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.
Topic
The Late Heavy Bombardment (LHB) — a roughly four-billion-year-old episode of intense cratering on the Moon — and a debate about its origin and extent.
Framework
Present Debate. The author lays out three competing theories about the LHB, then describes a single piece of new evidence and the cautious conclusion researchers can draw from it.
Main Point
Three views about the LHB compete (asteroid/comet breakup, no cataclysm at all, and a brief Earth-Moon-only cataclysm). A recent Mars rock found on Earth tentatively supports the view that the LHB extended throughout the inner solar system, but more samples are needed before any conclusion is firm.
P1: What the LHB is
Lunar samples revealed that the Moon's major craters formed in a vigorous bombardment ~4 billion years ago — the LHB. Projectiles that hit the Moon should also have struck Earth, with profound consequences: until the LHB ended, life couldn't have survived.
P2: Three theories
Theory 1: a single large body (asteroid or comet) broke up and peppered the inner solar system. Theory 2: there was no cataclysm; the Moon's evidence is just the tail end of a long, declining bombardment, with later impacts erasing earlier ones. Theory 3: there was a sharply defined cataclysm, but it was brief and confined to the Earth-Moon system.
P3: A Mars rock provides new (limited) support
A rare meteorite, evidently a Mars rock knocked from Mars and landed on Earth, has been dated to about 4 billion years old — which would mean Mars and the Moon were both being bombarded at the same time. That suggests the LHB extended through the inner solar system. But the author cautions that scientists need many more such rocks, and ideally surface samples from other planets, before drawing firm conclusions.