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Difficulty: MediumEvaluating the Impact of New Evidence

### Origin of the Moon

Scientists debate how the Moon was formed. Two primary hypotheses have been proposed:

Hypothesis 1
The Moon formed from the debris of a collision between the early Earth and a Mars-sized protoplanet. The impact ejected mostly the silicate-rich outer mantles of both bodies into orbit, where they accreted to form the Moon. Because the iron cores of both bodies merged to remain with Earth, the Moon was left with a very small iron core.

Hypothesis 2
The Moon formed independently in a different region of the solar nebula and was later captured by Earth's gravitational field during a close planetary flyby. This explains why the Moon has a much lower bulk density than Earth, as it formed from materials in a region of the nebula that was naturally depleted of iron.

New Evidence
Analysis of lunar rock samples reveals that the oxygen isotope ratios (17O/16O^{17}\text{O}/^{16}\text{O} and 18O/16O^{18}\text{O}/^{16}\text{O}) of lunar rocks are identical to those of Earth's mantle. Bodies that form in different regions of the solar nebula generally possess distinct, unique oxygen isotope signatures.

Based on this information, how does the new evidence affect the two hypotheses?

  1. It supports Hypothesis 1 and weakens Hypothesis 2.Answer
  2. B
    It weakens Hypothesis 1 and supports Hypothesis 2.
  3. C
    It weakens both Hypothesis 1 and Hypothesis 2.
  4. D
    It supports both Hypothesis 1 and Hypothesis 2.

Answer

The correct answer states that the new evidence supports Hypothesis 1 and weakens Hypothesis 2.
The correct answer explains that the new evidence supports the collision model (Hypothesis 1) and weakens the capture model (Hypothesis 2). According to the new evidence, bodies that form in different regions of the solar nebula possess distinct oxygen isotope signatures. Because the Moon and Earth's mantle have identical oxygen isotope ratios, they must have formed from the same reservoir of material. This supports Hypothesis 1, which claims the Moon formed from Earth's ejected mantle, and weakens Hypothesis 2, which claims the Moon formed independently in a different nebular region.

Step-by-Step Solution

1
Identify the core premise of Hypothesis 1 and Hypothesis 2 regarding the Moon's origin of material.
Hypothesis 1 states the Moon formed from Earth's mantle debris (shared material source). Hypothesis 2 states the Moon formed in a different region of the solar nebula (distinct material source).
Understanding the expected chemical/isotopic relationship between Earth and the Moon for each hypothesis is necessary to evaluate the new evidence.
2
Analyze the new evidence concerning oxygen isotope ratios.
Lunar rocks have oxygen isotope ratios identical to Earth's mantle, and planetary bodies from different regions of the nebula have distinct signatures.
This establishes that identical isotope ratios indicate a shared origin of material, while different regions of formation lead to different ratios.
3
Determine the impact of the new evidence on both hypotheses.
Because Earth and the Moon have identical ratios, they likely share a material source, which supports Hypothesis 1 (debris collision) and weakens Hypothesis 2 (independent formation in a different region).
This directly answers how the evidence supports or weakens each hypothesis based on the logical consistency of their claims with the new data.

Key Concept

Evaluating how new experimental or observational findings support or weaken existing scientific hypotheses.
Estimated Time:1m 30s
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Topics

Evaluating the Impact of New Evidence

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