The question stem says the argument relies on the assumption that blank.

So this is a necessary assumption question.

Let's start by figuring out why the premises don't prove the conclusion,

because that'll help us understand what the argument needs to assume.

So, in the beginning of the year 2003, we found methane in the atmosphere of Mars.

Now we know that methane is a fragile compound that falls apart

when it's hit by the ultraviolet radiation in sunlight.

So if methane is hit by the UV rays in sunlight, then that means methane is going to fall apart.

And it's not clear exactly what fall apart means,

but I'm thinking it means something like it'll be destroyed,

or it'll be transformed into something that is not methane,

like maybe its constituent elements.

So I'm not totally sure how to interpret that just yet.

Now based on this fact and the first sentence, the author concludes,

so, any methane in the Martian atmosphere must have been released into the atmosphere relatively recently.

Interesting.

We found methane in early 2003, and because methane is fragile and will fall apart when hit by UV rays,

that means any methane that's in the Martian atmosphere must have been released very recently.

Why does the author believe that the methane in the Martian atmosphere must have come about recently?

Well, let's understand how the author is interpreting the second sentence here.

Now I'm going to just draw a visual of that, but I'm not saying you have to diagram it.

This is just for the purpose of explanation.

We know that when methane is hit by UV rays,

so when methane is hit by sunlight and the UV rays in sunlight, that methane will fall apart.

It seems the author is interpreting that as if it means that if methane is hit by sunlight,

then you wouldn't be able to detect it anymore.

And so that's why the author believes that the fact that we found methane and were able to detect methane

means that that methane we found was not hit by sunlight.

The author is essentially trying to use the contrapositive of that idea,

that if the methane has not fallen apart, or in the author's mind that means,

or in other words, if it was detectable in the atmosphere, then that means it was not hit by sunlight.

And then the author is making this additional assumption that if it's not hit by sunlight,

that implies that the methane must have been released relatively recently.

Now this argument, I think, is going to be pretty hard to follow,

so don't take my visual here as a suggestion that we should have unpacked exactly what the author was assuming

before we went to the answers.

But this is the logic of the argument.

And in fact, let me spell out the other assumption here.

The author is assuming that the fact that we can detect methane implies that it has not fallen apart.

If we can detect methane in Mars' atmosphere, it hasn't fallen apart yet.

And then the author is using the premise to then get to the idea that that methane has not been hit by sunlight yet.

And then the author is making this additional assumption, so it must have been released relatively recently.

So let's take a look at these answers now, and remember, we're looking for what is necessary to the argument.

Well, no, that's not necessary, because the author could very well agree that Mars did have methane prior to 2003.

It's just that that methane eventually was hit by sunlight and fell apart, so we couldn't detect it anymore.

There's nothing wrong with methane happening before 2003.

All that the author is saying is, we did find methane in 2003, and so this methane that we found must have come relatively recently.

Maybe 50 years ago, 1,000 years ago, there was methane, but that methane's not currently in the atmosphere.

That's consistent with the author's position, so A is not necessary.

Let's consider B now, which says, all methane in the Martian atmosphere is eventually exposed to sunlight.

Now, I think this answer sneaks up on you, but if you negate answer choice B, I think you actually can understand why it's necessary.

The negation of B is, what if not all methane in the Martian atmosphere is eventually exposed to sunlight?

In other words, the negation is, what if there is some methane that is never exposed to sunlight?

Well, if there's some methane that's never exposed to sunlight, that opens the possibility that the methane we detected in Mars' atmosphere

might be the methane that is never exposed to sunlight.

And in that case, this methane actually could have been around for a very long time.

Much more time than relatively recently.

You see, the author's logic here is that we found the methane, and it must have come relatively recently, because otherwise, it would be undetectable.

When methane's hit by sunlight, it falls apart, and in the author's mind, that makes it undetectable.

But as part of that reasoning, the author is assuming that the sun, and let's say this is the sun here,

the author's assuming that the sun actually will hit all methane on Mars.

That's why the author thinks the fact that we can detect it right now means it must have come relatively recently.

Meaning, the sun hasn't yet had a chance to hit it and dissolve it.

But if you negate answer choice B, what that would mean is there's some methane that might actually never be hit by the sun.

What if for some reason, this methane is just never going to come in contact with the sun?

Or UV rays?

If that's the case, then it's possible this methane has been around for millions of years, not relatively recently,

and that would undermine the argument.

That's why answer choice B is necessary.

And if you look at this visual of the argument's logic down here, what B is describing is this assumption.

That if it hasn't been hit by sunlight, it must have happened relatively recently.

Another way to express this idea is that if the methane has not been released relatively recently,

meaning if it has been around a long time, then it would have been hit by sunlight.

And that's essentially what B is describing.

That eventually, all methane is hit by sunlight.

If it hasn't been hit by sunlight yet, then that means it must have been released recently.

Now if you're still having trouble understanding why B is necessary, take a look at this analogous argument here.

The argument goes, I saw a spill in aisle 5 of the grocery store.

Spills are cleaned up when employees see them.

So, the spill happened recently.

Does it make sense that in order to make this argument, there's an assumption that all spills in this grocery store are eventually seen by an employee?

That's why the author thinks that the spill must have happened recently.

The author's thinking, we must have seen the spill in between the time that it first came about,

and we're seeing that spill in the short moment before the employee sees that spill.

Because if the employee sees it, then the spill would be cleaned up, and then we wouldn't be able to see it anymore.

So the fact that we're seeing the spill in the author's mind here is evidence that an employee has not yet seen it.

And then that, in the author's mind, means it must have happened recently.

Because if it happened over a longer period of time, an employee would have seen the spill.

The logic of this argument is essentially the same as the logic in the argument in the stimulus.

So that's why B is necessary.

If it were not true, then the author could not reach this conclusion that the methane must have been around relatively recently.

Because it could be around for a much longer time if the sunlight might not eventually hit the methane.

C says methane cannot be detected until it has started to fall apart.

Well, no, that's not what's necessary.

What's necessary is that if it has fallen apart, then you can't detect it.

Or in other words, if you can detect it, then that means it hasn't fallen apart.

But what C is saying is, you can't detect it until it has fallen apart.

Or in other words, if you wanted to view C as a diagram, what it's saying is,

if it has not fallen apart, then that means you cannot detect it.

But that's just a twisted version of this idea.

In the author's mind, the fact we can detect it means it's still there.

In other words, it hasn't fallen apart.

That's not the same as saying, if you can detect it, then it has fallen apart.

Which is what C is saying.

So that's why C is not necessary.

the methane that the scientists detected had been exposed to ultraviolet radiation?

Well, no, this is the opposite of what the author is assuming.

In the author's mind, the fact that we found this methane means it hasn't been exposed to UV radiation and sunlight.

And then the author makes a second reasoning move and assumes that that means it must have come about relatively recently.

But the author is not assuming that this methane was exposed to ultraviolet radiation.

Because that would mean that the methane would have fallen apart.

So D is not describing something that's part of the author's reasoning.

But that's not necessary because the author says methane falls apart when hit by UV rays and sunlight.

The author didn't make an exception for Earth and methane in Earth's atmosphere.

So that's why E is not necessary.