Logical Reasoning

PT131 · S2 · Q25 About 3 billion years ago

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About 3 billion years ago, the Sun was only 80 percent as luminous as it is currently.

Conclusion

There was probably way more CO2 in Earth's atmosphere 3 billion years ago.

Evidence

The sun was only 80% as bright, which in today's atmosphere would mean frozen oceans. Since we know the oceans were liquid, the atmosphere must have had more greenhouse gases, like CO2 and methane.

Evaluation

This seems like a bit of an arbitrary choice. We know we need there to have been a higher level of greenhouse gases in the atmosphere 3 billion years ago, but why does it specifically have to be more CO2? Maybe it was more methane? Maybe it was more of some other greenhouse gas?

Goal

Look for an answer that addresses the possibility that the atmosphere had higher greenhouse gases, via some alternative to CO2 like methane or some other greenhouse gas.

25.

Which one of the following, if true, weakens the argument?

  1. Sufficient heat to keep the

    Strengthens / Opposite

    This rules out a different way for the dimmer Sun to coexist with a liquid ocean.

    11% picked this

  2. Correct

    Geological studies indicate that there

    Why this is right

    This lets us say, "Yes, author, there was a higher level of greenhouse gases 3 billion years ago, but that's because there was more methane, not because there was more CO2".

    Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.

    57% picked this

  3. Geological evidence indicates that the

    Strengthens / Opposite

    Like choice (A), this rules out a different way that the dimmer Sun could coexist with liquid oceans.

    12% picked this

  4. The increase in the Sun's

    Out of Scope

    Out of Scope: "increasing complexity of life" Nobody cares about the complexity of life, (D). We're just trying to solve the mystery of "how were the oceans still liquid if the Sun was dimmer?"

    5% picked this

  5. Because the distance from Earth

    Strengthens, if Anything

    If the Sun were previously closer to the Earth, so that 80% as luminous still meant we were getting the same level of radiation we do nowadays, then that would have been an alternate explanation for how a dimmer Sun could coexist with a liquid ocean. But this answer rules out that possibility.

    15% picked this

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