Geometric albedo

Geometric albedo is the ratio of a body's brightness at zero phase angle to that of a perfectly reflecting, diffusely scattering flat disk with the same cross-section. It can exceed one when a body scatters light strongly toward the observer.

Phase angle: The angle between the directions from an illuminated body to the light source and to the observer. Geometric albedo compares brightness at the zero-phase viewing geometry.
Bond albedo: The fraction of a body's total incident radiation that it reflects in all directions and wavelengths. Unlike geometric albedo, it accounts for reflected energy over every direction and phase.
Astronomical photometry: The measurement of the brightness and intensity of astronomical objects. Brightness measurements near opposition can constrain geometric albedo.
Opposition surge: A sharp increase in the brightness of a rough or particulate surface near zero phase angle. It can raise measured opposition brightness and therefore affect inferred geometric albedo.
Moon: Earth's natural satellite, a rocky body that reflects sunlight and shows prominent phase-dependent brightness. Its measured geometric albedo provides a familiar example of a dark airless surface.
Lambertian reflectance: A model of diffuse reflection in which apparent brightness is independent of viewing direction. The reference disk scatters light diffusely according to this idealization.
Spherical albedo: The fraction of incident light reflected by a sphere into all directions, at a specified wavelength. It integrates reflection over viewing directions rather than measuring brightness at zero phase.
Absolute magnitude: A measure of an object's intrinsic brightness defined at a standard distance and viewing geometry. Together with size, absolute magnitude helps determine a body's geometric albedo.
Coherent backscatter: A wave-interference effect that enhances light returned toward its source after multiple scattering. This mechanism can contribute to an opposition surge in particulate surfaces.
Enceladus: A moon of Saturn whose icy surface and active plume system make it exceptionally reflective. Its high geometric albedo illustrates how fresh ice can strongly return visible light.

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