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Difficulty: HardEvaluating Inter-Passage Relationships and Function

Passage A

For decades, the dominant model for the origin of Earth’s surface water has been late-delivery via volatile-rich impactors. Proponents of this theory point to the deuterium-to-hydrogen (D/H) ratio of Earth’s oceans, which closely matches that of carbonaceous chondrites—asteroids from the outer main belt. Comets, once favored candidates, have largely been ruled out because their D/H ratios are typically twice as high as Earth's. According to the late-veneer model, Earth formed hot and dry, having lost its primordial volatile budget due to high-energy collisions, including the Moon-forming impact. Water was subsequently re-introduced during an intense bombardment phase roughly 3.9 billion years ago. This narrative provides a straightforward, chemically consistent explanation for the abundance of water on our planet's surface.

Passage B

While the meteorite-delivery hypothesis offers a neat explanation for surface ocean chemistry, it overlooks the vast reservoir of water locked deep within Earth's mantle. Recent mineralogical studies of ringwoodite—a high-pressure polymorph of olivine found in the transition zone—suggest the mantle contains up to three times the volume of water currently in the oceans. This water bears an isotopic signature that deviates from carbonaceous chondrites, pointing instead to primordial hydrogen captured directly from the solar nebula during the planet's primary accretion. This ingassed hydrogen would have reacted with iron in the early mantle to form water, which was subsequently released through volcanic outgassing. Surface oceans, then, are not an accidental cosmic deposit, but rather the slow leak of our planet’s internal geological engine.

Which of the following statements best describes the relationship between the two passages?

  1. Passage B challenges the completeness of Passage A’s model by presenting evidence of internal geological processes that contributed to Earth's water budget.Answer
  2. B
    Passage B directly refutes the chemical evidence presented in Passage A by showing that carbonaceous chondrites have a different deuterium-to-hydrogen ratio.
  3. C
    Passage B provides empirical support for Passage A's timeline of water accumulation while suggesting a different source for surface oceans.
  4. D
    Passage B details the specific mineralogical composition of the carbonaceous chondrites discussed in Passage A.

Answer

Passage B challenges the completeness of Passage A’s model by presenting evidence of internal geological processes that contributed to Earth's water budget.
The correct answer is that Passage B challenges the completeness of Passage A's model. While Passage A argues that Earth's water was delivered externally by meteorites, Passage B highlights that this model overlooks mantle water (up to three times the ocean volume) and explains that this water comes from primordial hydrogen captured during early accretion and released via internal volcanic outgassing.

Step-by-Step Solution

1
Analyze Passage A to identify its main argument and supporting evidence.
Passage A argues that Earth's water came from space via meteorites (carbonaceous chondrites) about 3.9 billion years ago, supported by matching deuterium-to-hydrogen (D/H) ratios.
Understanding the core thesis of the first passage is necessary to compare it to the second.
2
Analyze Passage B to identify its relationship to Passage A's argument.
Passage B admits the meteorite hypothesis works for surface oceans but argues that it 'overlooks' a massive amount of mantle water that has a different isotopic signature and was captured during early accretion, leaking out over time.
This identifies how Passage B expands the scope of the discussion to find limitations in Passage A's model.
3
Synthesize the relationship between the two passages.
Passage B does not entirely reject Passage A's explanation of surface ocean chemistry, but it shows it is incomplete because it ignores mantle water and the internal outgassing process.
This points to the option describing Passage B as challenging the completeness of Passage A's model using internal processes.

Key Concept

Evaluating Inter-Passage Relationships and Function
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Topics

Evaluating Inter-Passage Relationships and Function

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