EARTH
The Late Heavy Bombardment
~3.9 billion years ago (existence disputed)
The Late Heavy Bombardment is a hypothetical brief surge of asteroid impacts, about 3.9 billion years ago, whose existence is now disputed.
The Late Heavy Bombardment is the hypothesis that asteroid impacts surged briefly and violently about half a billion years after the planets formed. It is based on the rocks that the Apollo missions returned from the Moon, because the measured ages of these rocks cluster narrowly. Textbooks have taught it for fifty years. The cluster may be an artefact of sampling, an effect of where the rocks were collected, rather than the trace of a real surge. The Apollo landing sites lie within reach of the material ejected by one very large impact, which formed the Imbrium basin. That single impact could have reset the ages of rocks across the whole sampled region, so that they all give its date.
Why it matters
Scientists rarely call into question so openly a hypothesis as established as the Late Heavy Bombardment. The dispute shows that a date may reflect the method that produced it, here the choice of samples and the dating technique, rather than the object dated.
Dating & uncertaintyDisputed
Boehnke and Harrison (2016) modelled an impact rate that declines steadily, with no peak at all. Their model still produces apparent peaks in the ages once argon is allowed to leak from the crystals being dated. The dating counts the argon that accumulates in a crystal, so a crystal that has lost argon appears younger. Harrison and colleagues (2024) compared dating methods applied to the same lunar rocks. They conclude that the hypothesis may not be testable with the samples available.
Earth history as one day
03:22:5920 h 37 min before midnight
if Earth’s 4.54 billion years were compressed into a single day