Detection of Complex Hydrocarbons in Titan Using High-Resolution Cross-Correlation Spectroscopy

Introduction

High‑resolution spectroscopy (HRS) has become one of the most powerful tools for characterising planetary atmospheres, enabling the detection of molecules whose individual spectral lines are too weak to be identified directly [1, 2]. Within this framework, high‑resolution cross‑correlation spectroscopy (HRCCS) has emerged as a particularly effective technique for isolating faint atmospheric signatures by correlating observed spectra with modelled molecular templates [1, 2]. HRCCS has been widely used to detect atomic and molecular species in exoplanet atmospheres and is increasingly being applied to constrain metallicities and elemental ratios such as C/O [3–6].

A major challenge for HRCCS is its strong dependence on the availability and quality of high‑resolution opacity data. Traditional template construction relies on radiative transfer models using line-by-line molecular spectra, but many molecules of atmospheric and astrobiological interest lack complete or accurate line lists [7, 8]. This limitation restricts the chemical space accessible to HRCCS, particularly for larger molecules, for which obtaining line-by-line molecular spectra is significantly more challenging. In contrast, laboratory absorption cross‑sections are far easier to obtain and are available for a much wider range of molecules, even though they are measured at specific temperature and pressure conditions and, thus, are not as universally applicable as line lists [7, 8].

Targets in the solar system offer an ideal environment for addressing these challenges, since their atmospheric compositions are better constrained than those of exoplanets, and their spectra typically have higher signal‑to‑noise ratios [9, 10], allowing new HRCCS methodologies to be tested and validated under controlled conditions. Titan, the largest moon of Saturn, is particularly well suited for this purpose. Its atmosphere hosts a rich hydrocarbon chemistry and decades of observations from Cassini‑Huygens and ground‑based facilities have established Titan as one of the best‑characterised planetary atmospheres in the solar system [11, 12], making it an ideal testbed for developing and validating molecular detection techniques.

In this work, we apply HRCCS to CRIRES+ K‑band observations of Titan (1.99–2.48 µm) to evaluate the performance of molecular cross‑section‑based templates as an alternative to traditional line‑list‑based models. Our goal is to assess whether laboratory cross‑sections can be used to construct reliable HRCCS templates for molecules that currently lack high‑resolution line lists. We focus on hydrocarbons as a test case, given their importance for Titan’s photochemistry and their relevance for exoplanet atmospheres.

Results

Applying HRCCS to the CRIRES+ dataset, our analysis recovers methane (CH₄), its isotopologue ¹³CH₄, and acetylene (C₂H₂) in the near‑infrared spectrum of Titan. Yet, the most significant result of this work is the first HRCCS detection of ethane (C₂H₆), obtained at a peak significance of 5.17σ [Figure 1]. Ethane is a key product of methane photochemistry [12], but no high‑resolution line list currently exists for this molecule. As a result, traditional line‑list‑based HRCCS searches cannot target C₂H₆. The detection reported here is made possible exclusively using cross‑section‑based templates, demonstrating that laboratory absorption cross‑sections can serve as an alternative for HRCCS template construction when line lists are unavailable.

Figure 1. Cross-correlation results for ethane (C₂H₆). Left panel: CCFs obtained from 2000 Monte Carlo realisations. For each Monte Carlo realisation, the normalised spectrum was perturbed within its flux uncertainties and the CCF recomputed [14]. The black-solid line shows the median CCF, while the dashed line represents the autocorrelation of the template used to compute the cross-correlation. The shaded region marks the velocity interval used to estimate the noise statistics. Right panel: distribution of the central-peak SNR values. This constitutes the first detection of ethane using HRCCS.

To assess the fidelity of the detections, we performed cross‑correlations between hydrocarbon templates to test for potential spectral degeneracy [10]. In the 3.0–3.5 µm region dominated by fundamental C–H stretching modes, many hydrocarbons exhibit similar band shapes and line‑density patterns, making them difficult to distinguish [13, 10]. However, the CRIRES+ K‑band (1.99–2.48 µm) probes overtone and combination bands whose higher‑order vibrational transitions produce more molecule‑specific line patterns. Consistent with this expectation, the cross‑correlations of ¹³CH₄, C₂H₂, and C₂H₆ against the CH₄ template show no significant peaks at the rest‑frame velocity of Titan, confirming that the detections reported here are not artefacts of template degeneracy.

Conclusion

This work establishes Titan as a benchmark for developing and validating HRCCS detection strategies and demonstrates that cross‑section‑based templates can be used to detect multiple hydrocarbons in the atmosphere of Titan, including species for which no high‑resolution line lists exist. The successful detection of ethane provides a proof‑of‑concept for extending HRCCS to a wider range of molecules, particularly those relevant to photochemistry, atmospheric evolution, and prebiotic chemistry.

Furthermore, this approach is directly applicable to future observations with ground‑based high‑resolution spectrographs and to the highest‑resolution modes of JWST, offering new opportunities for detecting and characterising molecules in both solar system and exoplanet atmospheres.

Acknowledgments

This work was financially supported by LA/P/0056/2020 (IMS DOI https://doi.org/10.54499/LA/P/0056/2020) and CQE UID/00100/2025 (https://doi.org/10.54499/UID/00100/2025), UID/PRR/100/2025 (https://doi.org/10.54499/UID/PRR/00100/2025) and UID/PRR2/00100/2025 (https://doi.org/10.54499/UID/PRR2/00100/2025) funded by national funds through FCT/MECI (PIDDAC). The authors also acknowledge funding by the Portuguese Foundation for Science and Technology (FCT) through project UID/04434/2025, and project ORIGINS (2022.05284.PTDC). RS acknowledges funding through the FCT PhD fellowship 2024.02527.BD.

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