Hi All

SpaceDaily reports that the migration of Uranus/Neptune about 700 million years after they formed led to the Late Heavy Bombardment according to models by Harold Levison and colleagues.
Levison has this paper - he calls it a fairytale - and other, submitted, works online at his homepage.

In this particular case the simulations neatly form a scenario, but the fine details are fuzzy. According to oligarchic planet formation modelling Jupiter and Saturn formed from 10-15 Earth mass cores BUT they didn't form alone. At least 4 "ice giant" cores formed between 6-10 AU from the Sun - but not further out. Formation time is way too high and the outer part of the disk probably rapidly lost gas due to UV from giant stars in the Sun's birth nebula.

For about ~ 700 myr the 4 planet system remains stable, but then a resonance between Jupiter and Saturn - which accreted gas at the expense of Uranus and Neptune - cause the system to go chaotic and spit the two remaining cores out to ~ 20 and 30 AU. This also sends thousands of hefty planetesimals inwards to bash the inner planets.

Adam