Could the Lunar “Late Heavy Bombardment” Have Been Triggered by the Formation of Uranus and Neptune?

Abstract

We investigate the hypothesis that the so-called Late Heavy Bombardment (LHB) of the Moon was triggered by the formation of Uranus and Neptune. As Uranus and Neptune formed, which we assume occurred at the epoch of the LHB, they scattered neighboring icy planetesimals throughout the Solar System. Some of these objects hit the Moon. Our integrations show that the Moon would have accreted about 6×10 21 g, if we assume that the Uranus–Neptune region initially contained 5 times the current mass of these planets in the form of small solid objects. In addition, Mars would have accumulated ∼6×10 22 g of icy material, which could have supplied its putative early massive atmosphere. However, Earth would likely have accreted only ∼7×10 22 g of water, or ∼5% of its oceans, through the mechanisms studied here. The numerical experiment that we have performed on the behavior of Uranus–Neptune planetesimals shows very good agreement with current constraints on the LHB. The influx of Uranus–Neptune planetesimals onto the Moon could have lasted for a time as short as 10 or 20 million years. The dynamical transport of the Uranus–Neptune planetesimals during this process would have caused Jupiter and Saturn to migrate. This migration, in turn, would have destabilized objects in the jovian Trojan swarms and the asteroid belt. Thus, not only would Uranus and Neptune planetesimals have struck the Moon, but asteroids would have as well. We find that the Trojan asteroids of Jupiter could not have contributed a large percentage of material to the LHB, but the asteroid belt could, in principle, have contributed to, or even dominated, the LHB. Although this model appears to explain the LHB well, it requires that fully formed Uranus and Neptune not appear in the trans-saturnian region until some 700 million years after the formation of the Earth.