Planet Earth in reflected and polarized light

III. Modeling and analysis of a decade-long catalog of Earthshine observations

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748, Garching near Munich, Germany
² Meteorologisches Institut, Ludwig-Maximilians-Universität München, Munich, Germany
³ Rayference, Rue d’Alost 7, 1000 Bruxelles, Belgium
⁴ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, Northern Ireland, UK
⁵ European Southern Observatory, Santiago, Chile
⁶ Laboratoire Lagrange, Observatoire de la Côte d’Azur, CNRS, Université Côte d’Azur, Nice, France

^★ Corresponding author: giulia.roccetti@eso.org

Received: 31 May 2025
Accepted: 24 July 2025

Abstract

Earthshine observations offer a unique opportunity to study Earth as an exoplanet seen from the Moon. As the Sun-Earth-Moon geometry changes, Earth can be observed as a spatially unresolved exoplanet at different phase angles, providing important context for future observations of Earth-like exoplanets. Here, we present a catalog of Earthshine polarization spectra obtained with FORS2 on the VLT, covering diverse scenes, surface conditions, cloud properties, and weather patterns for over a decade. For the first time, we model this extensive dataset in detail using a homogeneous modeling framework. Previous efforts to model some of these spectra struggled to reproduce the observed polarization continuum, even with advanced 3D radiative transfer models incorporating satellite-derived surface and atmospheric data. We improve upon this with a state-of-the-art 3D model that includes subgrid cloud variability, wavelength-dependent surface albedo maps, and an accurate treatment of ocean glint. Our simulations successfully reproduce most observed spectra to a much higher precision than previously possible. Additionally, our statistical analysis reveals that the spectral slope in the visible can distinguish between ocean and mixed surfaces in both reflected and polarized light, which is not possible using broadband filters alone. Polarized light at large phase angles, beyond the Rayleigh scattering regime, is particularly effective in differentiating oceans from land, unlike reflected light. While the vegetation red edge (VRE) is more pronounced in reflectance, it remains detectable in polarization. We also identify correlations between cloud optical thickness and the polarized spectral slope, and between cloud cover and broadband B-R differences in reflected light, demonstrating the diagnostic power of these observations. This catalog and its modeling highlight the potential of polarization for characterizing Earth-like exoplanets. From polarization alone, we can infer oceans, vegetation, and an active water cycle, key indicators of a habitable planet.