Why the difference between the terrestrial and Jovian planets?
Recall that, as the solar nebula formed, we would expect that in the inner solar system the solar radiation would not allow various ices to form, but farther out they could condense onto dust particles, etc. We will see that the icy parts of comets tend to boil off to form the coma and tail at a distance from the Sun typically somewhere between Jupiter’s and Mars’ orbits. So we can imagine the small planetismals in the early inner solar system as being like rocks and those in the early outer solar system as being like dirty snowballs (but containing not only water ice, but CO2 ice, etc.) The outer planets not only grew from different materials, but grew to masses such that they could accrete and retain gaseous material directly from the solar nebula. Thus Jupiter, for example, has a “rocky” core only slightly higher in mass than the Earth; most of its mass is hydrogen, and, in fact, its composition closely mirrors that of the Sun.
