Earthshine with High-Resolution Infrared Spectroscopy

Biosignatures in the near-infrared spectrum of Earth’s atmosphere include the simultaneous presence of O2, CH4, and H2O molecular absorption bands. These molecules are important tracers of biological and atmospheric processes on Earth and provide useful analogs for exoplanet studies. High-resolution infrared spectra of Earthshine were obtained with the CRIRES+ instrument at the Very Large Telescope, which provides a resolving power of R ≈ 86,000–110,000 for full-slit illumination. At this level, individual absorption lines can be resolved and studied in detail. Earthshine spectra contain the integrated light of Earth’s illuminated surface and atmosphere reflected by the Moon. Therefore, they are considered similar to the unresolved spectra that would be observed from an exoplanet. In this work, Earthshine spectra were first divided by Sky spectra to minimize telluric effects and isolate the true disk-integrated Earth signal. The molecular absorption lines were then fitted with Gaussian profiles, and their properties were studied statistically through amplitude distributions and Kolmogorov–Smirnov and Anderson–Darling tests. The results show CH4 and O2 absorption features in the H band. Overall, this study shows that high-resolution Earthshine spectroscopy can be used to detect biosignature-related molecular features in disk-integrated spectra of Earth, and that statistical analysis of absorption-line properties provides valuable understanding of their detectability under exoplanet-like observing conditions.