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The Lunar Late Heavy Bombardment

The small craters can be saturated on the oldest surfaces,
but probably not the basins.
There are more than 40 basins (D>300km) on the Moon.
- They are all older then ~3.8Gyr (Wilhelms 1987).
- The only good ages we have are for the two youngest:
  ......Imbrium (3.85Ga) and Orientale (3.82Ga).
- Nectaris is either ~3.9Ga or 4.1Ga.
This has led to controversy of whether the lunar basin forming impacts 3.9Ga were:
1. The tail end of terrestrial planet accretion.
2. A spike in the impact rate. (Tera et al. 1974)
Two recent advances strongly support the idea of a spike:
- Bottke et al.(2006) show terrestrial-region impactors don't last 700Myrs.
  - They find that we can rule out (1) to >3σ based on Imbrium and Orientale alone.
- D. Trail & S. Mojzsis looked at very old terrestrial samples and see no impacts at ~4.0Ga
If a spike, the most likely cause is a change in the dynamical state of the planetary system.