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Jupiter’s gravity attracts comets and asteroids

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vishalwin Master | Next Rank: 500 Posts Default Avatar
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Jupiter’s gravity attracts comets and asteroids

Post Sun Sep 27, 2015 5:58 am
og-15 RC-3

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.






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


Why option B is wrong? The portion highlighted in bold in above passage suggests that gravity is responsible for both inner and outer moons. Although the rate is different.

Can anyone please help me.


Thanks & Regards,
Vishal

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Post Wed Sep 30, 2015 10:52 am
I'm not sure that I understand your question. The passage states:
Quote:
...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.
Geological activity explains why there is NO cratering on Io. However, Io does receive bombardment from comets and asteroids - bombardment that explains the craters on the other moons. Therefore, we can infer that Io has been hit by a lot of comets and asteroids, causing craters, but that those craters were covered up / shaken off by the resurfacing from geological activity.

Answer choice B talks about craters on all 4 moons. The reference to gravity in line 25 is specifically about the INNER moons, and thus not relevant to all 4.

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vishalwin Master | Next Rank: 500 Posts Default Avatar
Joined
18 Sep 2015
Posted:
274 messages
Followed by:
1 members
Upvotes:
12
Test Date:
Oct 2016
Target GMAT Score:
750
GMAT Score:
530
Post Wed Sep 30, 2015 9:57 am
Hi,

As per the passage, degree of geological activity is used to explain cratering on Io. As stated in Question. 11

"Gravity is mentioned to explain why it's paradoxical that Io receives more bombardment but has fewer craters visible."

Can you please have a look at this again.

Also, I didn't understand OG's explanation for option B "Gravity is mentioned for inner moons and not for outer moons"

Thanks & Regards,
Vishal

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Post Tue Sep 29, 2015 5:39 am
Here's something the GMAT likes to do to trick people on RC questions: it gives answer choices that are factually true, but that don't answer the question.

Here is the full text of the passage:

Quote:
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.
You left off a key piece of the sentence: EVEN THOUGH Jupiter's gravity attracts...

The smaller inner moons (Io and Europa) receive MORE bombardment, but surprisingly have FEWER craters. This is because volcanic activity keeps resurfacing their surfaces.

So, while B is factually correct - there are craters on all 4 moons - it doesn't answer the question of WHY the author mentions gravity. Gravity is mentioned to explain why it's paradoxical that Io receives more bombardment but has fewer craters visible.

The answer is A.

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