M. R. P. Coelho, R. Rianço-Silva, D Goncalves, P. Machado, Z. Martins

Abstract
High-resolution cross-correlation spectroscopy (HRCCS) is a powerful technique for detecting molecules whose individual spectral lines are too weak to be identified directly, but its sensitivity is limited by the availability and quality of high-resolution opacity data. Many molecules of atmospheric and astrobiological interest lack complete line lists, restricting traditional template-based searches. In this work, we use Titan as a controlled testbed to develop and validate a new cross-section--based methodology for HRCCS template construction. We analyse K-band CRIRES+ observations of Titan (1.99--2.48 $\mu$m), and compute cross-correlation functions using both line-by-line and cross-section-based templates. Our analysis recovers known hydrocarbons such as methane (CH$_4$) and acetylene (C$_2$H$_2$), and yields the first HRCCS detection of ethane (C$_2$H$_6$) with a significance of SNR$_{\text{peak}}=5.17\pm 0.07$. The ethane detection was made possible exclusively through cross-section--based templates, as no high-resolution line list currently exists for this molecule. These results demonstrate that cross-section-based template construction is a practical and powerful strategy for extending HRCCS to molecules that currently lack reliable line lists, and establish Titan as a benchmark for calibrating molecular detection techniques that can be applied to both Solar system and exoplanet atmospheres. Future applications of this approach to other ground-based high-resolution spectrographs, as well as to JWST's highest resolution modes and next-generation facilities such as the ELT, could significantly expand the inventory of molecules detectable in planetary atmospheres.

Keywords
Molecular spectroscopy / High-resolution cross-correlation spectroscopy / Atmospheres / Titan / Data Methods

RAS Techniques and Instruments
Volume 5, Number rzag068, Page 30
2026 September

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