nice020.html

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).
- Historically, the issue of the nature of the bombardment comes down to the ages of three.
- The only good, and agreed upon ages we have are for the two youngest:
  ......Imbrium (3.85Ga) and Orientale (3.82Ga).
- Nectaris was though to be 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)
  - To get a spike either need to: 1) create new impact population or
    .................................................. 2) the orbits of the planets must have changed.
  - Four basic dynamical scenarios have been proposed:
    1. Break up of an asteroid (Zappala et al 1998) - No: Requires 1000X mass of Ceres.
    2. Co-orbitals of the Moon (Cuk & Gladman 2008) - No: Can't last long enough.
    3. Dynamical instability of a 5^th terrestrial planet in the asteroid belt (Chambers 2007)
    4. Influx of comets from the outer Solar System (Wetherill 1975)
      - Triggered by the migration of the giant planets (Levison et al. 2001)
      - Best model to date is the so-called Nice model.