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Difficulty: HardDetermining Author's Primary Purpose

In 1972, astronomers Carl Sagan and George Mullen pointed out a contradiction that had long quieted in the margins of stellar evolution theory: the Faint Young Sun Paradox. According to standard astrophysical models, during the Archean eon—between 3.8 and 2.5 billion years ago—the Sun’s output was only about 70 percent of its current intensity. Under such conditions, liquid water should have frozen globally, turning the early Earth into a sterile, ice-bound wasteland. Yet, geological evidence—such as sedimentary ripple marks and ancient pillow lavas—unequivocally demonstrates that liquid oceans existed and life began during this very epoch.

Sagan and Mullen proposed that a dense greenhouse atmosphere, rich in ammonia and methane, kept the planet warm. However, subsequent photolysis models revealed that solar ultraviolet radiation would have rapidly destroyed atmospheric ammonia, leaving scientists to search for alternative warming agents, such as carbon dioxide and carbon-isotope variations. While various geochemical feedback loops involving the carbonate-silicate cycle have since been proposed to explain how Earth maintained its temperate climate, the paradox remains a central calibration tool for climatologists. By studying how the early Earth escaped a permanent deep freeze despite a weak young Sun, researchers gain vital insights into the long-term stability of planetary habitability and the potential constraints on life elsewhere in the cosmos.

Which of the following best describes the primary purpose of the passage as a whole?

  1. Outline a scientific contradiction regarding early Earth and explain how efforts to resolve it contribute to the study of planetary habitability.Answer
  2. B
    Detail the geochemical mechanisms of the carbonate-silicate cycle that allowed liquid water to persist on the surface of the early Earth.
  3. C
    Argue that Sagan and Mullen’s early greenhouse model provides the most accurate explanation for the existence of ancient sedimentary rocks.
  4. D
    Assert that standard models of solar evolution are fundamentally incorrect and must be revised using modern geological data.

Answer

Outline a scientific contradiction regarding early Earth and explain how efforts to resolve it contribute to the study of planetary habitability.
The correct answer accurately captures the global focus of the passage. The author begins by introducing the Faint Young Sun Paradox (the contradiction between a weak young Sun and the presence of liquid water on early Earth), goes on to discuss various scientific attempts to explain it, and concludes by stating how this ongoing research provides valuable insights into the study of planetary habitability.

Step-by-Step Solution

1
Analyze the first paragraph to identify the central issue presented.
The passage introduces the Faint Young Sun Paradox, which is a contradiction between standard solar models (predicting a frozen early Earth) and geological evidence (proving liquid water and life existed).
Determining the main topic of the passage is the first step in identifying its primary purpose.
2
Evaluate the second paragraph to determine how the author develops the topic.
The author outlines proposed solutions to the paradox (ammonia/methane greenhouse warming), explains why those initial models failed (ammonia destruction by photolysis), and mentions current theories (the carbonate-silicate cycle).
Tracking the development of the arguments helps separate local supporting details from the primary message.
3
Analyze the conclusion of the second paragraph to identify the final takeaway.
The author concludes by noting that studying how Earth resolved the paradox helps climatologists calibrate models and understand planetary habitability and constraints on life in the cosmos.
The conclusion of a passage often defines the author's primary intention for writing the text.
4
Match the synthesized breakdown against the answer options.
The option discussing outlining a contradiction and explaining how efforts to resolve it contribute to the study of planetary habitability captures the entire scope of the text.
The correct option must encompass the global purpose of the entire passage rather than focus on narrow details or exaggerate claims.

Key Concept

Determining Author's Primary Purpose
Estimated Time:2m 0s
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Determining Author's Primary Purpose

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