QUESTION:
If Jupiter's composition is different than that of the planets closer to
the sun, why do Jupiter's moons have iron, and why is Jupiter like a solar
system within the solar system?

ANSWER from Robert West on Jan. 17, 1996:  
The planet Jupiter grew so large when it formed four and a half billion
years ago that its strong gravity was able to hold the light gases like
hydrogen and helium which were abundant in the early solar system but were
pushed away by the sun's energy after the planets formed. The smaller,
inner planets (Mercury to Mars) were not able to hold on to the light
gases, and were left with mostly rocky material and iron. Because Jupiter
and its moons retained all the elements, they also retained iron and other
rocky and icy materials. The moons of Jupiter lost any early atmosphere
they may have had because they were not big enough to hold on to it, so
they, like the inner planets, are mostly rocky and icy. As the moons of
Jupiter formed, the closest of the large moons (Io) was warmed by the heat
of the early Jupiter much more so than were the more distant moons.
Because Io formed in a warmer region near Jupiter, not as much ice was
able to condense on its surface, and it is made mostly of rocky material,
whereas more and more ice was able to form and be part of the moons
(Europa, Ganymede, and Callisto) more distant from Jupiter. The different
compositions of these moons reflect the conditions of their formation,
much like the compositions of the planets refect the conditions of their
formation, with Mercury being closest to the Sun, and having almost no
ice, whereas Earth has liquid water and ice because it is farther from the
sun where the temperature is cooler. So we can say that Jupiter and its
large moons are similar to the sun and planets, like a little solar system
within the solar system.