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Difficulty: EasyIdentifying Underlying Assumptions and Premises

Two students discuss the source of water on Earth.

Student 1
Earth's water came entirely from icy comets that bombarded the planet during its early history. Comets are composed largely of water ice, and as they collided with the young, hot Earth, the ice melted, vaporized, and eventually condensed to form the oceans. This explains why the ratio of deuterium (a heavy isotope of hydrogen) to normal hydrogen in Earth's oceans matches the ratio found in comets in the outer solar system.

Student 2
Earth's water did not come from comets, but rather from within the Earth itself. During the planet's formation, water was trapped inside rocks in the mantle. Volcanic eruptions released this water as vapor into the early atmosphere, which then cooled and fell as rain to form the oceans. The deuterium-to-hydrogen ratio in the oceans matches that of mineral-bound water found in ancient meteorites, which were the building blocks of early Earth, not comets.

Based on the passage, Student 1's explanation of the origin of Earth's water relies on which of the following assumptions?

  1. Water vapor released by impacting comets remained on Earth rather than escaping into space.Answer
  2. B
    Water was released from Earth's mantle through volcanic eruptions.
  3. C
    Comets contained no deuterium or hydrogen.
  4. D
    All comets in the solar system collided with Earth during its early history.

Answer

Water vapor released by impacting comets remained on Earth rather than escaping into space.
The correct answer identifies a necessary condition for Student 1's hypothesis to function: the vaporized water from the comets must have stayed on Earth and condensed rather than escaping into space. Without this assumption, the mechanism described by Student 1 could not result in the formation of Earth's oceans.

Step-by-Step Solution

1
Identify Student 1's main argument.
Student 1 argues that Earth's oceans were formed when ice from colliding comets melted, vaporized, and condensed.
Understanding the mechanism proposed is necessary to find its underlying assumptions.
2
Analyze what must be true for this mechanism to work.
If the vaporized water escaped into space instead of condensing, oceans could not have formed from comet impacts.
An assumption is an unstated premise required for the conclusion to be valid.
3
Evaluate the options to find this required premise.
The option stating that water vapor remained on Earth is a necessary condition for Student 1's hypothesis.
This confirms the correct option while eliminating choices that represent the opposing viewpoint or are logically inconsistent.

Key Concept

Identifying Underlying Assumptions and Premises
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Topics

Identifying Underlying Assumptions and Premises

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