LSAT Question Explanation
PT 105, Section 4, Question 5
WeakenArgument structure
The most reliable way to detect life on another planet would be to determine if the planet's atmosphere has methane.
Methane disappears from a planet's atmosphere unless it is replenished by life forms.
Explanation
This is a weaken question, so I'm looking for flaws.
While it seems like measuring methane levels in an atmosphere could play a role in detecting life (since life is necessary for maintaining the presence of methane) this conclusion goes too far. I'm skeptical whether we can conclude that determining if there's methane is the most reliable way to determine if there's life on a planet.
For example, what if not all life forms produce methane? Then you might measure an atmosphere, find no methane, and conclude there's no life... even if there exists a life form on that planet that doesn't emit methane. Not very reliable, eh?
Or what if there's just another method that's even more reliable than measuring methane? This conclusion is way too strong based on the evidence that we have.
Answer choices
This doesn't really weaken. While it shows the presence of other methods for detecting life, it doesn't indicate that those methods are more reliable than detecting methane.
This shows that the methane detecting method isn't that reliable, because if there's no methane on a planet we won't actually know whether or not there's life.
This tempts a lot of students because they think "we can't measure it, so this method must not be reliable." But the details of the stimulus matter in this case. The conclusion says that this method would be the most reliable, not that it currently is. Our inability to measure it now doesn't mean that it wouldn't be the most reliable method for determining the presence of life if we could execute it.
This doesn't weaken even though it sounds similar to (B). Maybe even if the amounts are small we could still detect the methane.
This doesn't change anything about if methane detection would be the best method for determining if there's life on a planet. In the future we may detect methane on many more planets.