World Builders™                                                                      Session Three  --  Climatology             



The albedo of a planet or moon tells us how much of the light that falls on it is sent back into space.


A scientist asks, "How much of the radiation that falls on a planet is reflected back out into space?"

The albedo is important because it tells us how much of the sun's energy actually gets to the surface of the planet or moon. This affects the planet's temperature.

Most of this radiation comes from the nearest star, but radiation may also come from other sources. For example, the earth receives reflected radiation from the moon.

Albedo is measured between 0 and 1.

     A low albedo planet absorbs most of the radiation that falls on it.  It does not send much of the radiation back into space, and so it does not shine brightly.  

     The surface of a low albedo planet is dark and rough.  It may be composed of dark stones or solidified lava or a heavy covering of dark colored vegetation.  These kinds of surfaces absorb tmost of he radiation that falls on them.  They soak up heat and light..

     The albedo of such a world might be 0.02.

     If the albedo were 0.02 the planet or moon would reflect back only 2% of the  incoming radiation. Only 2% of the radiation would be reflected back into space.  Although we think of the radiation as light, it also includes other radiation, such as infrared rays (heat) and gamma rays that can kill life forms

low albedo planet

  This is a low albedo planet. Notice that a lot of radiation is going in, and only a little radiation is being reflected back into space. This planet could have a thick atmosphere, which would tend to scatter and hold radiation.


    A high albedo planet or moon reflects a lot of light.  A moon with a mirror surface would reflect back everything, and so have an albedo of one, but it seems unlikely that there would be any moons or planets like that.  

     However, some surfaces are very reflective.  Clean ice and snow reflect most of the radiation that falls on them.  Clouds can also reflect incoming light and heat.  For example, Venus has dense high clouds that reflect about 72% of the radiation that falls on it.  The clouds are made of droplets and tiny crystals of sulfuric acid.  The drops of acid act like tiny mirrors and send radiation back into space. Venus is the brightest planet in our solar system with an albedo of about 72%.  It shines very brightly in the night sky.   Venus is a high albedo planet with an albedo of .72.

     Here you see a high albedo planet.  A lot of radiation is 
going in, and a lot of radiation is being reflected.  Perhaps the radiation is being reflected by clouds, as on Venus, or perhaps the planet has no atmosphere and is made of  some light colored material.  

Albedo is not the same all over a world or moon. On earth, forests have an albedo of about 5%, absorbing much of the incoming radiation. Ice and snow send most of the radiation back, and have a high albedo. Scientists suggest that one result of global warming will be a melting of the polar ice caps. The disappearance of these highly reflective areas will add to the warming of the planet.

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© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .