7) Explain why the moons of the outer planets are made of rock and
ice while their parent planets are mostly hydrogen and helium.
When the solar system formed, the outer planets grew large enough for
their gravity to hold down hydrogen and helium, so they scooped up these
gases from the solar nebula around them. These gases became part of the
planet, thus increasing its mass, increasing its gravity further, and
allowing it to incorporate even more of the gases. The moons never got
large enough for this process to begin before the Sun entered its
T Tauri phase and blew the remaining nebula away.