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Difficulty: HardIdentifying Core Claims and Hypotheses

### Faint Young Sun Paradox

During the Archean Eon, approximately 4 billion years ago, the Sun's energy output was only about 70% of its current value. Under these conditions, global temperatures should have dropped below the freezing point of water, yet geological evidence shows that liquid oceans existed. Two scientists present opposing hypotheses to explain this phenomenon.

Scientist 1
The liquid oceans were maintained by a powerful greenhouse effect. Early Earth's atmosphere contained extremely high levels of carbon dioxide (CO2CO_2) and methane (CH4CH_4). Intense volcanic outgassing released CO2CO_2 at rates far exceeding modern levels, while the lack of exposed continental crust limited silicate weathering, a process that removes CO2CO_2 from the atmosphere. Simultaneously, early methanogenic bacteria in the anaerobic oceans produced abundant CH4CH_4. Together, these gases trapped outgoing infrared radiation, raising surface temperatures enough to prevent global glaciation.

Scientist 2
Greenhouse gases alone cannot explain the liquid oceans because high atmospheric concentrations of CO2CO_2 would have led to extensive cloud cover, increasing planetary albedo (reflectivity) and reflecting the limited sunlight back into space. Instead, the primary driver of warming was a low global albedo. Because early Earth lacked continental landmasses, it was almost entirely covered by dark liquid oceans. Since water absorbs far more solar radiation than land or ice, the ocean-dominated planet absorbed a high fraction of the dim solar energy, warming the surface above freezing.

According to the hypothesis of Scientist 2, which of the following conditions was a necessary prerequisite for early Earth to absorb enough solar energy to maintain liquid oceans?

  1. The absence of large continental landmasses, which kept planetary albedo lowAnswer
  2. B
    The condensation of thick cloud layers, which prevented infrared radiation from escaping
  3. C
    The presence of methanogenic bacteria, which altered the absorption spectrum of the atmosphere
  4. D
    The rapid weathering of silicate rocks, which regulated atmospheric temperature

Answer

The absence of large continental landmasses, which kept planetary albedo low
The correct answer is correct because Scientist 2 explicitly states that a lack of continental landmasses left the planet covered by dark liquid oceans, resulting in a low planetary albedo that allowed it to absorb more solar energy.

Step-by-Step Solution

1
Identify the scientist whose viewpoint is referenced in the question.
The question asks about the hypothesis of Scientist 2.
Focusing on the correct viewpoint prevents misidentifying claims.
2
Locate Scientist 2's explanation for how early Earth maintained liquid oceans.
Scientist 2 claims that a low planetary albedo due to a lack of continental landmasses allowed the dark oceans to absorb solar radiation.
This establishes the core mechanism proposed by Scientist 2.
3
Evaluate the options to find the prerequisite that enables this mechanism.
The absence of large continental landmasses directly supports the low planetary albedo required for solar absorption.
This matches Scientist 2's claim and identifies the correct answer.

Key Concept

Identifying the core hypothesis of a scientific viewpoint by analyzing its proposed mechanism and prerequisites.
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Topics

Identifying Core Claims and Hypotheses

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