On Sun, 3 Oct 1999 you wrote:

> Where would I be able to find out more about albedo's? How would I be able
> to figure out the albedo of the planet's in our system?

There is an online definition of albedo (which, for readers of the website
who may not know, is a measure of how much light an object reflects)
available at:
http://zebu.uoregon.edu/~js/glossary/albedo.html
The albedo of a planet can be obtained by taking the amount of light
reflected from a planet and dividing it by the amount of light incident
upon a planet. The tough part is figuring each of these quantities out.
The amount of light incident upon a planet is fairly straightforward to
calculate. You take the energy output of the sun (its luminosity, which
is about 4x10^27 Watts), divide that by the surface area of a sphere at
the planet's distance from the sun (if the distance from the planet to the
sun is a, then the surface area is 4*pi*a^2). This gives you the umber of
Watts per square meter at a planet's orbit. Now, you multiply this by the
cross-section of the planet (pi*radius^2) to get the total energy falling
on the planet. To get the total energy reflected from the planet, you
need to make careful observations.
Most good text books on the planets contain tables with the
albedos of planets and their moons.

> Secondly, The albedo is the percentage of light reflected off a planet's
> surface and the rest of the light is absorbed heating the planet's surface.
> Does this mean that planet's that are very cold have high albedo's and
> planets that are very hot have low albedo's?

Your thoughts here are correct for a barren chunk of rock, but things are
a bit more complicated for real planets. First, a planet's distance from
the sun also has an important effect on its temperature. The planet
Mercury is close to the sun, and receives much more energy per unit area
then more distant planets. So, although Mercury and asteroids may reflect
the same percentage of light (and thus have the same albedo), Mercury
receives a lot more energy from the sun in the first place, and therefore
gets very hot. Hot enough to melt lead, in fact.

Also, a planet's atmosphere has a lot to do with its temperature. Venus
has the highest albedo of any planet, but its atmosphere is full of
greenhouse gases, and so what little energy Venus does absorb is trapped
in the planet, and so Venus is the hottest planet in the solar system.

Finally, Jupiter puts out more energy than it receives from the sun, as it
still contains a lot of energy left over from its formation. So, naively,
its albedo is higher than 1! However, it is easy to separate the excess
energy of Jupiter (which is infrared light) out from the incident solar
energy (which is mostly visible light), and so Jupiter's true albedo is
calculable. As you can see, albedos are quite complex!

Thanks for writing!

Sincerely,
Kurtis Williams

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