When a large body strikes a planet or moon, material is ejected, thereby creating a hole in the planet and...
GMAT Reading Comprehension : (RC) Questions
Source: Official Guide
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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.
Ques. 1/3
According to the passage, the gravitational compensation referred to in the highlighted text is caused by which of the following?
Solution
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| 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. | • What it says: When something big hits a planet or moon, stuff gets thrown out, making a hole and removing mass from that spot. • What it does: Introduces the basic concept of impact craters - sets up the fundamental cause-and-effect relationship. • Source/Type: Scientific fact - describing a physical process. • Connection to Previous Sentences: This is the opening sentence, establishing our foundation concept. • Visualization: Imagine a 50-mile-wide asteroid hitting the Moon. The impact throws billions of tons of rock into space, leaving a crater 200 miles across. Where all that rock used to be, there's now empty space. • What We Know So Far: Big impacts = holes + missing mass • What We Don't Know Yet: What this has to do with gravity, what specific examples we'll discuss |
| 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. | • What it says: The missing mass from sentence 1 causes weaker gravity in that area. • What it does: Explains the logical consequence of the first sentence - connects missing mass to gravity effects. • Source/Type: Scientific explanation - cause and effect. • Connection to Previous Sentences: This builds directly on sentence 1. Sentence 1 said "material ejected + hole + mass deficit." NOW sentence 2 tells us: "mass deficit = gravity anomaly." This is NOT new complexity - it's the natural next step in the logical chain. • Visualization: That same 200-mile crater from before: if you stood at the edge with a sensitive gravity meter, it would read slightly lower in the center of the crater because there's less rock mass beneath you to pull you down. • Reading Strategy Insight: This sentence EXPLAINS the first sentence rather than adding complexity. Feel confident - you're building understanding step by step. • What We Know So Far: Impact → missing mass → lower gravity in that area |
| 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. | • What it says: So logically, all the big craters on the Moon should have lower gravity, since they're just big holes. • What it does: Applies the established principle to a specific case (Moon craters) and sets up a logical expectation. • Source/Type: Author's logical reasoning ("One would expect"). • Connection to Previous Sentences: This is pure restatement and application! Sentences 1-2 established "impact holes = lower gravity." NOW sentence 3 simply says "Moon craters are impact holes, so they should have lower gravity." This is the SAME idea in a specific context. • Visualization: Think of the Moon's famous large craters like Mare Imbrium or Mare Orientale - those massive circular basins. Based on what we learned, each should show up as a "gravity dimple" when measured. • Reading Strategy Insight: Feel relieved here - this is simplification, not new complexity! The author is just saying "Hey, this principle we just learned should apply to Moon craters." • What We Know So Far: General principle + specific application to Moon basins |
| 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 it says: But actual 1994 spacecraft data shows many Moon basins DON'T have lower gravity - some even have HIGHER gravity. • What it does: Introduces the central puzzle/contradiction that the passage will solve. • Source/Type: Scientific data/evidence from specific mission. • Connection to Previous Sentences: This creates a direct contrast with sentence 3's expectation. Sentence 3: "should have lower gravity." Sentence 4: "actually don't have lower gravity." This is the classic setup: logical expectation vs. surprising reality. • Visualization: Scientists mapping lunar gravity expected to see: Basin A (weak gravity), Basin B (weak gravity), Basin C (weak gravity). Instead they found: Basin A (normal gravity), Basin B (STRONG gravity), Basin C (weak gravity). • Reading Strategy Insight: This contradiction is GOOD news for readers - it means the rest of the passage will solve this puzzle. You now know exactly what question the passage will answer. • What We Know So Far: Theory vs. Reality mismatch established - this is our central problem to solve |
| 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. | • What it says: Scientists think that long ago, when stuff got blasted out of the Moon, heavier material from deep inside immediately moved up to fill the space, balancing out the gravity. • What it does: Provides the solution to the puzzle from sentence 4. • Source/Type: Scientists' hypothesis/speculation. • Connection to Previous Sentences: This directly solves sentence 4's contradiction. Sentence 4 said "Why don't these basins show lower gravity?" Sentence 5 answers: "Because heavier material moved in to replace what was lost." This is problem → solution, not new complexity. • Visualization: Picture a 3-layer cake: light frosting on top, regular cake in middle, dense fruit filling at bottom. A meteor punches out some frosting and cake. Immediately, the dense fruit filling bubbles up to replace what was lost. Net result: the surface now has the same total "weight" as before. • Reading Strategy Insight: This is the "aha!" moment - the passage is becoming CLEARER, not more complex. The mystery is being solved. • What We Know So Far: Ancient impacts → material ejected → but heavier material rose up → gravity stayed normal |
| Later, however, as moon grew cooler and less elastic, rebound from large impactors would have been only partial and incomplete. | • What it says: But later, as the Moon cooled and became more rigid, this "filling in" process happened only partially. • What it does: Introduces a time-based contrast - explains why the compensation process changed over time. • Source/Type: Scientific reasoning/explanation. • Connection to Previous Sentences: This adds a time dimension to sentence 5's explanation. Sentence 5: "early impacts had full compensation." Sentence 6: "later impacts had only partial compensation." This is elaboration, not contradiction. • Visualization: Early Moon = hot, flexible rubber ball that bounces back completely when dented. Later Moon = cold, stiff clay that only partially springs back when dented. • Reading Strategy Insight: This refines our understanding rather than complicating it. We're learning about the TIMELINE of this process. • What We Know So Far: Early Moon (hot) = full compensation; Later Moon (cool) = partial compensation |
| Thus today such gravitational compensation probably would not occur: the outer layer of moon is too cold and stiff. | • What it says: So today, this gravity-balancing process probably wouldn't happen because the Moon's surface is too cold and rigid. • What it does: Provides the logical conclusion and brings the timeline to the present. • Source/Type: Author's logical conclusion ("Thus"). • Connection to Previous Sentences: This completes the timeline from sentence 6. We now have: Early Moon (full compensation) → Later Moon (partial compensation) → Modern Moon (no compensation). This is the natural endpoint of the cooling process described earlier. • Visualization: If a major asteroid hit the Moon tomorrow, it would create a crater with genuinely lower gravity because the Moon is now like a frozen, brittle shell that can't "heal" itself. • Reading Strategy Insight: Perfect! This ties everything together. The passage has taken us from the basic principle, through the historical puzzle, to the complete explanation. You should feel MORE confident now, not less. • What We Know So Far: Complete timeline understanding - from early compensation to modern non-compensation, explaining the mixed gravity data from 1994 |
2. Passage Summary:
Author's Purpose:
To explain why lunar impact basins don't show the gravity patterns that scientists expected to find, by describing how the Moon's changing physical properties over time affected how it responded to asteroid impacts.
