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Difficulty: HardIdentifying Points of Disagreement

### Origin of Earth's Water

Scientists discuss the origin of Earth's water and the mechanisms by which the oceans were formed.

Hypothesis 1
Earth’s water was delivered primarily by carbonaceous chondrite asteroids from the outer asteroid belt after Earth’s accretion was complete. The deuterium-to-hydrogen (D/HD/H) ratio of Earth's surface oceans (~1.5×10−41.5 \times 10^{-4}) is identical to that of carbonaceous chondrites, whereas comets have much higher ratios and the primordial solar nebula has a much lower ratio (~2.1×10−52.1 \times 10^{-5}). Furthermore, during the early accretion phase, Earth’s surface was molten and temperatures were too high to retain volatile water; any water present during this phase would have vaporized and escaped into space.

Hypothesis 2
Earth’s water is endogenous, originating from the primordial solar nebula and retained within the mantle during Earth's accretion. High pressures within the growing planet prevented water from escaping. Over geological time, volcanic activity outgassed this primordial water to form the oceans. Deep mantle mineral samples exhibit D/HD/H ratios significantly lower than those of surface oceans, aligning closely with the primordial solar nebula. Asteroid impacts occurred too late to account for the bulk of Earth's interior water.

For each key physical or chemical aspect of Earth's water history listed on the left, which description on the right correctly identifies the point of disagreement between Hypothesis 1 and Hypothesis 2?

  • Primary chemical or physical source of Earth's waterHypothesis 1 argues for delivery via outer asteroid belt meteorites, while Hypothesis 2 argues for endogenous accumulation from the solar nebula.
  • Timeframe of water delivery or accumulationHypothesis 1 claims water arrived post-accretion as a 'late veneer', while Hypothesis 2 claims water was incorporated concurrently with the accretion of the planet.
  • Expected bulk deuterium-to-hydrogen (D/HD/H) ratio of Earth's waterHypothesis 1 predicts a higher ratio (~1.5×10−41.5 \times 10^{-4}) matching carbonaceous chondrites, while Hypothesis 2 predicts a lower ratio (~2.1×10−52.1 \times 10^{-5}) matching solar nebular gas.
  • Fate of water volatiles during early Earth accretionHypothesis 1 asserts that early heat caused all water to vaporize and escape into space, while Hypothesis 2 asserts that high pressures retained water within the mantle.

Answer

Primary source matches Hypothesis 1: Asteroids / Hypothesis 2: Nebula; Timeframe matches Post-accretion / Concurrent; D/H ratio matches High / Low; Volatile retention matches Vaporization / Retention.
Each aspect represents a fundamental disagreement between the two hypotheses: Hypothesis 1 views Earth's water as exogenously delivered via asteroids after Earth formed, which is reflected in a high D/HD/H ratio and is predicated on the idea that early water could not survive Earth's hot accretion phase. Hypothesis 2 views Earth's water as endogenously trapped from the solar nebula during accretion, which is reflected in a lower mantle D/HD/H ratio and is predicated on the idea that mantle pressures prevented early water from escaping.

Step-by-Step Solution

1
Analyze the core claim of Hypothesis 1 regarding the origin, timing, isotopic signature, and physical retention of Earth's water.
Hypothesis 1 claims water came post-accretion from carbonaceous chondrites with a high D/HD/H ratio of ~1.5×10−41.5 \times 10^{-4} because early volatile water escaped into space due to intense heat.
Establishes the baseline parameters for the first viewpoint.
2
Analyze the core claim of Hypothesis 2 regarding the same variables.
Hypothesis 2 claims water is endogenous, incorporated during accretion from the solar nebula with a low D/HD/H ratio (~2.1×10−52.1 \times 10^{-5}) and retained in the mantle under high pressure.
Establishes the baseline parameters for the second viewpoint.
3
Compare the claims for each specific variable to identify the exact points of disagreement.
The viewpoints conflict on the source (asteroids vs. solar nebula), timeline (post-accretion vs. during accretion), isotopic ratio (high vs. low), and early retention mechanism (loss to space vs. mantle retention under pressure).
Allows mapping of the left-hand items to the right-hand items based on their points of conflict.

Key Concept

Identifying points of disagreement between scientific hypotheses regarding the source, timing, isotopic composition, and preservation of Earth's water.
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Topics

Identifying Points of Disagreement

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