Question: Why are all the moons of the outer planets icy, and the moons of the inner planets rocky? Because, I heard it's not just to do with their distance from the sun?

Ian Stephenson answered on 23 Jun 2014:

I’m sure there are a bunch of reason, and but lets start with an easy one:

There aren’t going to be any ice planets close to the sun – its too hot.

So the next thing to to would be to ignore icy planets, and just look at the distribution of rocky ones, which changes the question to why are there more rocky planets close to the sun? I don’t know specifically, but one thing to consider is that the rocky planets form when smaller pieces of rock for into a larger lump. As the solar system formed there are may have been more rocks closer in. However also consider what would happen if there were the same number of small rocks at each distance… the ones close in would be close together, so would have a good chance of merging. The ones further out are spread over a much larger area (imaging 10 ants on the surface of a golf ball, vs 10 and on a beach ball), so are less likely to form….