| MadSci Network: Astronomy |
The simple answer is that the planets are different colors because they are made of different materials. Mercury is grey because its surface is made of the same kind of material that the surface of the moon and Earth are made of - greyish-colored rock such as granite and basalt. Venus is yellow-grey because of the amount of sulfur and sulfuric acid contained in the heavy clouds that surround it. Earth is blue, brown-green, and white for reasons you already know. Mars is red-brown because of the large amount of iron oxide (rust) contained in its rocks and soil. Jupiter and Saturn are similar in color - oranges, yellows, and browns - because their clouds are made of similar materials. We do not know for sure, but these colors probably come from a material such as sulfur, which, depending on how it is mixed with other elements in these planets' atmospheres, can produce a wide range of colored compounds. Uranus and Neptune are also similar in color, and for the same reason - their clouds are made of similar materials. Both Uranus and Neptune have a gas called methane in their outer cloud layer. Methane soaks up red light and lets only blue through, so both these planets look blue. What about Pluto? Well, we don't know what color Pluto is, because we have not yet been able to get a close enough look. But be patient - NASA is currently planning a mission called Pluto Express to fly by that planet in the year 2010. I can't wait!
By the way, if you are interested in finding out more about the planets, and have access to the internet, you should check out The Nine Planets, at this address: http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html
Try the links in the MadSci Library for more information on Astronomy.