Logical Reasoning

PT16 · S3 · Q13 There are just two ways a moon

A free, expert breakdown of this official LSAT Logical Reasoning question.

Save & drill this skill — build targeted practice sets from questions like this one
Video walkthroughs — watch every question solved step by step
81 official LSATs — as questions, timed sections & full-length tests

There are just two ways a moon could have been formed from the planet around which it travels: either part of the planet’s outer shell spun off into orbit around the planet or else a large object such as a comet or meteoroid, struck the planet so violently that it dislodged a mass of material from inside the planet.

Background

Two ways a moon can form

1. part of the planet's outer shell spun off into orbit and became the moon

2. a comet or meteoroid slammed into the planet, dislodging a mass of material from inside the planet, which then becomes the moon

In the case of Earth's moon, it doesn't seem to be made of materials that resemble Earth's outer shell.

Evaluate

That last fact heavily suggests that Earth's moon was formed the 2nd way, with something crashing into earth and ejecting material from inside the planet out into orbit, where it then coalesced into the Moon.

However, this question stem is asking us to support the idea that the moon wasn't formed from a piece of the Earth. Both 1 and 2 involve a piece of the Earth (they only differ in terms of whether that piece came from the outer shell or from inside the planet).

So it's pretty confusing what we're supposed to be doing here. It seems like if we're saying that the Earth's moon was not formed from a piece of the Earth then we're denying that either of the "just two ways" occurred.

Does that mean we're supposed to argue that ?

Goal

If it did form from a piece of Earth, then we'd have to think that it formed from one of the 2 ways named. So if we're going to argue that the moon didn't form from a piece of Earth, then it seems like we need an answer to shoot down the 2nd way (i.e. tell us that Earth has not been hit by a comet / meteoroid, or tell us that the Moon's materials do not resemble materials from inside the Earth).

Or we need to suggest that the moon didn't form from the planet Earth at all. It formed from Mars's materials (for example) and then somehow floated over to us and got locked in an orbit with us

13.

If the statements above are true, which one of the following, if also true, would most help to justify drawing the conclusion that Earth’s moon was not formed from a piece of the Earth?

The moons of some planets
No Impact
This is about other moons, but we only care about the story of Earth's moon.
7% picked this
Correct
Earth’s moon consists primarily of
Why this is right
This is ruling out "Way #2". We had already ruled out Way 1, that the moon was formed from an outer shell, by saying the Moon's materials don't match the outer shell of Earth. This helps us rule out Way 2, that the moon was formed when something big slammed into Earth and ejected a mass of material from inside the planet. If the Moon doesn't seem to be made out of stuff that matches the inside of Earth, then it doesn't seem to have been made via Way 2. Thus, since we've ruled out "outer shell" and "ejected internal mass", we're supporting the idea that the Earth's moon was not made out of a piece of the Earth.
Skill tested: Strengthen · how this choice captures the argument's function is the move to repeat next time.
67% picked this
Earth’s gravity cannot have trapped
Opposite
If Earth's gravity could trap a meteoroid and turn it into its moon, that would satisfy our purposes nicely! That would help us argue, "the moon didn't form out of a piece of Earth; the moon is just a meteoroid that was flying by and got trapped in our orbit.
7% picked this
The craters on the surface
Weakens, if Anything
The fact that the moon has been struck by many thousands of large meteoroids increases the probability that Earth might have been struck by many thousands of large meteoroids, which increases the probability of Way #2 of how the moon formed. If a large meteoroid slammed into Earth and ejected a mass of material that then became the moon, it would support the conclusion that "the moon is made out of a piece of the Earth".
18% picked this
Comets and large meteoroids normally
Weakens, if Anything
Mainly, I wanted to avoid the irony of writing "No Impact" for an answer that is helping us assess whether comets or meteoroids have impacted the surface of Earth. This answer really does nothing, but if anything it's suggesting that since comets and meteoroids move at really high speeds, a collision with Earth would be more likely to be the violent type of collision that could eject material. Since this answer drifts toward making Way #2 seem more plausible, it's going against our purposes.
1% picked this

Continue the review in LSAT Lab

Save this question, watch the video walkthrough, and drill similar questions in your LSAT Lab account.

Save and practice this skill

LSAT Lab

Turn this review into a targeted study plan.

Save this question, drill more like it, watch the video walkthrough, and track your progress in your LSAT Lab account.

Start practicing free