Examkin

Question

Difficulty: MediumDistinguishing Main Point from Subsidiary Points

For decades, the standard model of early solar system dynamics included a period known as the Late Heavy Bombardment (LHB), a hypothesized epoch approximately 3.9 billion years ago during which the inner planets experienced a sudden, dramatic spike in asteroid and comet impacts. This consensus emerged primarily from radiometric dating of impact melt rocks returned by the Apollo missions; the samples, collected from diverse lunar equatorial regions, consistently yielded ages tightly clustered around the 3.9-billion-year mark. Because the Apollo landing sites were widely separated, planetary scientists inferred that the rocks recorded dozens of distinct basin-forming impacts occurring nearly simultaneously, implying a system-wide cataclysm.

However, recent re-evaluations of both lunar geology and the Apollo samples have cast significant doubt on the LHB hypothesis. The original consensus rested on the assumption that the impact melts collected at each Apollo site were produced by localized, independent cratering events. Yet, advanced geochemical fingerprinting and high-resolution computer modeling of ejecta trajectories suggest a different narrative. These studies indicate that the massive impact that formed the Imbrium basin—one of the youngest and largest craters on the Moon—was energetic enough to distribute its debris globally.

Consequently, researchers now argue that the widely separated Apollo sites were almost certainly contaminated by ejecta from the single Imbrium event. If the impact melts dated by scientists predominantly originate from this one colossal collision, the clustering of their ages ceases to be evidence for a sudden flurry of distinct impacts. Instead, the data more plausibly points to a single dominant event that obscured the broader cratering record. Thus, rather than experiencing a delayed, violent spike in impacts, the inner solar system likely underwent a gradual, continuous decline in bombardment following its initial formation.

Which one of the following most accurately expresses the main point of the passage?

  1. A
    Radiometric dating of impact melt rocks collected from widely separated lunar landing sites consistently yields ages clustered around 3.9 billion years ago.
  2. B
    The geological history of the inner solar system is characterized by a gradual, continuous decline in asteroid collisions rather than by sudden, catastrophic spikes.
  3. C
    Planetary scientists traditionally assumed that the formation of the Imbrium basin caused a sudden, dramatic spike in asteroid and comet impacts.
  4. The theory that the inner solar system underwent a sudden spike in meteor impacts is undermined by evidence that lunar sample ages reflect contamination from a single massive collision.Answer
  5. E
    Advanced geochemical fingerprinting and computer modeling demonstrate that ejecta from the Imbrium impact was distributed globally across the Moon.

Answer

The correct answer is the option stating that the theory of a sudden spike in meteor impacts is undermined by evidence of lunar sample contamination from a single collision.
The correct answer effectively captures the comprehensive argument of the passage. It identifies the traditional view (a sudden spike in impacts) and summarizes how the new evidence (contamination from a single massive collision) challenges that view, perfectly mirroring the author's synthesis in the final paragraph.

Step-by-Step Solution

1
Analyze the passage's structure to locate the author's central argument.
The passage introduces a traditional theory in paragraph 1, examines its faulty foundational assumption in paragraph 2, and synthesizes a new conclusion in paragraph 3.
Identifying the rhetorical structure prevents confusing background context or intermediate evidence with the main thesis.
2
Synthesize the conclusion presented in the final paragraph.
The author concludes that Apollo data points to contamination by a single dominant event rather than a system-wide spike, refuting the Late Heavy Bombardment hypothesis.
The final paragraph explicitly draws the overarching implication of the new evidence discussed earlier.
3
Evaluate the options against the synthesized conclusion, filtering out subsidiary points.
Only the correct option captures both the traditional theory and the specific evidence that refutes it. Other options either state intermediate evidence, distort the facts, or generalize too broadly.
The main point must encompass the entire logical arc of the passage, not just individual true statements.

Key Concept

Distinguishing Main Point from Subsidiary Points
Estimated Time:2m 0s
Rate this question

Topics

Distinguishing Main Point from Subsidiary Points

More questions from this topic