LSAT 132, Section 3, RC Passage 2, Moon Craters

PASSAGE TEXT: A vigorous debate in astronomy centers on an epoch in planetary history that …

Paragraph Summaries

1. The late heavy bombardment was identified from Moon rocks and crater evidence. If debris hit the Moon around four billion years ago, it probably hit Earth too, which matters because life couldn’t have survived during that period.
2. Scientists disagree about what the LHB was. Some think it was debris from a broken asteroid or comet, some think it was just the end of a long declining bombardment, and some think it was a brief local event within the Earth-Moon system.
3. A possible Mars rock found on Earth supports the idea that the bombardment extended beyond the Earth-Moon system. But scientists need more rocks or planetary samples before deciding how widespread the LHB really was.

Analysis

The question being debated here is:

Was the LHB a real, widespread cataclysm, or are we misinterpreting the Moon’s evidence?

The second paragraph is where most of the action happens. It can be easy to mix up the theories so I’ve written an overview.

Theory 1: Solar-system-wide cataclysm

These scientists think the LHB was a genuine disaster. A large asteroid or comet orbiting the Sun broke apart and sprayed debris throughout the inner solar system (Mercury, Venus, Earth, Moon, Mars, etc.).

Picture someone smashing a piñata in the middle of a room. Debris goes everywhere. If this theory is right, you should expect evidence of impacts on many bodies besides the Moon.

Theory 2: There was no special LHB at all

This group thinks the name “late heavy bombardment” is misleading.

Their view is that the inner solar system was already experiencing a continuous, gradually declining bombardment over billions of years. The Moon only seems to show a dramatic late event because later impacts were so intense that they erased evidence of earlier ones.

This is more like painting over old layers of paint. The older history gets covered up, making it look like only the most recent layer existed.

Under this theory, there wasn’t a sudden catastrophe around 4 billion years ago. It was just the tail end of a long process.

Theory 3: Real cataclysm, but only local

This group agrees with Theory 1 on one point: there really was a sharply defined cataclysmic cratering event.

But they disagree about its extent.

They think the debris came from a body that broke apart within the Earth-Moon system itself. Since the debris was local, it would have been swept up quickly and wouldn’t have spread throughout the entire inner solar system.

Think of setting off a firecracker inside one room instead of in the whole house (btw, don’t do that …)

______

Okay, so that’s the theories. Now the Mars rock. It’s important because it potentially distinguishes Theory 1 from Theory 3.

If the rock really came from Mars and was blasted off Mars at about the same time as the Moon’s LHB, then two separate planetary systems experienced bombardment simultaneously.

That starts to look a lot less like a local Earth-Moon event and a lot more like a solar-system-wide event.

But notice the author’s restraint. One rock doesn’t settle the full debate. The wording repeatedly signals caution:

• “if the rock is indeed from Mars”
• “tiny piece of evidence”
• scientists need “many more such rocks”

This is why questions about the author’s attitude should usually point toward cautious support, not certainty.

A lot of the questions here are really testing whether you can keep the three theories straight.

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