The Detection-vs-Retrieval Challenge: Titan As An Exoplanet

Cassini’s observations of Titan’s atmosphere are exemplary benchmarks for exoplanet atmospheric studies owing to (1) their precision and (2) our independent knowledge of Titan.

Leveraging these observations, we perform retrievals (i.e., analyses) of Titan’s transmission spectrum to investigate the strengths/limitations of exoplanet atmospheric retrievals with a particular focus on the underlying assumptions regarding the molecular species included in the retrieval.

We find that multiple hydrocarbons can be “retrieved” depending on the selection made ahead of a retrieval. More importantly, we find that the estimates of other parameters such as the abundance of key absorbers like methane can be biased by ∼0.5 dex (by a factor of ∼3) due to such choices.

This shows that beyond the possible misidentification of a molecular feature (e.g., current debate surrounding dimethyl sulfide, DMS, in K2-18 b), the implicit molecular detections made pre-retrieval to avoid retrieving for hundreds of molecules at a time can bias a large range of parameters. We thus recommend sensitivity analysis to assess the dependencies of atmospheric inferences on such selections in tandem with complementary information (e.g., chemistry models) to support any pre-retrieval selection.

Finally, we introduce an independent path to constrain the dominant atmospheric constituent, even when lacking observable absorption feature (e.g., H₂ and N₂) through the scale height.

Molecules may have many lookalikes. Overview of the absorption cross sections for hydrocarbons taken from PNNL (Sharpe et al. 2004) and included in HITRAN2024 database (Hargreaves et al. 2025). The y-axis provides the logarithm of intensity, with each absorption cross section offset for display purposes. A minimum intensity of 1.0×10−21 cm² /molecule has been applied for each molecule. — astro-ph.EP

Prajwal Niraula, Julien de Wit, Robert Hargreaves, Iouli E. Gordon, Clara Sousa-Silva

Comments: Comments welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2506.12144 [astro-ph.EP] (or arXiv:2506.12144v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2506.12144
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Submission history
From: Prajwal Niraula
[v1] Fri, 13 Jun 2025 18:05:18 UTC (6,901 KB)
https://arxiv.org/abs/2506.12144

Astrobiology,