Albedo

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Iapetus, a moon of Saturn, with an albedo ranging from 0,03 to 0,5

An optical term. A measure of reflectivity.[1]

Terrestrial albedo

Sample albedos
Surface Typical
albedo
Fresh asphalt 0.04
Conifer forest 0.08 to 0.15
Deciduous trees 0.15 to 0.18
Bare soil 0.17
Green grass 0.25
Desert sand 0.40
New concrete 0.55
Ocean ice 0.5–0.7
Fresh snow 0.80–0.90

Albedos of typical materials in visible light range from up to 0.9 for fresh snow to about 0.04 for charcoal, one of the darkest substances. Deeply shadowed cavities can achieve an effective albedo approaching the zero of a black body. When seen from a distance, the ocean surface has a low albedo, as do most forests, whereas desert areas have some of the highest albedos among landforms. Most land areas are in an albedo range of 0.1 to 0.4. The average albedo of Earth is about 0.3. This is far higher than for the ocean primarily because of the contribution of clouds.

Earth's surface albedo is regularly estimated via Earth observation satellite sensors such as NASA's MODIS instruments on board the Terra and Aqua satellites.

Earth's average surface temperature due to its albedo and the greenhouse effect is currently about 15° C. If Earth were frozen entirely (and hence be more reflective) the average temperature of the planet would drop below −40° C. If only the continental land masses became covered by glaciers, the mean temperature of the planet would drop to about 0° C. In contrast, if the entire Earth is covered by water—a so-called aquaplanet—the average temperature on the planet would rise to just under 27° C.

Astronomical albedo

Astronomically, it is the percentage of the total illumination of a planet or satellite that is reflected from its surface. Albedo is a major factor contributing to the brightness of a planet in the night sky.

The albedos of planets, satellites and asteroids can be used to infer much about their properties.

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, whereas unusually high radar albedo is indicative of high metal content in asteroids.

Enceladus, a moon of Saturn, has one of the highest known albedos of any body in the Solar System, with 99% of EM radiation reflected. Another notable high-albedo body is Dwarf Planet Eris, with an albedo of 0.96. Many small objects in the outer Solar System and asteroid belt have low albedos down to about 0.05. A dark surface is thought to be indicative of a primitive and heavily space weathered surface containing some organic compounds.

The albedo values for the planets are:

See also

Weblinks

Notes and References

  1. The term was introduced into optics by Johann Heinrich Lambert in his 1760 work Photometria