Albedo is a ratio of scattered to incident electromagnetic radiation power, most commonly light. It is a unitless measure of a surface or body's reflectivity. The word is derived from the Latin for "white".

The albedo of planets, satellites and asteroids can be used to infer much about their properties. The study of albedos, their dependence on wavelength, lighting angle ("phase angle"), and variation in time comprises a major part of the astronomical field of photometry. For small and far objects that cannot be resolved by telescopes, much of what we know comes from the study of their albedos. For example, the absolute albedo can indicate the surface ice content of outer solar system objects, the variation of albedo with phase angle gives information about regolith properties, while unusually high radar albedo is indicative of high metallic content in asteroids.

Enceladus, a moon of Saturn, has the highest known albedo of any body in the solar system, with 99% of EM radiation reflected, while many objects in the outer solar system and asteroid belt have low albedos down to about 0.05. Such a dark surface is thought to be indicative of a primitive and heavily space weathered surface.

The overall albedo of the Moon is around 12%, but it is strongly directional and non-Lambertian, displaying also a strong opposition effect [1]. Such reflectance properties of moon regolith are different to those of any terrestrial terrains, but common on airless rocky solar system bodies.

Two common definitions of astronomical albedos are the geometric albedo and the Bond albedo, and their values can differ significantly — a common source of confusion.


See also

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