#13 - There are just two ways a moon could have been formed

[maximbasu]

Hello,
I chose D as the correct answer while the correct answer was B.

I reasoned B was wrong because it just mentioned the inside of the Earth and not other parts of it, such as the surface. I reasoned D was correct because craters are evidence of meteoroids, hence, Earth's moon was created by meteoroids.

Thank you, Maxim.

[Adam Tyson]

The stimulus lays out two ways for a moon to be formed from the material of the planet it circles - 1) from the outer shell and 2) from the interior (after being struck by a meteor, perhaps). It then lays out that the first way is unlikely - the moon is not made of the same stuff as the Earth's outer shell, so it probably wasn't method no. 1.

The stem asks us to strengthen a claim, not present in the stimulus but only in the stem, that the moon was not formed from the Earth's material. We already know it probably wasn't from the shell, so we must eliminate the other possibility, that it was formed from a portion of the Earth's interior. That's what answer B does - it tells us that the moon isn't like that stuff, either, so we can infer that it probably wasn't formed from that material.

The issue raised in D is not relevant to how the moon was created in the first place. What good does it do us to know that since its creation it has been hit by meteoroids? Does that do anything to help explain how the moon was formed? Remember, we want to strengthen the idea that it was NOT formed by meteoroids hitting the earth and dislodging a chunk of the earth's interior, so the presence of meteoroids is, if anything, harmful rather than helpful.

A simple analogy might help: There are only two ways to get into my house - either a door or a window. Nobody climbed in through a window. I want to strengthen the idea that nobody is in my house. What would you say? I would say that nobody came through the door, since that is the only remaining option.

[NotSureWhy]

Question: In the stimulus, the passage says "There are just two ways..." and in the stem, it says "if the above are true." I was pretty confused here, as the stem almost read like a typo. I diagrammed the stimulus as:

outer shell spun off or large object, which is equivalent to:
NOouter shell spun off > large object
NOlarge object > outer shell spun off

As a result, I was really thrown off by the phrase "would most help to justify the conclusion that Earth's moon was not formed from a piece of Earth." If, as the stem says, "the above are true," and the above stimulus says 'there are only two ways," how was it even possible that there is an option where it wasn't from Earth?

I ask this because I'm unsure how to approach a similar situation on the test. When confronted with something like this, do you have advice for how to proceed? Option (B) seems to defend the idea that the moon could not have formed from a large object dislodging materials inside Earth, but I was operating under the assumption that it could have only been one of two ways.

Put differently, though I appreciate you analogy and it makes a lot of sense, everything from the stimulus and stem seems to suggest that someone is in your house. What do I do when confronted with something like this come test day? Or is this just an old problem and things like this don't occur anymore?

Thanks in advance,
NotSureWhy

[Paul Marsh]

Hi NotSureWhy! The first portion of the stimulus reads, "There are just two ways a moon could have been formed from the planet around which it travels". I bolded the second part of that phrase because it's extremely important and can be skipped over if you're reading to quickly. It's important because it is telling us that there are only two ways Earth's moon can be made from the Earth's material...but there are potentially a lot of other ways that the Earth's moon could be made from something else in the universe! Like what? Well, maybe part of another planet drifted off and entered the Earth's orbit. Or maybe asteroids clumped together and entered the Earth's orbit. I don't know, I'm not a astronomer! But it's important to realize that while the stimulus is limiting us to two ways that the moon can be made of Earth's material, it is not limiting us to how it can be made of anything else.

I'm guessing that skimming through that first part of the stimulus too quickly might have led to your confusion! (Try it again and see if it makes more sense, and then come back and read the rest of this explanation).

If we were to try and diagram the stimulus using conditional reasoning, it would look a little more like this:

Moon formed from material of planet it orbits around Outer shell spun off OR Large object dislodged inside material

And the contrapositive would be:

Outer shell spun off AND Large object dislodged inside material Moon formed from material of planet it orbits around

The question stem asks us to justify the conclusion that Earth's moon was not formed from material of Earth. Look at the contrapositive above - the necessary condition is the exact same as the conclusion that the question stem is asking us to justify! In order to reach that necessary condition, all we have to do is rule out the two possibilities contained in the sufficient condition. The stimulus tells us the Earth's moon has material different material than the Earth's outer shell, so that rules out the "Outer shell spun off" possibility. So all we have to do is look for an answer choice that will rule out the second possibility - that the moon was formed from material inside the Earth dislodged by a large object. Answer Choice (B) does just that. With both possibilities eliminated for how a moon could be formed from material of the planet it's orbiting, we can successfully conclude that the Earth's moon was not formed from a piece of the Earth - it must have been formed from something else.

Hope that helps!