The Earth-Moon system during the Late Heavy Bombardment period

TL;DR

This study links Earth's early iridium enrichment to cometary impactors during the Late Heavy Bombardment, providing direct geochemical evidence that supports the lunar cratering rate and suggests comets delivered significant water to Earth.

Contribution

First direct geochemical evidence indicating LHB impactors were likely comets, aligning Earth's iridium record with lunar cratering data and advancing understanding of early Earth's impact history.

Findings

Iridium enrichment at Isua matches lunar cratering rate if impactors were comets.

Impact of comets could have delivered a km-deep ocean to Earth.

Supports cometary origin of LHB impactors and their role in Earth's water delivery.

Abstract

The Late Heavy Bombardment (LHB) period is the narrow time interval between 3.8 and 3.9 Gyr ago, where the bulk of the craters we see on the Moon formed. Even more craters formed on the Earth. During a field expedition to the 3.8 Gyr old Isua greenstone belt in Greenland, we sampled three types of metasedimentary rocks, that contain direct traces of the LHB impactors by a seven times enrichment (150 ppt) in iridium compared to present day ocean crust (20 ppt). We show that this enrichment is in agreement with the lunar cratering rate, providing the impactors were comets, but not if they were asteroids. Our study is a first direct indication of the nature of the LHB impactors, and the first to find an agreement between the LHB lunar cratering rate and the Earth's early geochemical record (and the corresponding lunar record). The LHB comets that delivered the iridium we see at Isua will at the same time have delivered the equivalent of a km deep ocean, and we explain why one should expect a cometary ocean to become roughly the size of the Earth's present-day ocean, not only in terms of depth but also in terms of the surface area it covers.