In early 2003, scientists detected methane in the atmosphere of Mars

Premise: Scientists detected methane in Mars' atmosphere in 2003. Methane is fragile and breaks down when exposed to ultraviolet radiation.
Conclusion: The methane detected must have been released into the atmosphere recently.
The task is to identify the assumption necessary for the conclusion to be valid.

Analyzing Each Option:
(A) Mars had no methane in its atmosphere prior to 2003

Analysis: This is irrelevant to the argument because the argument is about the presence of methane at the time of detection, not the presence of methane before 2003. The argument could hold even if methane existed before 2003 but had just been replenished recently.
Conclusion: This is not necessary.

(B) All methane in the Martian atmosphere is eventually exposed to sunlight

Analysis: This assumption is key because the conclusion relies on the idea that the methane in the atmosphere would eventually fall apart due to exposure to ultraviolet light, making it necessary for the methane to be released recently. If some methane could avoid exposure to sunlight, it could persist for a long time, weakening the conclusion that it must have been released recently.
Conclusion: This is necessary.

(C) Methane cannot be detected until it has started to fall apart

Analysis: The detectability of methane is not discussed in the argument. Whether or not it has begun to fall apart does not affect the reasoning behind the conclusion that methane must have been released recently.
Conclusion: This is not necessary.

(D) The methane that the scientists detected had been exposed to ultraviolet radiation

Analysis: This supports the idea that the methane is fragile and will break down, but it is not a necessary assumption. The methane could have been detected before or after being exposed to UV radiation without affecting the logic of the argument.
Conclusion: This is not necessary.

(E) Methane in Earth's atmosphere does not fall apart as a result of exposure to ultraviolet radiation

Analysis: The behavior of methane on Earth is irrelevant to the argument, which focuses on the behavior of methane in Mars' atmosphere. The argument does not make any comparison with Earth's methane.
Conclusion: This is not necessary.

Correct Answer: (B)
Explanation: The argument's conclusion hinges on the assumption that methane cannot persist in Mars' atmosphere for long because it would be exposed to ultraviolet radiation and break down. Therefore, the assumption that all methane in the atmosphere is eventually exposed to sunlight is necessary to support the conclusion that the methane detected must have been released recently.

Since you havn't asked specific question I will share my inputs for all. The blue text is the conclusion.
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In early 2003, scientists detected methane in the atmosphere of Mars. Methane is a fragile compound that falls apart when hit by the ultraviolet radiation in sunlight. So any methane in the Martian atmosphere must have been released into the atmosphere relatively recently.

The argument relies on the assumption that:

(A) Mars had no methane in its atmosphere prior to 2003 - WRONG. Prior to 2003 is beyond scope of the passage. Even if it has to be the answer it needs further assumptions to be made to support it. Doing so is a blunder. Hence wrong.
(B) All methane in the Martian atmosphere is eventually exposed to sunlight - CORRECT. Slight problem with "all" but it's best among the lot. If not exposed to sunlight then conclusion stands to lose.
(C) Methane cannot be detected until it has started to fall apart - WRONG. This is not a necessity for detectability. Also, detectability is not the core of the passage.
(D) The methane that the scientists detected had been exposed to ultraviolet radiation - WRONG. 2nd best choice here. So, is it necessary that the detected methane needs to exposed to UV radiation. Since it touches the "exposed" part it confuses us when it all does is conditionalise the situation which need not be true.
(E) Methane in Earth's atmosphere does not fall apart as a result of exposure to ultraviolet radiation­ - WRONG. Irrelevant.

Answer B.

We're looking for an assumption on which the argument RELIES -- other words, something that MUST be assumed in order for the argument to hold.

Choice (A) can be eliminated with a simple example: maybe there was some methane in Mars's atmosphere 2000 years ago. Imagine that the fragile methane was hit by ultraviolet radiation from the sun, and after, say, 1-2 years, all of that methane had fallen apart.

Now we're back to a clean slate, with no more methane in Mars's atmosphere. So the author's argument regarding the methane discovered in 2003 still holds, even if (A) is not assumed.

(B), on the other hand, must be assumed. If there's some methane in the Martian atmosphere that is NOT eventually exposed to sunlight, that methane would NOT fall apart. In that case, we'd have no idea whether the methane from 2003 was released recently or a long time ago.

I hope that helps!