Planetary bodies differ from one another

A free, expert breakdown of this official LSAT Logical Reasoning question.

TopicsMust be True

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Stimulus

Planetary bodies differ from one another in their composition, but most of those in the Solar System have solid surfaces. Unless the core of such a planetary body generates enough heat to cause volcanic action, the surface of the body will not be renewed for millions of years. Any planetary body with a very cold moon belonging to Jupiter, have solid icy surfaces with very few meteorite craters.

What this question is testing

Must be True

Premise

The argument lays out a chain. If a solid-surface planet doesn't have a hot, volcanically active core, its surface doesn't get renewed. And if the surface doesn't get renewed, meteorite craters pile up over millions of years.

Then we're told about Europa: it's old, very cold, has a solid icy surface — but very few craters.

Anticipate

Run the chain backward. Europa doesn't have lots of craters. So its surface must have been renewed. So it must have a core hot enough for volcanic action.

That's a little surprising, since Europa is described as very cold. But "cold" is about the surface; the chain says the core is what matters.

Goal

The answer should say: at least one very cold planetary body has a core that generates enough heat for volcanic action.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question

If the claims above are true, which one of the following must, on the basis of

Answer choices

Out of Scope4% picked this
The Earth’s Moon does not have an
The stimulus uses the Moon as a generic example of what a heavily pockmarked surface looks like, but it never says the Moon does or doesn't have ice. We can't infer this from the stimulus.
4%
Reversal16% picked this
If a planetary body does not have a heavily pockmarked surface, its core does not generate enough heat
The stimulus says: no volcanic core -> not renewed -> pockmarked. The contrapositive (what we can derive) is: not pockmarked -> volcanic core. This answer takes the contrapositive of the second link only — not pockmarked -> core doesn't generate volcanic heat — which is actually a flat reversal of the contrapositive's direction. The stimulus does not let us conclude that absence of pockmarking means absence of volcanic activity.
16%
Unsupported17% picked this
Some planetary bodies whose cores generate enough heat to cause volcanic action do not have
The stimulus tells us nothing about planetary bodies that do have volcanic cores but lack solid icy surfaces. This claim about a category — bodies with volcanic cores that don't have icy surfaces — has no support either way from the stimulus.
17%
Unsupported5% picked this
Some of Jupiter’s moons are heavily pockmarked by
The stimulus mentions Europa, one of Jupiter's moons. From that, we can't conclude anything about Jupiter's other moons. Some of them might be heavily pockmarked, but the stimulus gives us no information about them.
5%
Correct58% picked this
Some very cold planetary bodies have cores that generate enough heat to
Why this is right
This is the contrapositive at work. Europa is described as very cold, solid-surfaced, and not heavily pockmarked. Working backward through the chain: not pockmarked -> surface was renewed within millions of years -> core generates enough heat for volcanic action. So Europa is a very cold body with a volcanic-heat core, which means at least one very cold body has such a core. That's exactly what (E) claims.
Skill tested: Must be True · how this choice captures the argument's function is the move to repeat next time.
58%

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