Passage Summary: Since methane disappears quickly unless living things are making more of it, finding it on a planet is a great sign that life is present.

Conclusion: The most dependable method for identifying life on other planets is searching for atmospheric methane.

Reasoning: Methane is chemically unstable and vanishes from atmospheres unless it is being continuously produced by living organisms.

Analysis: The author assumes a strict 'one-to-one' relationship where only biological life can replenish methane. To weaken this, we need to find a non-biological way that methane could be pumped into an atmosphere. If geological events like volcanoes or chemical reactions between rocks and water can also produce methane, then the gas is no longer a 'reliable' indicator of life. Look for an answer choice that introduces a non-living source for methane replenishment.