Inferring and Interpreting the Visual Geometric Albedo and Phase Function of Earth

Abstract: Understanding reflectance-related quantities for worlds enables effective comparative planetology and strengthens mission planning and execution. Measurements of these properties for Earth, especially its geometric albedo and phase function, have been difficult to achieve due to our Terrestrial situation -- it is challenging to obtain planetary-scale brightness measurements for the world we stand on. Using a curated dataset of visual phase-dependent, disk-averaged observations of Earth taken from the ground and spacecraft, alongside a physical-statistical model, this work arrives at a definitive value for the visual geometric albedo of our planet: 0.242 (+0.005/-0.004). This albedo constraint is up 30--40% smaller than earlier, widely-quoted values. The physical-statistical model enables retrieval-like inferences to be performed on phase curves, and includes contributions from optically thick clouds, optically thin aerosols, Rayleigh scattering, ocean glint, gas absorption, and Lambertian surface reflectance. Detailed application of this inverse model to Earth's phase curve quantifies contributions of these different processes to the phase-dependent brightness of the Pale Blue Dot. Model selection identifies a scenario where aerosol forward scattering results in a false negative for surface habitability detection. Observations of phase curves for Earth at redder-optical or near-infrared wavelengths could disentangle ocean glint effects from aerosol forward scattering and would help with understanding the utility of phase curve observations for the under-development Habitable Worlds Observatory.