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Zorluk: OrtaIdentifying Underlying Assumptions and Premises

### Origin of Earth's Oceans

Scientist 1
Earth's water was delivered during the planet's formation by carbonaceous chondrite meteorites originating from the asteroid belt. These meteorites contain up to 20% water by weight bound within their mineral structures. Because the planetesimals that formed in Earth's orbital zone were too hot to retain volatile water, water had to be delivered by chondrites originating beyond the "snow line" (the boundary beyond which water ice could condense). Thus, the deuterium-to-hydrogen (D/HD/H) ratio of Earth's oceans reflects the D/HD/H ratio of these chondrites.

Scientist 2
Earth's oceans were formed much later, during the Late Heavy Bombardment (approximately 3.9 billion years ago), by water-rich comets originating from the outer Solar System. Comets consist of up to 80% water ice and would have deposited vast quantities of water upon impacting the young Earth. The volatile elements on early Earth were completely vaporized and lost to space during the giant impact that formed the Moon. Therefore, Earth's current water inventory must have been delivered post-impact by these outer-system comets.

Scientist 2's argument regarding the cometary origin of Earth's oceans relies on which of the following underlying assumptions?

  1. Earth's mantle did not retain significant amounts of water deep underground during and after the Moon-forming impact.Cevap
  2. B
    The deuterium-to-hydrogen ratio of Earth's current oceans is identical to that of outer-system comets.
  3. C
    The Late Heavy Bombardment occurred approximately 3.9 billion years ago.
  4. D
    Carbonaceous chondrites and outer-system comets contain the same percentage of water ice by weight.

Cevap

Earth's mantle did not retain significant amounts of water deep underground during and after the Moon-forming impact.
Scientist 2 argues that the Moon-forming impact completely vaporized and removed Earth's volatile elements, meaning comets must have delivered the water later. This argument relies on the underlying assumption that no substantial reservoir of water survived the impact in Earth's interior (such as the mantle) to later form the oceans. If the mantle had retained water, cometary delivery would not be the only possible source for Earth's current water inventory.

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1
Identify the core claim made by Scientist 2.
Scientist 2 claims that Earth's water must have been delivered post-impact by comets because the Moon-forming impact vaporized and lost all pre-existing volatiles.
Understanding the premise of the argument is necessary to identify what implicit assumptions support it.
2
Evaluate what conditions must be true for Scientist 2's conclusion to hold.
If Earth's interior (like the mantle) had retained substantial water despite the surface vaporization, that water could have outgassed to form the oceans. Therefore, Scientist 2's conclusion requires the assumption that no such internal water reservoir survived.
An assumption is an unstated premise required for the conclusion to remain valid.
3
Compare this assumption with the options provided.
The option regarding the mantle not retaining water matches the required assumption, while other options are either explicit facts, related only to Scientist 1, or factually incorrect details.
This confirms the correct option among the distractors.

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Identifying Underlying Assumptions and Premises
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Identifying Underlying Assumptions and Premises

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