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

TL;DR

This paper accurately determines Earth's visual geometric albedo and phase function using ground and spacecraft data, revealing a lower albedo than previously thought and analyzing the effects of clouds, aerosols, and ocean glint on planetary brightness.

Contribution

It introduces a physical-statistical model for interpreting Earth's phase curves, providing a definitive albedo value and insights into scattering processes affecting planetary reflectance.

Findings

Earth's geometric albedo is approximately 0.242, 30-40% lower than earlier estimates.

Aerosol forward scattering can mimic ocean glint, affecting habitability assessments.

Phase curve analysis can disentangle surface and atmospheric contributions at different wavelengths.

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 (0.4--0.7 um) 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, 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.