Re: Why are there moons?

This is an interesting question that actually still isn't completely 
resolved.  First we have to consider that not all planets have moons.  
Mercury and Venus do not have moons.  So another question could be "Why do 
some planets have moons and others do not?"

But even when we look at the planets that do have moons we see a variety of 
the types of moons.  Some moons are nothing more that large asteroids that 
orbit a planet.  Other moons are as large as some planets and have 
atmospheres and geologic activity.

Some of the moons of Mars, Jupiter, and Uranus are obviously captured 
asteroids that are orbiting those planets.  Of course, another question is 
"Where did the asteroids come from?"  Unfortunately, I don't have an answer 
for that.  I don't think anyone has really figured that one out yet.

Other moons, like Titan and Europa, which are like planets all on their own 
are not as easy to explain.  It is likely that they are the remnants of the 
material that formed the planets which they orbit.  But, because we aren't 
certain how the planets formed, we can only guess as to how the moons of 
those planets formed.

In the case of our moon, the most studied satellite in the solar system, we 
do have a better picture of its formation.  The idea is that soon after the 
Earth formed and began to cool, it was struck by another large object.  This 
object was so big and hit with such great force that a huge amount of 
material flew out into space.  That material finally settled into the moon 
that we see today.  Some of the evidence that we have for this is comparing 
the composition of the Earth and the moon.  There are some elements and 
compounds that are found on both the moon and Earth.  But, there are also 
some elements and compounds that are unique to the moon and unique to the 
Earth.  This suggests that two different bodies collided allowing for some 
mixture of materials.

Of course, because no one was around to witness this event, we can't be 
certain if our idea is correct.  But, it is the best explanation for the 
evidence before us.