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Never ending Universe

Sabtu, 06 September 2008

Albedo

Albedo

The term albedo (Latin for white) is commonly used to applied to the overall average reflection coefficient of an object. For example, the albedo of the Earth is 0.39 (Kaufmann) and this affects the equilibrium temperature of the Earth. The greenhouse effect, by trapping infrared radiation, can lower the albedo of the earth and cause global warming.

The albedo of an object will determine its visual brightness when viewed with reflected light. For example, the planets are viewed by reflected sunlight and their brightness depends upon the amount of light received from the sun and their albedo. Mercury receives the maximum amount of sunlight, but its albedo is only 0.1 so it is not as bright as it would be with a higher albedo.

In more technical treatments of albedo, such as that of de Pater and Lissauer, a distinction is made between "bond albedo" and "geometric albedo", the numbers quoted above being geometric albedos. The geometric albedo is defined as the amount of radiation relative to that from a flat Lambertian surface which is an ideal reflector at all wavelengths. The bond albedo is the total radiation reflected from an object compared to the total incident radiation from the Sun. The bond albedo for the Earth is given as 0.29 by de Pater and Lissauer, compared to their value of 0.37 for the geometrical albedo.

The planetary albedos given by de Pater and Lissauer are summarized in the following table:

Planet
Mercury
Venus
Earth
Moon
Mars
Pluto
Geometric Albedo
0.138
0.84
0.367
0.113
0.15
0.44-0.61
Bond Albedo
0.119
0.75
0.29
0.123
0.16
0.4
Planet
Jupiter
Saturn
Uranus
Neptune
Bond Albedo
0.343 +/-0.032
0.342+/-0.030
0.290+/-0.051
0.31+/-0.04

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