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Difficulty: HardIdentifying Core Claims and Hypotheses

### Origin of Earth's Water

How Earth acquired its water remains a subject of active scientific debate. Three scientists present different hypotheses regarding the source and timing of water delivery to the early Earth.

Scientist 1
Earth formed in a region of the solar nebula that was too hot for water vapor to condense. Consequently, the proto-Earth was initially dry. Earth’s water was delivered late in the accretion process, specifically during a "late veneer" phase after the Moon-forming impact. This water was carried by carbonaceous chondrite asteroids originating from the outer solar system, which were rich in hydrated minerals. The isotopic ratio of deuterium to hydrogen (D/HD/H) in Earth's oceans closely matches that of these carbonaceous chondrites, confirming they are the primary source.

Scientist 2
Earth's water is primordial, meaning it was incorporated while the planet was still growing in the presence of the solar nebula. As the proto-Earth grew, its gravity captured a dense, hydrogen-rich nebular atmosphere. This hydrogen dissolved directly into the molten magma ocean. Once dissolved, the hydrogen reacted with iron oxides (FeOFeO) in the magma, producing water (H2OH_2O) and metallic iron (FeFe). The iron sank to form the core, leaving the synthesized water in the mantle. Therefore, Earth's water was synthesized *in situ* prior to the Moon-forming impact, and no late-stage outer solar system bombardment was necessary.

Scientist 3
Earth's water was not delivered by asteroids, nor was it synthesized in a surface magma ocean. Instead, hydrogen gas was trapped inside the iron-rich core during the initial separation of the core and mantle. Over geological time, this hydrogen has steadily diffused upward into the lower mantle. In the mantle, the hydrogen reacts with silicate minerals under high-pressure conditions to synthesize water molecules. This deep-mantle water is slowly transported to the surface via mantle plumes and volcanic outgassing. This ongoing chemical synthesis is the primary contributor to Earth's surface water.

Which of the following statements best describes the core hypothesis of Scientist 3 regarding the origin and delivery of Earth's surface water?

  1. A
    Water molecules were trapped in the core during planetary differentiation and are gradually released to the surface through mantle plumes.
  2. B
    Water was delivered to the mantle by carbonaceous chondrites and is continuously released through volcanic outgassing.
  3. Water molecules are continuously synthesized in the mantle through reactions between silicate minerals and hydrogen gas diffusing from the core.Answer
  4. D
    Water was synthesized in a primordial magma ocean and has been slowly diffusing out of the core over geological time.

Answer

Water molecules are continuously synthesized in the mantle through reactions between silicate minerals and hydrogen gas diffusing from the core.
The correct answer accurately represents the core hypothesis of Scientist 3. Scientist 3 argues that hydrogen gas was trapped in the core during differentiation and subsequently diffuses into the lower mantle, where it reacts with silicate minerals to synthesize water molecules. This matches the description of the chemical reaction and delivery mechanism outlined in the third viewpoint.

Step-by-Step Solution

1
Analyze Scientist 3's description of where the primary components of water originate.
Scientist 3 claims that hydrogen gas was trapped inside the iron-rich core during the initial separation of the core and mantle.
To identify the initial source material according to the specific viewpoint.
2
Determine the location and chemical mechanism of water synthesis described by Scientist 3.
The hydrogen diffuses upward into the lower mantle, where it reacts with silicate minerals under high-pressure conditions to synthesize water molecules.
To pinpoint how and where water is actually formed, distinguishing it from options that claim water itself was trapped in the core or formed in the magma ocean.
3
Match these findings to the answer choices to identify the core claim.
The choice stating that water is continuously synthesized in the mantle through reactions between silicate minerals and hydrogen gas diffusing from the core matches Scientist 3's hypothesis.
To select the correct option representing Scientist 3's core model.

Key Concept

Identifying core claims and hypotheses in conflicting scientific viewpoints
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Topics

Identifying Core Claims and Hypotheses

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