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

Question

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 
 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 
 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 
 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

https://gmatclub.com/forum/download/file.php?id=67197

AdityaHongunti · Accepted Answer

4 mins 46 seconds : including 2 min 10 sec reading

Divide passage in 2 paras

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 Clementine basins have no anomalously low gravity and some even have anomalously high gravity. 
  
(summary - An expected consequence of a phenomenon is challenged by recent findings.... prethinking - author may try to explain why such data was otained or he may try to put another thesis to explain the data obtained ...)

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.
 
  (summary - an explanation to the unexpected findings... )

TOPIC - Lunar deficit.
PURPOSE- to explain the unexpected finding about a phenomenon...
Main idea - There is a possibilty that the unexpected finding may be the result of upwelling of mass from moon.

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

According to the passage, the  gravitational compensation  referred to in the highlighted text is caused by which of the following?
Relevant text - 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
 - When there is a hole ( deficit) , mantle rises up , and fills the void ..this phenomenon is COMPENSATION...
What causes the compensation?? - the rising of mantle...

(A) A deficit of mass resulting from the creation of hole in lunar surface
- this does not cause anything. This AC is telling about what happen when IMPACTORS collide...

(B) The presence of material from the impactor in the debris created by its impact
- The presence of material does not cause anything.. infact when the impactor makes an impact and the released debris has this material ...OOS

(C) The gradual cooling and stiffening of the Moon's outer surface
-This explains why compensation  MAY not  happen now

(D) The ejection of massive amounts of debris from the moon's crust
-ejection will result in debris in space... nothing else

(E) The rapid upwelling of material from the lunar mantle
- CORRECT ANSWER - rapid ( almost immediately - from passage) upwelling - see explanation...

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

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
-relevant text -  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.

speculation - impact >> deficit (void) >> mantle rises up >> NO GRAVITATIONAL ANAMOLY ... 
so if this process is true what will be true??

(A) were not formed early in the Moon's history- CORRECT
Look carefully at the end of 2nd passage ( 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.) .. so this means   NOW   MOON IS TOO STIFF AND COLD and hence gravitational compensation would not have happened .. if it would have happened it would be comoensated with mantle.. LOOK AT THE TIME PERIODS ... NOW - no compensation .. Earlier time - compensation possible... now the Q says there IS A LARGE DEFICIT ... so if there is a large deficit NOW then it wldnt have formed earlier , because if it did there would be compensation.. but now the moon is stiff and hence no compensation..

(B) were not formed by the massive ejection of crustal debris- 
- There are many ways to form a deficit ... passage never says "only" impactors create

(C) are closely surrounded by other impact basins with anomalously low gravity- OOS
- NO IDEA

(D) were created by the impact of multiple large impactors
- There are many ways to form a deficit ... passage never says "only" impactors create

(E) were formed when the moon was relatively elastic
- MOST COMMON WRONG AC - why we fall for this AC?? because we read only the last part and think that now the moon is stiff and cold so deficits would not have formed but earlier, the moon was elastic so deficit was possible ... SO basically we just messed up the entire para .. the elasticity explanation is for GRAVITATIONAL COMPENSATION and not possible deficit.... the deficits can form all the time .. it is the compensation that cannot

CrackverbalGMAT · Answer

Hey! 
Passages like these are tricky because they are so full of details that keeping track of the main idea can become disorienting. 
And if you rely on memory of the passage from the first phase of reading alone - while answering the questions, chances are that you will probably pick some option that "sounds good" or "looks good" rather than the option that IS substantiated by the passage!

Let's look at the passage and see what parts were important and needed focus

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 Clemenstine spacecraft show that many of these  Clementine 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.

The passage talks about a phenomenon ; shows an example where this does not hold good - and gives explanation for that. 
That's pretty much all the passage is concerned about

Ajeeth 
CrackVerbal - Verbal Expert

CrackverbalGMAT · Answer

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

The passage starts by discussing how impacted bodies in space show gravitational anomalies.

It then discusses the moon and its many craters - it then delves into explaining WHY the moon does not exhibit these negative anomalies.

Option C is correct as the passage does provide possible explanation of a puzzling finding about lunar impact basins (puzzling because the moon has lot of impacted areas, which should ideally show negative gravity anomalies, yet does not show any negative lunar anomalies)

Option E is wrong since the Moon DOES NOT show negative gravity anomalies.

Hope that clarifies things!

CrackverbalGMAT · 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

The Scope for Question 1 rests here " 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...." 
This says that huge amounts of dense debris is ejected upwards. Option E is therefore correct.

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

From the information (scientist's speculation) in the passage we know that if large impacts happened in the early stages, the impact areas would have quickly been covered with mass ejected from the mantle. Therefore the inference that can be made is that these ring shaped craters were formed much later.

Therefore Option A is correct.

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.

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?

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

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.

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

rak08 · Answer

Hi  KarishmaB ,

I inferred the it exactly how you wrote,
in that case why is it that Q2 answer can't be E, that previously "more elastic"

guddo · Answer

Because  (E) says “formed when the moon was relatively elastic,”  and the passage says  elastic early moon impacts got compensated  by mantle rebound, leaving  no negative anomaly  (or even higher gravity). The  most negative  anomalies would happen when the moon is cold and stiff, so compensation does not occur. Hence not (E).

egmat · Answer

The confusion is understandable, but choice  E  actually says the OPPOSITE of what the passage implies.

Let's trace the logic carefully:

- When the Moon was young and ELASTIC, mantle material rushed up to fully compensate for ejected debris. Result?  NO  gravity anomaly.
- When the Moon became COOLER and STIFFER (less elastic), the rebound was only partial. Result?  Negative gravity anomaly  remains.

Now the question asks: which basins have the MOST SIGNIFICANT negative gravity anomalies?

Those would be the ones where compensation was LEAST complete — meaning the Moon was at its STIFFEST,  not  its most elastic.

Choice E says the opposite : it claims these basins formed when the Moon was "relatively elastic." But if the Moon were elastic, the mantle would have rebounded fully, and there would be NO significant negative anomaly at all!  This is a common mistake — watch out for answer choices that reverse the relationship described in the passage.

Choice A works because saying they were "not formed early" is equivalent to saying they were formed LATER, when the Moon had already cooled and stiffened.  That's exactly when you'd get incomplete compensation and  therefore  the most significant negative gravity anomalies.

Key Insight:  "Elastic" in the passage is associated with FULL compensation (no anomaly), while "stiff/cool" is associated with INCOMPLETE compensation (negative anomaly). Choice E gets this relationship backwards.

Answer: A

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

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

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