#23 - On Earth, biological activity leads to a change in the ratio of isotope

[Administrator]

Complete Question Explanation

In this stimulus, we are introduced to an argument against the likelihood of life on Mars.  The author of the stimulus gives us an interesting fact about isotopes on Earth. Biological activity on Earth leads to a change in the ratio of one isotope to another.  We don't need to know anything about isotopes, what they are, or what they signify here.  Remember that this is not a test of outside knowledge.   Any information they do not provide in the stimulus is either extremely basic OR not necessary.  Here, it's completely unnecessary.  We just need to know there's a change in a measurable chemical that indicates biological life is present.  

The argument continues by telling us that there's a meteorite of Martian origin that exhibits the same ratio of isotopes as was found in pre-life Earth. The author draws the conclusion that the isotope ratio on the meteorite suggests that it is unlikely that life occurred on Mars.

Our question is a weaken-except question, meaning that four of the answer choices will weaken the conclusion, and one answer choice will not weaken the conclusion.  The one answer choice that does not weaken will be our correct answer, but it is easier for us to run through the answer choices and eliminate those answer choices that DO weaken.   You must always read all four answer choices in this question type to make sure that you have correctly identified the one and only answer choice that either strengthens or has no impact on the argument.

It's not unusual to see a weaken-except question for causal arguments.  Causal relationships are inherently weak, and the test makers have many options for weakening these relationships.  In this question, we want to weaken the relationship between the isotope ratio in the meteorite and the likelihood of life on Mars.  

Weaken, Except. The correct answer choice is (C).

Answer choice (A): This answer choice suggests that the cause (life) can occur without the effect (specific isotope ratio). It doesn't give a specific example of when that would happen, but it introduces the possibility that the cause could occur without the effect. It doesn't weaken the argument very much, but it does not have to destroy the argument. It just has to give a bit of doubt.

Answer choice (B): This answer choice weakens the argument by suggesting an additional cause. We can't just look at the isotope ratio in isolation but instead recognize that there are a number of environmental and climate factors that could impact that ratio, and those would be different on Earth and on Mars.

Answer choice (C): This is the correct answer choice. This answer choice does not weaken the argument. By stating that the ratio of the isotope in the meteorite is the same as on Mars at the time it left, it links the meteorite as we found it on Earth to the conditions on Mars. That makes it possible for us to learn information about what Mars was like based on the condition of the meteorite.

Answer choice (D): This answer choice weakens the relationship between the cause (life) and the effect (isotope ratio). It shows that even on Earth, where we know the cause exists, we don't always see the ratio expected.

Answer choice (E): This answer choice weakens by bringing in concepts of time. By suggesting that the isotope ratio on Mars has not been static, it means that we can't draw a conclusion about life on Mars over time from a snapshot of the ratio seen in a single meteorite. Just because the meteorite might show there was no life on Mars at the time of the meteorite it doesn't prove that life didn't occur on Mars after the meteorite left.

This explanation is still in progress. Please post any questions below!

[BMM2021]

Hi,

Could this answer be explained? I didn't find myself initially liking any of the answers. I chose D because it didn't immediately appear to have any relevance to the argument (and I had erroneously talked myself out of C because it didn't weaken or strengthen the argument, which should have been the reason to choose it), but I wanted to understand how/why D weakens the argument. Also, is there a quick way to interpret the impact of answer D? I have a few theories of how it weakens, but I didn't arrive it them in a manner efficient for the LSAT. Any tips, or should I just hope to be smarter next time?

Thanks.

[Beth Hayden]

Hi BMM,

The idea behind the argument is that biological activity changes the ratio of these isotopes, and since the meteorite does not show that changed ratio, it must mean that it didn't come from a place with life. There are a few problems with that argument that you can use to weaken it.

Answer choice (D) weakens by theorizing that maybe biological activity doesn't always change the isotope ratio in all minerals. If that's true, there might be some mineral samples in a place where there is biological activity that don't show the changed ratios--their absence is not enough to definitively conclude there isn't life on the planet.

Consider a similar argument: The ocean makes sand on the beach wet, so since we found some sand that is dry, it must not have come from the beach. You can weaken that argument by saying that the tide only touches a small fraction of sand on the beach, and so some sand will stay dry. Not a perfect comparison but I hope that helps!

Beth

[lynsi123456789]

Beth: Your explanations are extremely helpful. Can you help me understand the rest of the question?
In particular, I”m struggling with answer choice A

[Rachael Wilkenfeld]

Hi lynsi123456789,

The key idea here is a causal idea. Life CAUSES a shift in the ratio between S-32 and S-34. Let's look at the rest of the answer choices.

Answer choice (A): The stimulus goes from because we don't see the isotope S-32 in the right ratio, we don't have life on Mars. This answer choice weakens that connection by saying that there are other forms of life that do not result in more S-32.

Answer choice (B): This answer choice weakens the stimulus by providing an alternate-ish cause. It's not just life that impacts the isotope ratio, it's also climate. Mars has a different climate than Earth, so the different ratio could be explained by that as opposed to differences in the existence of life. By adding an additional cause to the relationship, it weakens the causal explanation proposed in the stimulus.

Answer choice (C): This is the correct answer choice. This would actually potentially strengthen the argument, showing that the meteorite was similar to Mars as a whole, making it easier to draw conclusions about Mars as a whole from the meteorite. We are looking for the answer choice that does not weaken, so this would be our correct answer.

Answer choice (D): As Beth stated in her explanation, this answer choice suggests that we won't always see the ratios suggested in the stimulus. Another way of thinking about this is to say that it gives a situation where you have the cause (life) but not the effect (the expected ratio). It draws into question the reliability of drawing a broad conclusion about a planet based on a single meteorite sample.

Answer choice (E): This is another answer choice that suggests that the meteorite is not a reliable indicator of the planet Mars. By stating that it's different than Mars is currently, it suggests that you cannot use the single rock to determine what the ratio of the isotopes was on Mars over time.

Hope that helps!

[mkloo11]

Not sure if this tripped anyone else up, but just commenting in case it helps someone: I didn't understand the operative word"terrestrial" in answer choice D. Upon learning that it means "Earth", the whole thing connected for me. Not many samples from Earth (where life is known to be) indicate the presence of life. So, as other have said, the cause doesn't always lead to the effect.