The Late Heavy Bombardment
Was there a sudden storm of giant impacts 3.9 billion years ago, or a slow fading of them?
This page describes a scientific hypothesis that is still argued over. It sets out the case for it, the case against, and where the evidence stands today.
Key facts
- When
- proposed for about 4.1 to 3.8 billion years ago
- Where
- The Moon, Earth and the inner planets
- What hit
- many bodies tens to hundreds of kilometres across
- Era
- Deep time
If it happened as first described, the largest single impacts, those that made the great basins on the Moon, would each have released millions of times the energy of the Chicxulub impact.
The Moon's dark 'seas' fill basins hundreds of kilometres wide, each the scar of one such impact.
What happened
In the 1970s, scientists dating rocks melted by impacts in the Apollo samples found a surprise. Many of the ages bunched together at around 3.9 billion years. That suggested a short, violent spike in impacts long after the planets had formed, which became known as the Late Heavy Bombardment or the lunar cataclysm.
The idea was popular for decades, especially when computer models of the giant planets showed that a shift in their orbits could have scattered asteroids and comets inwards at about the right time.
Today it is genuinely debated. Many researchers now think the bunching of ages is partly an illusion: the Apollo landing sites lie near the huge Imbrium basin, so many samples may record that one event. Lunar meteorites, which come from random places on the Moon, show a more spread-out range of ages. Newer models also suggest the giant planets shifted much earlier. The alternative view is a steady decline, a long tail of impacts left over from planet-building, perhaps with smaller spikes.
Effects
Whichever version is right, the young Earth was hit far more often than today. Large impacts would have repeatedly heated the oceans and crust. Some scientists think this may have delayed or disrupted the start of life; others suggest impacts created hot, mineral-rich pools where the chemistry of life could begin.
The evidence
Ages of impact melt in Apollo and Luna samples, the dating of the great lunar basins, crater counts on the Moon, Mercury and Mars, and the ages recorded in meteorites from asteroids. New samples from the far side of the Moon, returned by China's Chang'e 6 mission in 2024, are helping to test the idea.
Did you know?
Earth must have suffered many more large impacts than the Moon in this period, because it is bigger and pulls harder, but plate tectonics and erosion have erased all of their craters.
The dark patches of the 'man in the Moon' are lava that later flooded basins blasted out in this era.
Related pages
How is impact energy measured? See energy explained, or try the impact calculator.