Summary of Passage Structure:
In this passage, the author walks us through a scientific puzzle and its solution in logical steps:
- First, the author explains the basic principle that when asteroids hit planets or moons, they create holes and remove mass, which should cause lower gravity in those areas.
- Next, the author applies this principle to the Moon, explaining that all lunar impact basins should logically show lower gravity since they are essentially large holes.
- Then, the author introduces surprising data that contradicts this expectation - many lunar basins actually have normal or even higher gravity than expected.
- Finally, the author resolves this puzzle by explaining that early in the Moon's history, it was hot and flexible enough for dense material to rise up and replace ejected material, but as the Moon cooled and became rigid over time, this compensation process became incomplete and eventually stopped working.
Main Point:
The reason many lunar impact basins don't show the expected lower gravity is that when the Moon was younger and hotter, dense material from deep inside could quickly rise up to replace the lighter material that got blasted away, keeping gravity levels normal. As the Moon cooled and became more rigid over time, this self-repair process became less effective.
3. Question Analysis:
The question asks us to identify what causes "gravitational compensation" as referred to in the highlighted text. From the passage analysis, we know that gravitational compensation is the process that explains why many lunar basins don't show the expected negative gravity anomalies.
Connecting to Our Passage Analysis:
From our passage analysis, we identified that sentence 5 provides the key explanation for gravitational compensation: "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." This sentence directly explains the mechanism - it's the upward movement of dense mantle material that creates the compensation effect.
The passage analysis also showed us the complete timeline:
- Early Moon (hot and flexible) - full gravitational compensation occurred
- Later Moon (cooler, less elastic) - partial compensation
- Modern Moon (cold and stiff) - no compensation would occur
Prethinking:
Based on our understanding from the passage analysis, gravitational compensation happens when dense material from the lunar mantle rises up to replace the lighter crustal material that was ejected during impact. This upward movement of heavier material balances out the mass deficit, preventing the expected negative gravity anomaly. The correct answer should identify this upward movement of mantle material as the cause of compensation.
Answer Choices Explained
A
A deficit of mass resulting from the creation of hole in lunar surface
Why It's Wrong:
- The deficit of mass is what creates the problem that needs compensation, not what causes the compensation itself
- This describes the initial condition (missing mass) rather than the compensating mechanism
- According to our passage analysis, gravitational compensation specifically counteracts this mass deficit
- Confusing the problem with the solution?
→ Remember that compensation fixes the mass deficit - it doesn't cause it - Mixing up cause and effect in the compensation process?
→ Focus on what creates the balancing effect, not what creates the original imbalance
B
The presence of material from the impactor in the debris created by its impact
Why It's Wrong:
- The passage doesn't suggest that impactor material contributes to gravitational compensation
- Material from the impactor would be negligible compared to the massive amounts of ejected lunar material
- Our passage analysis showed that compensation comes from lunar mantle material moving upward, not from impactor debris
- Thinking the impactor itself provides the compensating mass?
→ The impactor is tiny compared to the millions of cubic kilometers of ejected crustal material - Confusing different types of material mentioned in the passage?
→ Focus on the distinction between ejected crustal material and rising mantle material
C
The gradual cooling and stiffening of the Moon's outer surface
Why It's Wrong:
- Cooling and stiffening actually prevents gravitational compensation from occurring
- According to our passage analysis, this describes why modern impacts wouldn't show compensation
- This explains the timeline of when compensation stopped working, not what caused it to work
- Thinking cooling enables compensation rather than prevents it?
→ Re-read the timeline: early (hot) Moon had compensation, later (cool) Moon lost this ability - Confusing the conditions that allow compensation with compensation itself?
→ Heat and flexibility allowed compensation to occur, but the rising mantle material was the actual compensating mechanism
D
The ejection of massive amounts of debris from the moon's crust
Why It's Wrong:
- Ejection of debris is what creates the mass deficit that needs to be compensated
- This describes the problem that compensation solves, not the mechanism of compensation itself
- From our passage analysis, this is the initial step that creates the gravity anomaly, not what fixes it
- Thinking that ejection somehow causes its own compensation?
→ Ejection creates the deficit; compensation requires additional material to fill that deficit - Focusing on the dramatic debris ejection rather than the subtler compensating process?
→ Look for what replaces the ejected material, not what ejects it
E
The rapid upwelling of material from the lunar mantle
Why It's Right:
- This directly matches the mechanism described in the passage: "denser material from the moon's mantle rose up beneath the impactors almost immediately"
- The upward movement of dense mantle material is specifically what compensates for the ejected lighter crustal material
- This explains how the mass deficit gets balanced out, preventing negative gravity anomalies
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