Hi All

gb... the "Grand Bombardment" was a late event, apparently a dramatic spike in the cratering patterns. Prior the planets were apparently pretty quiet impactor wise. Just what it was in accretion terms is... not much. A few extra 100 km wide impactors messed up the superficial layers of the planets, but in terms of accretion it's the same as asteroid impacts in late geological times.

What caused it? If Levison & co. are correct then it was the time of the outward migration of Uranus and Neptune, which scattered the proto-Kuiper Belt. So no wonder a few impactors ventured our way, but it wasn't planetary formation style accretion, which draws material from orbits similar to the planets themselves.

Adam