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Difficulty: MediumEvaluating the Impact of New Evidence

### Origin of Earth's Water

Earth is unique among the inner planets for having vast surface oceans of liquid water. Scientists debate how Earth acquired its water during its formation. Two hypotheses have been proposed:

Hypothesis 1
Earth formed in a region of the solar system that was too hot for water to condense. Consequently, Earth was initially dry. Water was delivered to Earth late in its formation by comets from the cold outer solar system. Because comets are composed of up to 50% water ice, a relatively small number of cometary impacts could have easily filled Earth's ocean basins.

Hypothesis 2
Earth's water was delivered during its accretion phase by carbonaceous chondrite asteroids from the outer asteroid belt. These asteroids contain significant amounts of water bound in hydrated clay minerals. Since these asteroids formed closer to the Sun than comets did, their minerals stable at higher temperatures brought water directly to the developing Earth.

A new study measured the deuterium-to-hydrogen (D/HD/H) ratio—a standard chemical fingerprint for water sources—in Earth's oceans. The study found that Earth's ocean water has a D/HD/H ratio of 1.56×10−41.56 \times 10^{-4}. Measurements from outer solar system comets show an average D/HD/H ratio of 3.1×10−43.1 \times 10^{-4}, while carbonaceous chondrite asteroids show an average D/HD/H ratio of 1.59×10−41.59 \times 10^{-4}.

Based on this new study, how is the credibility of Hypothesis 1 and Hypothesis 2 affected, if at all?

  1. Hypothesis 1 is weakened and Hypothesis 2 is supported, because Earth's D/HD/H ratio matches that of carbonaceous chondrite asteroids but is much lower than that of outer solar system comets.Answer
  2. B
    Hypothesis 1 is supported and Hypothesis 2 is weakened, because Earth's D/HD/H ratio matches that of outer solar system comets but is much lower than that of carbonaceous chondrite asteroids.
  3. C
    Both Hypothesis 1 and Hypothesis 2 are supported, because the D/HD/H ratio of Earth's water is an average of the ratios of comets and asteroids, indicating both sources contributed equally.
  4. D
    Both Hypothesis 1 and Hypothesis 2 are weakened, because Earth's D/HD/H ratio of 1.56×10−41.56 \times 10^{-4} is significantly higher than the ratios of both comets and asteroids.

Answer

Hypothesis 1 is weakened and Hypothesis 2 is supported, because Earth's D/HD/H ratio matches that of carbonaceous chondrite asteroids but is much lower than that of outer solar system comets.
The correct answer states that Hypothesis 1 is weakened and Hypothesis 2 is supported because the D/HD/H ratio of Earth's oceans (1.56×10−41.56 \times 10^{-4}) is nearly identical to that of carbonaceous chondrite asteroids (1.59×10−41.59 \times 10^{-4}), supporting Hypothesis 2, while being roughly half that of outer solar system comets (3.1×10−43.1 \times 10^{-4}), which weakens the cometary delivery claim of Hypothesis 1.

Step-by-Step Solution

1
Identify the D/HD/H ratios provided in the new study for Earth's ocean water, comets, and asteroids.
Earth's ocean water has a ratio of 1.56×10−41.56 \times 10^{-4}; comets have a ratio of 3.1×10−43.1 \times 10^{-4}; asteroids have a ratio of 1.59×10−41.59 \times 10^{-4}.
This establishes the factual basis for comparing the new evidence with the predictions of the hypotheses.
2
Compare Earth's ocean water D/HD/H ratio directly to the values for comets and asteroids.
Earth's ratio (1.56×10−41.56 \times 10^{-4}) is extremely close to the asteroid ratio (1.59×10−41.59 \times 10^{-4}) and significantly lower than the comet ratio (3.1×10−43.1 \times 10^{-4}).
Determining similarity helps decide which source is a more likely origin for Earth's water.
3
Evaluate the impact of this comparison on both hypotheses.
Since the ratio matches asteroids, Hypothesis 2 (asteroid delivery) is supported. Since it differs significantly from comets, Hypothesis 1 (comet delivery) is weakened.
Aligning the physical evidence with the claims of each hypothesis reveals how their credibility changes.

Key Concept

Evaluating how new physical evidence supports or weakens competing scientific hypotheses.
Estimated Time:1m 15s
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Evaluating the Impact of New Evidence

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