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:
- The tail end of terrestrial planet accretion.
- 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:
- Break up of an asteroid (Zappala et al 1998) - No: Requires 1000X mass of Ceres.
- Co-orbitals of the Moon (Cuk & Gladman 2008) - No: Can't last long enough.
- Dynamical instability of a 5th terrestrial planet in the asteroid belt (Chambers 2007)
- 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.