Logical Reasoning

PT116 · S3 · Q13 Up until about 2 billion

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Up until about 2 billion years ago, the sun was 30 percent dimmer than it is now.

Given that ...

The sun, throughout Earth's history until about 2 billion years ago, was 30% dimmer

and that ...

Nowadays, a sun that dim would lead to frozen oceans

Why is it that ..

3.8 billion years ago, there was liquid water?

Evaluate / Goal

There must be something else that was also different about the Earth back then that would allow a dimmer Sun to still warm the Earth enough to keep water in its liquid state.

13.

Which one of the following, if true, most helps to resolve the apparent discrepancy described above?

  1. Correct

    Our atmosphere currently holds in

    Why this is right

    This offers us "something else that was different about Earth back then that would allow a dimmer Sun to still warm the Earth enough to keep water in its liquid state". Apparently back then, the atmosphere held in way more heat than it does nowadays. This gives us a way to explain how the Earth could end up having a pretty similar global temperature then/now, when it comes to keeping water in its liquid form. You could get your house to 65 degrees (Fahrenheit) by either blasting the heater, while all your doors and windows are open (that's modern Earth ... stronger heat source, worse heat preservation), or you could get your house to 65 degrees by running the heater 30% less strongly, but closing all the doors and windows so that the house better holds in the heat it acquires.

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

    66% picked this

  2. The liquid water present 3.8

    No Impact

    This sort of just creates more confusion. It certainly does nothing to explain how we had liquid water 3.8 billion years ago, despite having a 30% dimmer sun (which nowadays would cause the oceans to free). We have no idea how it was liquid 3.8 billion years ago or why it later froze. We might be able to guess that the reason it melted about 2 billion years ago is that the sun became as bright as it currently is at that point. But our job wasn't to explain why frozen water melted 2 billion years ago. Our job is to explain why liquid water existed 3.8 billion years ago.

    16% picked this

  3. A significant source of heat

    Unrelated to Goal

    This isn't going to help us explain how there was liquid water 3.8 billion years ago, when it doesn't even reference anything from around that time period. This is only talking about something that transpired around 2 billion years ago.

    13% picked this

  4. Evidence suggests that certain regions

    Unrelated to Goal

    This isn't going to help us explain how there was liquid water 3.8 billion years ago, when it doesn't even reference anything from around that time period. This is talking about stuff that happened less than 2 billion years ago.

    2% picked this

  5. When large portions of the

    Unclear Impact No Distinction

    We need a distinction between Earth from 3.8 billion years ago and Earth nowadays to resolve this paradox. This answer isn't providing any distinction, unless we know that there was some difference between then and now. In order for this answer to do the work we need it to do (give us a way "to warm the Earth" enough for their to be liquid water), then we would need to know that nowadays, large portions of the globe are ice-covered whereas 3.8 billion years ago, only the poles are ice-covered (so that even though less heat was coming in from a dimmer sun, more of it was absorbed by the earth). If anything, we suspect the opposite of that is true. But since we have no idea what parts of the globe were ice-covered 3.8 million years ago, we don't have any way to use this idea anyway.

    4% picked this

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