LSAT 108 | Section 2 | Logical Reasoning: Q20

LR Question 20 Explanation

QUESTION TEXT: One of the most important events for modern astronomy …

QUESTION TYPE: Necessary Assumption

CONCLUSION: The comet was composed of rock rather than ice.

REASONING: The collisions created formations that showed no signs of water, so:

• No water was involved in the collisions
• None of the fragments penetrated Jupiter’s lower atmosphere

ANALYSIS: The key jump is from No signs of water to Fragments didn’t reach Jupiter’s lower atmosphere.

Why does “no water” tell us anything about the lower atmosphere?

It only works if the lower atmosphere contains water and penetrating it would have produced detectable signs of water.

Without that link, the argument falls apart.

The structure is:

If fragments reached the lower atmosphere → we’d see signs of water.

We don’t see signs of water.

Therefore, fragments didn’t reach the lower atmosphere.

___________

1. This is tempting because the conclusion mentions rock rather than ice. But the argument already assumes we’re dealing with a comet, not trying to distinguish a comet from an asteroid. Whether comets usually contain ice doesn’t tell us that this particular comet had to be rocky.
2. CORRECT. If penetrating the lower atmosphere wouldn’t necessarily produce signs of water, then no sign of water would no longer let us conclude that it didn’t penetrate. We need to assume B for the argument to work.
3. The size of the explosion is irrelevant. The conclusion is based entirely on the absence of water, not on how large the explosions were.
4. The formations don’t all need to have exactly the same composition. Even if they differed somewhat, the argument only relies on the claim that none showed signs of water.
5. The difficulty of detecting deeper explosions is actually dangerous for the argument. If deeper explosions are harder to detect, then maybe water signatures were simply missed. This isn’t something the argument assumes.