When a large body strikes a planet or moon, material is ejected, there

Question

Source  :  GMATPrep Default Exam Pack

This passage was excerpted from material published in 1996.

When a large body strikes a planet or moon, material is ejected, thereby creating a hole in the planet and a local deficit of mass. This deficit shows up as a gravity anomaly: the removal of material that has been ejected to make the hole results in an area in slightly lower gravity than surrounding areas. One would therefore expect that all of the large multi-ring impact basins on the surface of earth's moon would show such negative gravity anomalies, since they are, essentially, large holes in lunar surface. Yet data collected in 1994 by the Clementine spacecraft show that many of these lunar basins have no anomalously low gravity and some even have anomalously high gravity.  Scientists  speculate that early in lunar history, when large impactors struck the moon's surface, causing millions of cubic kilometers of crustal debris to be ejected, denser material from the moon's mantle rose up beneath the impactors almost immediately, compensating for the ejected material and thus leaving no gravity anomaly in the resulting basin. Later, however, as moon grew cooler and less elastic, rebound from large impactors would have been only partial and incomplete. Thus today such  gravitational compensation  probably would not occur: the outer layer of moon is too cold and stiff.   1 
 E 
 1. According to the passage, the  gravitational compensation  referred to in the highlighted text is caused by which of the following?

(A) A deficit of mass resulting from the creation of hole in lunar surface
(B) The presence of material from the impactor in the debris created by its impact
(C) The gradual cooling and stiffening of the Moon's outer surface
(D) The ejection of massive amounts of debris from the moon's crust
(E) The rapid upwelling of material from the lunar mantle

2 
 A 
 2. The passage suggests that if the  scientists  mentioned in the highlighted text are correct in their speculations, the large multi-ring impact basins on the Moon with the most significant negative gravity anomalies probably

(A) were not formed early in the Moon's history
(B) were not formed by the massive ejection of crustal debris
(C) are closely surrounded by other impact basins with anomalously low gravity
(D) were created by the impact of multiple large impactors
(E) were formed when the moon was relatively elastic

3 
 C 
 3. The passage is primarily concerned with

(A) analyzing data from a 1994 exploration of lunar surface
(B) reconciling two opposing theories about the origin of lunar impact basins
(C) presenting a possible explanation of a puzzling finding about lunar impact basins
(D) discussing how impact basins on the Moon's surface are formed
(E) examining the claim that the moon's impact basins show negative gravity anomalies

ballest127 · Answer

Hi experts,

I'm quite confused with Q1.

The passage explains the process of  gravitational compensation  as follows:

1.large impactors struck the moon's surface
2.causing millions of cubic kilometers of crustal debris to be ejected
3.denser material from the moon's mantle rose up beneath the impactors almost immediately
4. compensating  for the ejected material and thus leaving no gravity anomaly in the resulting basin.

And Q1 ask what is the cause of  gravitational compensation ?

According to the process above, I think it is a   large impactors  (that struck the moon's surface)
is the main cause; without it , compensation will not occur.

However, the OA is (E): The rapid upwelling of material from the lunar mantle
I think E is more like a process than a cause of  gravitational compensation .

Yet,  large impactors   are not in the choices, so I choose (D) :The ejection of massive amounts of debris from the moon's crust, believing that
the loss of debris is what cause the moon to compensate.

So why D is incorrect?

Please explain.
Thank you.

ajetaajeta · Answer

Can Any one Explain? How to eliminate Option C and B Question no 3 The passage is primarily concerned with.

Why B is wrong...? 
Because of theories or Because It say Theories were about Origin of Lunar Impact Basins.

I narrowed down to B and C but I felt C is Half explanation of the passage.

Thanks!

ajetaajeta · Answer

In 3 Question. 
How one can eliminate B ?
Is it because of reconciling Or because of origin of lunar impact basin?

I felt option c is partial - It just tells about the last few lines.

rocky620 · Answer

ajetaajeta 
The passage discusses a general phenomenon/principle and its effect: 
 Phenomenon:  Striking of large body with a planet or moon.
 Effect:  Some material from the surface of the planet/moon is ejected, resulting in gravitational irregularities around the impact area.

Then the passage advances by the comparing the  similar phenomenon and effects  wrt moon's craters, Scientists findings - which are different from the general phenomenon - from  them  and providing probable explanations for these findings.

(B) reconciling  two opposing theories  about the origin of lunar impact basins 
 There is only one theory , which has opposing  results . So we cannot consider this.

(C) presenting a possible explanation of a puzzling finding about lunar impact basins 
As per the above passage map, this choice fits the bill.

As for other options:  A, D & E  represents only a part of the passage.

Hope, the explanation helps.

nikitamaheshwari · Answer

Later, however, as moon grew cooler and less elastic, rebound from large impactors would have been only partial and incomplete.

What does the line mean by saying "rebound from large impactors would have been only partial and incomplete."

I understand that early in the moon history, when large impactors hits a moon, then denser material within a moon comes up and fills up the space of the hole - Thereby compensating for the ejected material and thus leaving no gravity anomaly in the resulting basin.
But today, the moon has become elastic and cooler, so large impactors affect on moon is partial and incomplete?
Didnt really get this line.

Thanks

VeritasKarishma   GMATNinja

KarishmaB · Answer

Initially when moon was hot and elastic, impact was equalised by the transfer of denser material from its inside. Hence there were no gravity anomalies. Hence, the moon "rebounded" back. Whatever it lost was compensated for. 
The moon now is less elastic and cooler so materials cannot travel freely inside it. Hence when an impacter takes away a chunk of its surface now, the denser material from inside is not easily able to travel to the site to equalise gravity. Hence, "rebound" is not full now.

