Albedo

albedo (ălbē`dō), reflectivity of the surface of a planet, moon, asteroid, or other celestial body that does not shine by its own light. Albedo is measured as the fraction of incident light that the surface reflects back in all directions. A perfect reflector by definition has an albedo of unity, i.e., all the incident light is reflected; a body that reflects no light at all would have an albedo of zero. Real surfaces have albedos between these values. The albedos of planets, moons, and asteroids provide valuable information about the structure and composition of their surfaces. The dark regions on the earth's moon give it the very low average albedo of 0.07, while highly reflective clouds give Venus an albedo of 0.85, the highest of any body in the solar system.

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albedo

Fraction of light reflected by a body or surface, commonly used in astronomy to describe the reflective properties of planets, natural satellites, and asteroids. “Normal” albedo (the relative brightness of a surface when illuminated and observed from directly above) is often used to determine the surface compositions of satellites and asteroids. The albedo, diameter, and distance of such objects together determine their brightness.

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albedo [al′bē‚dō]

(nucleonics)

The reflection factor a surface, such as paraffin, has for neutrons.

(optics)

That fraction of the total light incident on a reflecting surface, especially a celestial body, which is reflected back in all directions.

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Albedo 

a characteristic of the reflecting properties of the surface of a body: the ratio of the flow of radiation scattered by a surface to the flow falling on that surface. There is true albedo (or diffuse albedo, Lambert albedo, the coefficient of diffuse reflection) and apparent albedo (or the brightness factor). True albedo is the ratio of the flow scattered in all directions by a plane element of a surface to the flow falling on that element. Apparent albedo is the ratio of the brightness of a surface illuminated by parallel bundles of rays to the brightness of an absolutely white surface (that is, a surface for which the ratio of the brightness to the illumination does not depend on the direction and for which the true albedo equals one) perpendicular to the illuminated beam.

In astronomy the concept of albedo is generalized and is considered as a characteristic of a non-self-luminous heavenly body as a whole. Spherical albedo (Bond’s albedo) is the ratio of light flow scattered by a body in all directions to the flow falling on the body. Geometric albedo is the ratio of the mean brightness of an object given a phase angle of zero to the brightness of an absolutely white plane screen perpendicular to the sun’s rays, placed in the same point and visible under the same solid angle as the object. Illustrative albedo differs from geometric albedo in that the mean brightness of an absolutely white sphere is the standard, rather than the brightness, of a planar screen.

Integral (energetic) albedo is also considered, for a whole flow of radiation, as are monochromatic albedo (in monochromatic light) and albedo in various regions of the spectrum, such as ultraviolet, visual, and infrared. The true visual albedo of the earth’s surface varies from 0.03 ( a water surface) to 0.9 (fresh snow, clouds). The albedo of vegetation in the visible range of the spectrum is 0.1–0.3 and reaches 0.9 in the infrared range. The spherical visual albedo of the earth, determined according to the earthlight on the moon, based on measurements with artificial cosmic bodies and also according to the calculation of the heat balance of the earth, is close to 0.45. The albedo of the planets and satellites lacking an atmosphere (Mercury, the moon) is usually low and close to 0.07; the albedo of planets with a dense cloud atmosphere (Venus, Jupiter, Saturn, Uranus) is close to 0.6; the albedo of Mars is approximately 0.15.

REFERENCE

Sharonov, V. V. Priroda planet. Moscow, 1958.

G. A. LEIKIN