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Difficulty: HardIdentifying Underlying Assumptions and Premises

### Origin of Earth's Water

Two hypotheses address the origin of Earth's surface oceans.

Hypothesis 1
During Earth's accretion from planetesimals in the inner solar nebula, the high temperatures from gravitational collapse and radioactive decay vaporized all local water. Because Earth's early gravitational field was too weak to retain these light volatile gases, this water vapor escaped into space, leaving the planet completely dry. Subsequently, during the Late Heavy Bombardment (≈4.0\approx 4.0 to 3.83.8 billion years ago), water was delivered to Earth's surface by carbonaceous chondrite meteorites. These meteorites are rich in water (up to 20%20\% by weight) and possess a deuterium-to-hydrogen (D/HD/H) ratio (≈1.5×10−4\approx 1.5 \times 10^{-4}) that is identical to that of Earth's modern oceans.

Hypothesis 2
Earth's water is primordial, originating from hydrated silicate minerals within the local planetesimals that accreted to form the planet. During the rapid accretion process, Earth's gravity was strong enough to retain the steam outgassed from its molten interior. Deep mantle reservoirs, insulated from surface vapor loss, preserved this primordial water. The D/HD/H ratio of this mantle water is lower (≈1.2×10−4\approx 1.2 \times 10^{-4}) than that of modern surface water. Tectonic recycling and the preferential escape of lighter hydrogen isotopes to space over billions of years have gradually increased the surface D/HD/H ratio to its modern value of 1.5×10−41.5 \times 10^{-4}.

According to the passage, Hypothesis 1 and Hypothesis 2 differ fundamentally in their assumptions regarding which of the following?

  1. Whether Earth's gravitational field during the accretion phase was strong enough to prevent outgassed water vapor from escaping into spaceAnswer
  2. B
    Whether the deuterium-to-hydrogen ratio of modern ocean water is approximately 1.5×10−41.5 \times 10^{-4}
  3. C
    Whether carbonaceous chondrites delivered water during the Late Heavy Bombardment
  4. D
    Whether tectonic recycling of surface water can alter the isotopic composition of the mantle over geological time

Answer

Whether Earth's gravitational field during the accretion phase was strong enough to prevent outgassed water vapor from escaping into space
The correct option is correct because it identifies the primary physical divergence in the initial states of the models: Hypothesis 1 assumes Earth's gravity was too weak to retain water vapor during accretion, while Hypothesis 2 assumes Earth's gravity was strong enough to prevent this vapor from escaping.

Step-by-Step Solution

1
Analyze Hypothesis 1's premise regarding early water.
Hypothesis 1 assumes that during accretion, Earth's gravity was too weak to prevent vaporized water from escaping into space, necessitating a later external source (meteorites).
Understanding the physical constraints assumed by the first model is required to compare it with the second.
2
Analyze Hypothesis 2's premise regarding early water.
Hypothesis 2 assumes that Earth's gravity was strong enough during accretion to retain the steam outgassed from the interior, allowing native water to form the oceans.
This establishes the opposing physical assumption held by the second model.
3
Identify the core difference in their underlying premises.
The two models disagree on Earth's gravitational capacity to retain volatile water vapor during accretion.
Comparing these premises yields the fundamental difference in the underlying assumptions of the two viewpoints.

Key Concept

Identifying Underlying Assumptions and Premises
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Identifying Underlying Assumptions and Premises

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