Examkin

Question

Difficulty: MediumIdentifying Underlying Assumptions and Premises

### Origin of the Moon

Two scientists present opposing viewpoints on the origin of Earth's Moon.

Scientist 1
The Moon was originally an independent planetesimal that accreted in a different region of the solar nebula than Earth. As this planetesimal passed close to Earth, it was captured by Earth's gravity and pulled into a stable orbit. When two planetary bodies form in different regions of the solar system, they accrete from different reservoirs of dust and gas, which possess distinct ratios of oxygen isotopes (18O^{18}\text{O} to 16O^{16}\text{O}). Therefore, the Moon must have a different oxygen isotope ratio than Earth.

Scientist 2
The Moon formed from the debris of a collision between the young Earth and a Mars-sized protoplanet. The high energy of this impact melted and vaporized both bodies, allowing their materials to mix thoroughly before condensing. Because the debris that formed the Moon was a well-mixed blend of Earth's mantle and the impactor, the Earth and the Moon must share nearly identical oxygen isotope ratios.

Based on the passage, Scientist 1's argument relies on which of the following assumptions about the early solar nebula?

  1. A
    The Earth and the Moon accreted from the same reservoir of dust and gas.
  2. The isotopic composition of dust and gas was not uniform throughout the solar nebula.Answer
  3. C
    Earth's gravitational field was significantly stronger during the accretion phase than it is today.
  4. D
    Oxygen isotopes were only present in the region of the solar nebula where the Earth formed.

Answer

The correct answer states that the isotopic composition of dust and gas was not uniform throughout the solar nebula.
The correct answer is correct because Scientist 1's hypothesis rests on the premise that forming in different regions of the solar system results in different oxygen isotope ratios. For this to occur, the solar nebula must have had a non-uniform distribution of oxygen isotopes. If the nebula were completely uniform, any planetesimal forming anywhere in the nebula would have the same isotopic composition, meaning a captured Moon would not have a different oxygen isotope ratio than Earth.

Step-by-Step Solution

1
Identify the core claim made by Scientist 1 regarding the Moon's composition.
Scientist 1 claims that because the Moon formed in a different region of the solar system than Earth, its rocks will have a different oxygen isotope ratio.
This establishes the link between a body's formation location and its isotopic signature.
2
Determine the unstated premise required for different regions to yield different isotopic signatures.
For different regions to yield different isotopic ratios, the raw materials (dust and gas) in those regions must have differed in their isotopic ratios.
An assumption is a necessary condition for a conclusion to follow logically from its premises.
3
Select the option that matches this required condition of spatial variation (non-uniformity) in the solar nebula.
The option stating that the isotopic composition of dust and gas was not uniform throughout the solar nebula matches this requirement.
If the nebula were uniform, different regions would have identical compositions, undermining Scientist 1's primary line of reasoning.

Key Concept

Identifying Underlying Assumptions and Premises
Rate this question

Topics

Identifying Underlying Assumptions and Premises

More questions from this topic