| MadSci Network: Astronomy |
Scientists have two major theories about the moons. The first one is the impact theory which basically means something big impacted the planet and broke off a portion of it. The other theory is that some of the moons are astroids that have gotten "captured" by the planets gravitational pull. Here are two websites that tell a lot about moons and have great pictures:
[Moderator's note: The two theories, by the way, depend on the size of the moon. In the case of the moons of Mars, or some of the smaller moons of the giant outer planets like Jupiter, it seems reasonable to think that these moons are captured asteroids. The larger moons of Jupiter, such as Ganymede and Callisto, and our own moon cannot be explained this way, though. Either they formed as seperate planets in their own right, and were later captured by their home planets, or our own moon may in fact have been formed if the Earth was hit, just after it formed, by something large enough to split a huge chunk off into space.
Planets like Jupiter and Saturn have lots of moons, by the way, because they are so much larger than the Earth that their huge gravity probably made it easier for them to capture small planets and asteroids when the solar system was forming.
Jim O'Donnell ]
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