Io and Europa, the inner two of Jupiter's four largest moons, are about the size of Earth's moon and are composed mostly or entirely of rock and metal. Ganymede and Callisto are larger and roughly 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. Jupiter's moons are, however, more "systematic": 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.

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 activity. In similar fashion, Callisto's surface is very heavily cratered from the impact of comets and asteroids; Ganymede, like Earth's moon, is heavily cratered in parts; Europa is very lightly cratered; and no craters have been detected on Io, even though Jupiter's 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 Io's high degree of geological activity, its surface undergoes more-or-less continuous volcanic resurfacing.

11. According to the passage, the difference in the amount of crate ring on Callisto's and Io's respective surfaces can probably be explained by the difference between these two moons with respect to which of the following factors?
(A) Size
(B) Ice content
(C) The rate of bombardment by comets and asteroids
(D) The influence of Jupiter's other moons
(E) The level of geological activity


12. Which of the following best describes the purpose of the second paragraph of the passage?
(A) To provide further evidence of the systematic variation in the characteristics of Jupiter's four largest moons
(B) To present a comprehensive theory to explain the systematic variation in the characteristics of Jupiter's four largest moons
(C) To explain the significance of the systematic variation in the characteristics of Jupiter's four largest moons
(D) To introduce facts that contradict conventional assumptions about Jupiter's four largest moons
(E) To contrast the characteristics of Jupiter's four largest moons with the characteristics of the planets of the solar system


13. The author's reference to Jupiter's gravity in line 25 serves primarily to
(A) indicate why the absence of craters on Io's surface is surprising
(B) explain the presence of craters on the surface of Jupiter's four largest moons
(C) provide an explanation for the lack of geological activity on Callisto
(D) contrast Jupiter's characteristics with the characteristics of its four largest moons
(E) illustrate the similarity between Jupiter's four largest moons and the planets of the solar system