A vigorous debate in astronomy centers on an epoch in planetary history that was first identified by analysis of rock samples obtained in lunar missions. Scientists discovered that the major craters on the Moon were created by a vigorous bombardment of debris approximately four billion years ago—the so-called late heavy bombardment (LHB). of Earth since, until the LHB ended, life could not have survived here.
Various theoretical approaches have been developed to account for both the evidence gleaned from samples of Moon rock collected during lunar explorations and the size and distribution of craters on the Moon. Since the sizes of LHB craters suggest they were formed by large bodies, some astronomers believe that the LHB was Earth-Moon system, because the debris from such an event would have been swept up relatively quickly.
New support for the hypothesis that a late bombardment extended throughout the inner solar system has been found in evidence from the textural features and chemical makeup of a meteorite that has been found on Earth. It seems to be a rare example of a Mars rock that made its way to more such rocks and perhaps obtain surface samples from other planets in the inner solar system.
What this question is testing
Topic
The author is explaining a real argument among astronomers: about 4 billion years ago, the Moon got pummeled with debris. The question is — was it just the Moon and Earth, or the whole inner solar system, and was it really a sudden burst or just the end of a long fade-out?
Framework
Present Debate. Three different positions are laid out side by side. The author doesn't pick a winner, but they do flag a piece of evidence that nudges things one direction.
Main Point
Here's the simpler version: scientists agree the Moon got hit with a lot of debris around 4 billion years ago. They disagree about three things — whether it was a sudden cataclysm or just a tapering off, where the debris came from, and how big a region was affected. A new piece of evidence (a Mars rock that landed on Earth and dates to that era) hints that other planets got hit too, which would favor the "whole inner solar system" view. But the author is careful: one rock isn't enough.
P1: What everyone agrees on
Lunar rocks tell us the Moon got hammered around 4 billion years ago. If the Moon got hit, Earth almost certainly did too — which is why life couldn't have started yet.
P2: Where they disagree (three theories)
One group: a big asteroid or comet broke apart and scattered debris across the inner solar system. Another group: nothing dramatic happened — what we're seeing on the Moon is just the end of a long fade. A third group: there was a sudden burst, but it only hit the Earth-Moon system, because the debris would have been swept up too quickly to spread further.
P3: The Mars rock
A meteorite found on Earth looks like it came from Mars and dates to about 4 billion years ago. If that's right, both Mars and the Moon were being hit at the same time — which supports the "whole inner solar system" view. The author closes with a polite caution: this is one rock. To really know, scientists need more rocks like it and ideally samples from other planets.
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