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 owing to our terrestrial situation—it is challenging to obtain planetary-scale brightness measurements for the world we stand on. Using a curated data set of visual (0. 4–0. 7 μ m) 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. ₀₀₄^+0. 005. This albedo constraint is up to 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, which implies that aerosol forward scattering can effectively mimic an ocean glint signature in broadband visual phase curves. Observations of phase curves for Earth at redder-optical or near-infrared wavelengths could disentangle ocean glint effects from aerosol forward scattering. Finally, a review of albedos and planetary photometry is provided, as well as a simple two-parameter fit to Earth’s visual phase curve to ease adoption into other tools.
Tyler D. Robinson (Thu,) studied this question.