon, 8 Sep 2025 09:58:51 UTC (2,899 KB)
https://arxiv.org/abs/2509.06496
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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