| Line | Io and Europa, the inner two of Jupiters four |
| largest moons, are about the size of Earths moon | |
| and are composed mostly or entirely of rock and | |
| metal. Ganymede and Callisto are larger and roughly | |
| (5) | half ice. Thus, these four moons are somewhat |
| analogous to the planets of the solar system, in which | |
| the rock- and metal-rich inner planets are distinct | |
| from the much larger gas- and ice-rich outer planets. | |
| Jupiters moons are, however, more systematic: | |
| (10) | many of their properties vary continuously with |
| distance from Jupiter. For example, Io is ice-free, | |
| Europa has a surface shell of ice, and while | |
| Ganymede and Callisto are both ice-rich, outermost | |
| Callisto has more. | |
| (15) | This compositional gradient has geological |
| parallels. Io is extremely geologically active, Europa | |
| seems to be active on a more modest scale, and | |
| Ganymede has undergone bouts of activity in its | |
| geological past. Only Callisto reveals no geological | |
| (20) | activity. In similar fashion, Callistos surface is very |
| heavily cratered from the impact of comets and | |
| asteroids; Ganymede, like Earths moon, is heavily | |
| cratered in parts; Europa is very lightly cratered; and | |
| no craters have been detected on Io, even though | |
| (25) | Jupiters gravity attracts comets and asteroids |
| passing near it, substantially increasing the | |
| bombardment rate of the inner moons compared | |
| to that of the outer ones. But because of Ios high | |
| degree of geological activity, its surface undergoes | |
| (30) | more-or-less continuous volcanic resurfacing. |
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