PrepTest 141, Section 4, Question 13
In early 2003, scientists detected methane in the(35 more words not shown)
The argument relies on the(2 more words not shown)
- AMars had no(7 more words not shown)
- Ball methane in(8 more words not shown)
- Cmethane cannot be(8 more words not shown)
- Dthe methane that(9 more words not shown)
- Emethane in Earth's(13 more words not shown)
Reading the argument
Sunlight breaks methane apart. Does the sunlight actually reach all the methane?
Imagine that part of Mars's atmosphere shields the gas from ultraviolet radiation—something like an atmospheric sunscreen. Or imagine methane remaining in a protected part of the atmosphere. The gas could be old and still survive if the thing that destroys it never reaches it.
The argument instead treats methane's continued presence as a sign of recent arrival. That reasoning needs exposure: methane can't remain safely hidden from sunlight indefinitely. Otherwise, finding it in 2003 wouldn't tell us that it had only recently entered the atmosphere.
What we’re looking for
We're looking for an assumption the argument needs, not a complete explanation of how methane is produced or detected. My prediction is that the sunlight will eventually reach the methane. That closes the particular escape route we noticed, even though it doesn't by itself give a precise timetable for the gas's destruction.
Working the answers
- A
Detection in 2003 doesn't establish that Mars had never had methane before then. There could have been earlier methane that broke apart and later methane that replaced it. The argument concerns how long any methane can remain, not whether methane existed before that date.
- BCorrect
If some methane could remain in the Martian atmosphere without ever encountering sunlight, the destruction mechanism wouldn't establish that all methane there was recently released. The argument needs to rule out that lasting shelter. Eventual exposure is a required part of the reasoning, rather than a guarantee by itself of the exact age of every methane molecule.
- C
The argument doesn't require detection to occur only after methane starts breaking apart. Scientists might detect newly released methane before it encounters the relevant radiation. No particular detection method is used to connect the sunlight premise with the conclusion about recent release.
- D
Having already been exposed is different from eventually being exposed. Recently released methane could still be waiting to encounter the radiation. The argument needs a reason old methane couldn't persist indefinitely, not a claim that the particular gas detected had already undergone exposure.
- E
What happens to methane in Earth's atmosphere isn't part of this inference about Mars. The same process could happen on Earth, or conditions there could differ, without changing the argument's need to establish exposure in the Martian atmosphere.
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