Soumanti Roy · Answer

I know this thread is old, but could anyone please clarify my doubt on Q2?
Does "negative gravity anomaly" mean very low gravity (i.e. when compensation did not happen) or gravity anomaly that did not happen (i.e. compensation at its best). Only in 1st case I can justify the Ans A. Please help me understand this.

ParamjitDasGMAT · Answer

Hi @Soumanti,

The passage mentions the following point wrt gravity anomaly during the early lunar history period --

"Scientists speculate that early in lunar history.... compensating for the ejected material and thus leaving no gravity anomaly in the resulting basin."

We can infer that during early lunar history, there would have been no negative gravity anomaly (because of the gravitational compensation). Therefore, if there were large multi-ring impact basins on the Moon with large negative gravity anomalies, they would not have been formed during this period. They must have been formed later when gravitational compensation did not occur -- hence, we also have the idea that several of these basins did not have low gravity anomalies.

We can have a gravity "anomaly" only when there is  NO  gravitational compensation. If there were something to compensate, then there wouldn't be an "anomaly" (exception)

Hope this helps.

Kushchokhani · Answer

Bunuel

There seems to be a typo in the passage- "Clementine basins" should be replaced with "lunar basins" in the middle of the passage.

Please check highlighted part in the screenshots for reference.

krndatta · Answer

KarishmaB  Ma'am,

What exactly does it mean when the passage mentions that  "the removal of the material that has been ejected to make the hole results in an area of slightly lower gravity than surrounding areas."

Yet data collected in 1994 by the Clementine spacecraft show that many of these lunar basins have no anomalously low gravity and some even have anomalously high gravity.

What is their meaning?

priyanshu14 · Answer

Dear  egmat   GMATNinja ,
I request you to share the detailed explanation of this question in your format.
Thanks in advance
Priyanshu

KarishmaB · Answer

Gravity exerted by an object depends on its mass and its radius. Since moon has mass lower than Earth, moon's gravity will be lower than Earth's (though its radius is lower too so the decrease will not be proportional). When a big crater is created on moon by say an asteroid, its mass in that area would be lower so gravity is expected to be lower there (at the crater). 
So this is what you would expect would happen on Moon too - lower gravity in areas of craters. 
But data shows that these craters on the moon do not have lower gravity and some even have higher gravity (this is unexpected)

Scientists explain this by speculating that in early phases, dense material moved from Moon's centre and made up for the lost mass. That is why craters do not have lower gravity.

But these details are irrelevant. You don't have to worry WHY 'the removal of the material that has been ejected to make the hole results in an area of slightly lower gravity than surrounding areas.'
You are given that it results in area of lower gravity and that is what you have to accept. 
The fact that moon does not have lower gravity in these areas is a paradox - an unexpected situation.

Elite097 · Answer

avigutman   CrackverbalGMAT  why not B in Q2?  KarishmaB   GMATNinja

We already know that when debris was ejected, there were no gravity anomolies so negative gravity was obviously not caused by ejection of debris.

avigutman · Answer

The boldfaced text above is incorrect,  Elite097 . According to the passage, some of the large multi-ring impact basins on the surface of earth's moon show negative gravity anomalies, while many have no anomalously low gravity and some even have anomalously high gravity. This is the text I'm referencing:

Redwhite387 · Answer

Can someone please explain this query by scofield1521

Redwhite387 · Answer

1. According to the passage, the gravitational compensation referred to in the highlighted text is caused by which of the following?
A) A deficit of mass resulting from the creation of hole in lunar surface
E) The rapid upwelling of material from the lunar mantle

IMO: Upwelling of the material itself is gravitational compensation. So what caused this is because of deficit of mass.
hole caused deficit -> deficit caused upwelling -> upwelling of material (Gravitational compensation)
Please explain in detail.

KarishmaB · Answer

1. According to the passage, the gravitational compensation referred to in the highlighted text is caused by which of the following?

(A) A deficit of mass resulting from the creation of hole in lunar surface
(B) The presence of material from the impactor in the debris created by its impact
(C) The gradual cooling and stiffening of the Moon's outer surface
(D) The ejection of massive amounts of debris from the moon's crust
(E) The rapid upwelling of material from the lunar mantle

"Gravitational compensation" is the  increase in gravity  to compensate for the decrease caused by deficit of mass due to asteroid hits etc. 
What causes the increase in gravity? "upwelling of the material from the mantle"
Hence (E) is correct.

TBT · Answer

Can someone explain why A in Q3. is wrong. I was confused between A and C.

Sajjad1994 · Answer

Explanation

3. The passage is primarily concerned with

Difficulty Level:   550

Explanation

The passage is primarily concerned with presenting a possible explanation of a puzzling finding about lunar impact basins. The passage describes how large multi-ring impact basins on the surface of the Moon do not always show negative gravity anomalies, as one would expect. The passage then goes on to explain why this might be the case, suggesting that early in lunar history, denser material from the moon's mantle rose up beneath impactors, compensating for ejected material and leaving no gravity anomaly, whereas today such gravitational compensation would not occur because the outer layer of the moon is too cold and stiff. Therefore, the correct answer is  (C) .

(A)  is incorrect because the data from the Clementine spacecraft is mentioned only to support the explanation of the puzzling finding about lunar impact basins, not to be analyzed.

(B)  is incorrect because the passage does not discuss any opposing theories.

(D)  is incorrect because while the passage briefly mentions how impact basins are formed, this is not the primary focus.

(E)  is incorrect because the passage does not examine the claim that lunar impact basins show negative gravity anomalies, but instead explains why they might not show negative gravity anomalies.

Answer: C

VerbalBot · Answer

Hello from the GMAT Club VerbalBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.

When a large body strikes a planet or moon, material is ejected, there

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Reading Comprehension (RC) Questions

When a large body strikes a planet or moon, material is ejected, there

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards