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

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[color=#ff0000] 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..[/color]

(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

Hello idinuv,
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!

Ajeeth Peo
Verbal Trainer,
Crack Verbal.

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.

Question 2
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.
Hope that helps,
Ajeeth
Verbal Tranier- Crack Verbal

Since I marked answer option for Q3 wrong , I want to discuss it.

I marked (C) and in my opinion it is wrong because of the words 'origin of lunar impact basins', when the author is concentrated on the gravity of the holes created in the impact and not the origin.

Please correct me if I am wrong.

Rgds,
TGC!

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.

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

Thanks Crack Verbal. Can anyone of you CrackVerbalGMAT kinjiGC manishkhare Harley1980

please help me with the below question -

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

I always have an issue dealing with this problem.

Any help is appreciated

Hello Steinbeck,
From the passage: "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."

Gravitational compensation refers to the above mentioned phenomenon.

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 over a period of time rebounds from the Moon's surface ,did not cause huge debris to be ejected. Consequently ,denser material did not rose from the crust.

Look at the Options .Only Option E refers to it .
_________________

If you come across a dense passage ,avoid the technical jargon as much as possible. Focus more on the author's intent .Look at the question first and then read the passage .
Answering the question will be easier
_________________

Took 4 mins 30 seconds to answer , including 2 mins 10 seconds to read
-The author explains how local deficits of mass are created on planets or on the moon
- The author then explains why certain multi ring impacts on the moon do not show properties they are expected to show

1."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"
Answer E

2.
"Scientists speculate that early in lunar history "....
"Thus today such gravitational compensation probably would not occur"
Answer A

3.
The author clearly wants to present an explanation on why the lunar surface does not exhibit certain expected properties. Hence option C is the best answer.
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I can help you on this question.
Ok let's refer to our passage.

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. 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.

From Q2 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?

So these scientists mentioned that in early time of the moon, when there is anything hit the moon really hard that that hit area should show gravity anomaly. But the moon was still elastic and its mantle rose up to compensate the gravity anomaly ---> No more gravity anomaly
If you keep reading the next sentence you will find that "Later, however, as moon grew cooler and less elastic, rebound from large impactors would have been only partial and incomplete."
From these sentences, you can make a guess without reading any answer choices that these whatever basins caused negative gravity anomalies ---> No compensation effect ---> Must occured when there was no compensation thing
That's how I got my answer "A"

Or if you wanna to use POE, we can take a look at each answer choices
(A) were not formed early in the Moon's history ---> Sound good. keep it.
(B) were not formed by the massive ejection of crustal debris ---> I look back to the passage and don't think this is really relevant. Out
(C) are closely surrounded by other impact basins with anomalously low gravity ---> the passage doesn't mention that gravity will be affected by surrounded basins. Out
(D) were created by the impact of multiple large impactors ---> Idk can't find any reference in the passage. Out
(E) were formed when the moon was relatively elastic ---> opposite to what are we looking for! Out

With POE. Answer is also "A" but this approach takes longer time.

Did you check this post? https://gmatclub.com/forum/when-a-large ... l#p2081853
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Hi workout

Though I got that OA is E for Q1 but whats wrong in option D.
Can you please help here

Thanks!

gmat1393



As quoted above, the ejection of massive amount of debris, option D, is the reason for the gravitational anamoly.



Again, as quoted above, the gravitational compensation is caused by the denser material from the moon's mantle that rose up which is option E.
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Trying to analyse Q2 .

If we read this part of the passage :
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. 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.

This says > basins with no low gravity i.e no negative gravity. Scientists speculate that something happened in early lunar history.

Now Q2 : talks of basins with negative gravity. So clearly this was not from early lunar history.

If these basins were from early lunar history, they wouldn't have negative gravity.

Point to note is that the basins mentioned in the Q2 and basins mentioned in the above mentioned part of the passage are different.

Hi BARUAH,

Sometimes your kind of analysis though it looks trivial, it is indeed apt. I hope you have used the concept of complement of information along with limiting the scope of what is given and what is not? Sometimes narrowing down the scope too much makes me select the wrong choice especially in RC. In the Q2's case, how can one say if basins with no gravity occurred in the early lunar history, basins with anomalous gravity were not formed?

Hello mallya12

Please go through the post below ( To me it has explained well) ..... but in case if you still have doubts ... let us know.
https://gmatclub.com/forum/when-a-large ... l#p2081853
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I read the link but couldn't understand much from it. Why are option D and E wrong?? Weren't the large multi-ring impact basins on the Moon with the most significant negative gravity anomalies created by the impact of multiple large impactors.

How to justify option A

Thanks

Hi, what I seemed to understand from this passage is quite opposite, actually the passage mentions that "Gravity anomaly is when a hole is created by impactors and that results in low gravity area around it" hence, negative gravity anomaly will be when hole is "compensated" and there no significant low gravity around the area. Please help me understand what am I missing here.