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Difficulty: EasyComparing and Contrasting Models

### Origin of Earth's Water

How Earth acquired its water is a subject of debate among planetary scientists. Two models describe different origins:

Model 1 (Asteroid Delivery)
Earth initially formed dry because the heat of the early Sun drove volatile compounds outward. Later, water-rich carbonaceous chondrite asteroids from the outer solar system collided with the cooling Earth, depositing water. The Deuterium-to-Hydrogen (D/HD/H) ratio of these asteroids matches the ratio found in Earth's current oceans (1.5×10−41.5 \times 10^{-4}).

Model 2 (Nebular Ingestion)
Earth acquired water during its formation. The proto-Earth was surrounded by hydrogen-rich solar nebula gas. This primordial gas was dissolved directly into the magma ocean of the growing planet, where the hydrogen reacted with iron oxides in the mantle to form water. This model predicts that early Earth water initially had a D/HD/H ratio of 0.2×10−40.2 \times 10^{-4}.

Based on the passage, match each statement about the origin of Earth's water to the model(s) it describes.

  • Water was brought to Earth by colliding carbonaceous chondrites after Earth formed.Model 1 only
  • Water was created by chemical reactions between dissolved solar nebula gas and mantle iron oxides.Model 2 only
  • Proposes a scientific mechanism explaining how Earth acquired its water.Both Model 1 and Model 2

Answer

The statement regarding colliding carbonaceous chondrites matches Model 1 only; the statement regarding solar nebula gas reactions matches Model 2 only; and the statement proposing a mechanism for the origin of Earth's water matches both Model 1 and Model 2.
The correct matches align with the unique characteristics and common purpose of the models: Model 1 attributes water to post-formation asteroid collisions, Model 2 attributes it to chemical reactions during formation, and both models serve to explain the origin of water on Earth.

Step-by-Step Solution

1
Analyze Model 1 to identify its key mechanism.
Model 1 explains water delivery via asteroid collisions after Earth formed, which matches the first statement.
To correctly categorize the carbonaceous chondrites statement.
2
Analyze Model 2 to identify its key mechanism.
Model 2 explains water formation via gas dissolving into the magma ocean and reacting with iron oxides, which matches the second statement.
To correctly categorize the chemical reactions statement.
3
Determine if both models address the overarching scientific question.
Both Model 1 and Model 2 are specifically introduced as models describing different origins of Earth's water.
To correctly categorize the third statement.

Key Concept

Comparing and Contrasting Models
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Topics

Comparing and Contrasting Models

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