**Passage** The origin of Earth's volatile elements, particularly water, remains a central… — ACT

Passage

The origin of Earth's volatile elements, particularly water, remains a central question in planetary science. Two scientists present competing hypotheses regarding the source and evolution of Earth's water.

Scientist 1
Earth accreted inside the "frost line"—the radial distance in the solar nebula where temperatures were cool enough for volatile compounds like water ice to condense. Consequently, the primordial materials that formed the proto-Earth were completely dry. Earth’s water was delivered during the late veneer phase, after core differentiation was complete, by carbonaceous chondrite meteorites originating from the outer solar system. The deuterium-to-hydrogen ( $D/H$ ) ratio of Earth's oceans has remained constant at approximately $1.5 \times 10^{-4}$ since this delivery. Because this value matches the $D/H$ ratio of carbonaceous chondrites, it serves as a pristine chemical signature of the late-accreting outer solar system material.

Scientist 2
Earth's water is primordial and was accreted directly from enstatite chondrite-like planetesimals in the inner solar system. Although temperatures were too high for water ice to condense, hydrogen was incorporated directly into the iron and silicate mineral lattices of the accreting planetesimals. The $D/H$ ratio of Earth's surface water has not remained constant. Initially, Earth's primordial water had a $D/H$ ratio of $1.0 \times 10^{-4}$ , identical to enstatite chondrites. Over billions of years, solar ultraviolet radiation photolyzed atmospheric water vapor, and the lighter protium ( $^1H$ ) isotope preferentially escaped Earth’s gravity compared to the heavier deuterium ( $^2H$ ). This selective escape of protium acted as the primary driver that gradually elevated the surface $D/H$ ratio to its current value of $1.5 \times 10^{-4}$ .

Based on the passage, Scientist 1 and Scientist 2 differ in their views regarding which of the following?

Whether the isotopic ratio of hydrogen on Earth's surface has changed significantly since the planet's formation.Answer
B
Whether the current $D/H$ ratio of Earth's ocean water is approximately $1.5 \times 10^{-4}$ .
C
Whether carbonaceous chondrites or enstatite chondrites served as the primary source of the volatile elements delivered to the outer solar system.
D
Whether the preferential escape of protium from the atmosphere is the independent variable that determines the temperature of the inner solar nebula.

Answer

Whether the isotopic ratio of hydrogen on Earth's surface has changed significantly since the planet's formation.

The correct answer is the option stating 'Whether the isotopic ratio of hydrogen on Earth's surface has changed significantly since the planet's formation.' Scientist 1 claims that the

D/H

ratio has remained constant at

1.5 \times 10^{-4}

since its delivery by carbonaceous chondrites. In contrast, Scientist 2 claims that the ratio has not remained constant, starting at

1.0 \times 10^{-4}

and gradually increasing to

1.5 \times 10^{-4}

due to the escape of lighter protium.

Step-by-Step Solution

Identify Scientist 1's position on the history of Earth's

D/H

ratio.

Scientist 1 states that the

D/H

ratio has remained constant at approximately

1.5 \times 10^{-4}

since it was delivered.

This establishes the baseline claim of the first hypothesis regarding the temporal stability of the isotopic ratio.

Identify Scientist 2's position on the history of Earth's

D/H

ratio.

Scientist 2 states that the

D/H

ratio has not remained constant, starting at

1.0 \times 10^{-4}

and rising to

1.5 \times 10^{-4}

over time.

This establishes the competing claim of the second hypothesis regarding the temporal stability of the isotopic ratio.

Compare the two positions to find the direct point of contradiction.

Scientist 1 claims the ratio has remained stable/constant, while Scientist 2 claims it has changed/increased over geologic time.

This direct conflict confirms that the stability of the ratio over time is the key point of disagreement.

Key Concept

Identifying Points of Disagreement

Estimated Time:2m 0s

Question

Answer

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Key Concept

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