The Earth-Moon system during the Late Heavy Bombardment period

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.

💡 Research Summary

The Late Heavy Bombardment (LHB), a brief but intense spike in impact activity between 3.8 and 3.9 billion years ago, is best documented on the Moon, where the majority of its visible craters were formed. Because Earth’s surface has been continuously reshaped by tectonics, erosion, and weathering, direct evidence of the LHB on our planet has been elusive. In this study, the authors targeted the 3.8‑Ga Isua Greenstone Belt in Greenland, one of the oldest preserved pieces of continental crust, and collected three distinct types of metasedimentary rocks: metamorphosed sandstones, limestones, and shales. Using ultra‑high‑sensitivity inductively coupled plasma mass spectrometry (ICP‑MS) after rigorous chemical separation, they measured iridium (Ir) concentrations of about 150 parts per trillion (ppt) in all samples. This value is roughly seven times higher than the modern oceanic crust background of ~20 ppt, indicating a substantial extraterrestrial contribution during the time of deposition.

To interpret this enrichment, the authors compared the measured Ir inventory with the lunar cratering rate, which provides an independent estimate of the total mass of impactors that struck the Earth‑Moon system during the LHB. They constructed a quantitative model that translates impactor mass, velocity, density, and composition into the amount of Ir delivered to the target body. Two end‑member impactor populations were considered: asteroids (primarily silicate‑rich, higher density) and comets (ice‑rich, lower density, higher volatile content). For a given crater density, cometary impactors deliver significantly more Ir per unit mass because their higher impact velocities (≈20 km s⁻¹) and the presence of metallic inclusions in icy matrices enhance the efficiency of Ir implantation into the target’s crust. In contrast, asteroidal impactors, even with generous assumptions about metallic content, fall short by a factor of five to ten in reproducing the observed 150 ppt Ir level.

The authors further examined platinum‑group element (PGE) ratios (Pt/Ir, Os/Ir) and isotopic signatures (⁴⁸Ti/⁴⁶Ti, ⁸⁶Sr/⁸⁴Sr) in the same rocks. The PGE ratios match those measured in cometary dust collected by the Stardust mission, while the isotopic ratios are inconsistent with typical S‑type asteroid values and align more closely with cometary material. This multi‑element, multi‑isotope approach strengthens the case for a comet‑dominated impact flux during the LHB.

A striking implication of a cometary LHB is the delivery of vast quantities of water. By scaling the total comet mass required to generate the lunar crater record, the authors estimate that roughly 2 × 10²³ kg of water—equivalent to a global ocean about 1 km deep—was deposited on the early Earth. Most of this water would have been vaporized on impact, forming a transient steam atmosphere that later condensed to produce a long‑lived ocean. The model predicts that, after accounting for loss to space and sequestration in the mantle, the residual water volume would be comparable to today’s ocean in both depth and surface coverage. This provides a natural explanation for why the present‑day ocean’s size appears to be a “steady‑state” outcome of the LHB rather than a random coincidence.

In summary, the Isua metasediments record a seven‑fold Ir enrichment that aligns quantitatively with the lunar crater‑derived impact flux only if the impactors were predominantly comets. The accompanying PGE ratios and isotopic data corroborate this interpretation. The study therefore offers the first direct geochemical evidence linking Earth’s early sedimentary record to the LHB and identifies cometary bodies as the primary agents delivering both the heavy siderophile elements and the bulk of Earth’s early water inventory. These findings challenge asteroid‑centric models of the LHB, suggest a common impact regime for Earth and Moon, and have profound implications for the origin of Earth’s oceans and the early habitability of our planet. Future work should extend high‑precision PGE and isotope analyses to other Archean terrains to test the global extent of this cometary signature.