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#13 - Up until about 2 billion years ago, the sun was 30

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Complete Question Explanation

Resolve the Paradox. The correct answer choice is (A)

Many LSAT questions contain multiple viewpoints or time periods. Whenever one of these questions
occurs, it is absolutely critical to keep track of which characteristics belong to which viewpoint or time
period. It is important to organize the information contained in this stimulus into clear and manageable
segments. One approach is to treat 2 billion years ago as the demarcation point between the modern
and ancient eras. It may seem counterintuitive to call 2 billion years ago the “modern era,” but the name
is used only to indicate a meaningful distinction between present conditions and earlier conditions.
Readers are told that at any time prior to 2 billion years ago, the sun was 30 percent dimmer than it is
now and any time since then, the sun has not been that much dimmer. One common feature of both eras
is that life and liquid water were present.

What, then, is the apparent discrepancy? The stimulus states that if the sun were as dim during the
modern era as it had been during the ancient era, there would not be liquid water today. So, in order
for liquid water to have been present during the ancient era, there must be some other factor which
counteracted the diminished heat from the sun during that era.

When scanning the answer choices, aggressively search for this factor. Also note that the discrepancy
in the stimulus is between conditions now and conditions 3.8 billion years ago. Therefore, the correct
answer choice is likely to include these two time periods. Only answer choices (A) and (B) describe two
different time periods. Both (C) and (D) are about one time period only and (E) discusses a principle
which is unrelated to any specific time period.

Answer choice (A): This is the correct answer choice. After reading the stimulus, many test takers
will prephase an answer that involves heat. They understand that the critical factor in explaining this
discrepancy is explaining how the earth could have been hot enough for presence of liquid water as early
as 3.8 billion years ago, despite the diminished sunlight. In this case, that factor is the amount of heat
retained by the atmosphere. In the modern era, a brighter sun allows liquid water to exist, despite lower
heat retention in the atmosphere. In the ancient era, higher atmospheric heat retention allowed liquid
water to exist, despite a dimmer sun.

Answer choice (B): This is the most frequently chosen incorrect answer choice. In this scenario, liquid
water that was present 3.8 billion years ago later froze, and then melted about 2 billion years ago.
However, the stimulus notes that the sun was 30 percent dimmer than it is now up until 2 billion years
ago. This mean that throughout the entire ancient era, including 3.8 billion years ago, the sun was not
generating as much heat as is now necessary for the presence of liquid water. Merely reaffirming that
there was liquid water at some point in the ancient era does not help resolve this apparent discrepancy.
Remember, to resolve the paradox the correct answer choice must present an explanation for how both
sides of the paradox could occur. Answer choice (B) does not give any explanation for how liquid water
could have existed 3.8 billion years ago, so it cannot be correct.

Answer choice (C): This answer choice seems promising because it addresses a source of heat other than
the sun. However, test takers should be wary of this answer choice because it only addresses one time
period. Specifically, it talks about ice sheets melting 2 billion years ago. An additional source of heat
at that time would explain how liquid water has been present throughout the modern era, but does not
account for the presence of liquid water during the ancient era (3.8 billion years ago).

Answer choice (D): This answer suggests that even after the sun became less than 30 percent dimmer
than it is now, the earth was still cold enough that certain regions remained frozen. Of course, this does
not explain how liquid water could have been present at all 3.8 billion years ago, when the sun was at
least 30 percent dimmer than it is now.

Answer choice (E): If this answer is true, the apparent discrepancy would become even worse. A
dimmer sun would generate less heat which would lead to lower temperatures. Lower temperatures
would probably increase the portion of the globe that is ice-covered. The ice would reflect more of the
sun’s heat, cooling the planet even further. This cycle would make it even more difficult to explain the
presence of liquid water 3.8 billion years ago under conditions which would preclude liquid water today.
LSAT Apprentice
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I'm looking at question 13, "Up until about 2 billion years ago, the sun was 30% dimmer..."

What's the best way to approach this problem? I tried pre-phrasing and chose B, but am seeing now that the best answer is A.

Any advice would be much appreciated.


Rachael Wilkenfeld
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This is a resolve the paradox question. In order to answer it, we first need to identify the surprising situation in the stimulus. In this question, we see that if the sun were as dim now as it was 2 billion years ago, the oceans would be frozen. However, we know that there was liquid water then. The question asks us to resolve that paradox.

To prephase, we don't need to know the exact answer. We just need to think about what sort of information we need. I would want to know what else was different 2 billion years ago. We know NOW if the sun was that dim, the oceans would freeze, but what about THEN? Was there another factor keeping the Earth warm? Was the Earth closer to the sun so the heat didn't have to travel as far?

With that in mind, we can look at the answer choices:

a) This answer looks good. It basically says that even though the sun was dimmer, the Earth was better at holding in warmth. Therefore, it makes sense that less heat would be required to warm the planet.

B) This answer doesn't really resolve the paradox. Why? Because our stimulus says "up until 2 billion years ago..." That means that we need to explain how we had liquid water all the way from 3.8 billion years ago through 2 billion years ago. Therefore, even though it talks about the sun warming back up, it still doesn't explain how liquid water could ever have been present if the sun was that dim.

C) This answer has a similar problem. It only addresses the situation 2 billion years ago, and not 3.8 billion years ago. Since we know from the fossils, there was liquid water 3.8 billion years ago we need an answer choice that explains how there was water that long ago.

D) This answer doesn't address the paradox. It says that the ice didn't completely melt by 2 billion years ago. It doesn't address how the melting occurred at all.

E) This would suggest that even less of the heat stayed in. If there was ice covering the Earth, this answer says that the Earth would require even more heat to melt.

Hope that helps!
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I'm confused because I rationalized B (my original answer) as wrong in an entirely different way than above. The key word to me is "now".

B requires the assumption that the effect of the sun being 30% dimmer "now" would be the same 3.8 billion years ago. We are only told what would happen "now" were the sun 30% dimmer, specifically that the oceans would be completely frozen. If we were told that "anytime that the sun is that dim, the oceans become completely frozen", then I imagine B would be correct.

Am I still off here?

I do understand why A is correct. We can resolve the discrepancy if we understand that today's atmospheric conditions are different (there is less heat retention now) than they were 3.8 billion years ago, so the effect of the sun being 30% dimmer would not be the same.
Rachael Wilkenfeld
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Hi Tomars,

Even if you read the word "now" as you are, answer choice (B) has some problems. Mostly, it doesn't provide an explanation of how there was liquid water with the dimmer sun. It just says that there was liquid water, and it froze, but it doesn't explain how the liquid water was able to form in the first place.

One way to think about this is to focus on the paradox. In the past, the sun was dimmer (and therefore, gave off less heat). But there was still liquid water. If it was that dim now, we wouldn't have liquid water on Earth. So what was different then? How did the Earth have had a liquid water then, but not with the same conditions now? Answer choice (B) doesn't provide that explanation.

Hope that helps!