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
(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.
Jupiter's 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, 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
(25) 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
(30) more-or-less continuous volcanic resurfacing.

Questions 11- 13 refer to the passage above.

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





OAs:

11. E
12. A
13. A