Late Heavy Bombardment

The Late Heavy Bombardment is a hypothesized episode of unusually intense asteroid and comet impacts on the Moon and inner planets about 4.1–3.8 billion years ago.

Lunar Impact Melt Rocks: Moon rocks melted by impacts and later collected or sampled from the lunar surface. Their radiometric ages underpin the proposed concentration of lunar impacts around 3.9 billion years ago.
Nice Model: A model in which the giant planets migrated from an initially compact configuration and destabilized small bodies. Its original version proposed that giant-planet migration triggered a late surge of impacts.
Apollo Program: NASA’s crewed lunar exploration program that landed astronauts on the Moon from 1969 to 1972. Apollo samples made precise dating of ancient lunar rocks possible.
Late Veneer: The addition of chondritic material to Earth’s mantle after core formation, inferred from highly siderophile elements. It records late accretion to Earth but does not require a sharp impact spike at 3.9 billion years.
Apollo 16: The fifth crewed NASA lunar landing mission, conducted in 1972 in the Moon’s Descartes Highlands. Its highland samples supplied impact-melt ages once treated as evidence for a bombardment spike.
Giant-Planet Migration: Changes in the orbital distances of giant planets caused by gravitational interactions with surrounding material. Migration is the dynamical process invoked to redirect distant planetesimals toward the inner Solar System.
Apollo 17: The final crewed NASA lunar landing mission, conducted in 1972 in the Moon’s Taurus–Littrow valley. Its samples contributed to the age record used to assess ancient lunar impact history.
Hadean: The earliest geological eon of Earth, extending from its formation to about 4.0 billion years ago. The proposed bombardment overlaps the closing part of the Hadean and may have affected its surface.
Impact Gardening: The continual mixing and fragmentation of a planetary surface by repeated impacts. It can mix ejecta from impacts of different ages, complicating the chronology inferred from lunar samples.
Resonance Crossing: The passage of planetary orbits through a condition where their orbital periods form a simple ratio. Changing resonances can destabilize small-body orbits and increase impact rates.

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