The Lunar Cataclysm: Reality of Mythconception?

Marc Norman¹ and Robert Duncan2

¹ Planetary Science Institute, Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
² College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 USA

Understanding the early impact history of the terrestrial planets is a priority goal for solar system exploration. More specifically, ascertaining whether or not the Earth and Moon experienced a cataclysmic Late Heavy Bombardment of impacting planetesimals ~3.9 billion years ago remains an open question with significant implications for the dynamical history of the Solar System, environmental conditions on the early Earth, and the development of crustal structures on Mars.

The idea of a Late Heavy Bombardment is controversial. Lunar impact melt breccias have crystallisation ages that cluster between 3.8 and 4.0 Ga (Norman et al., 2006; Figure 1), corresponding to an episode of intense crustal metamorphism defined by U-Pb isotopic compositions of lunar anorthosites. The late cataclysm interpretation invokes a spike in the mass flux to the Earth and Moon at precisely the time when the Earth’s oldest preserved continental crust was forming and life was emerging.  Alternatively the age distribution of lunar impact melts may reflect a steadily declining flux of impactors, with the older record being erased by younger events.

Relating lunar surface deposits to specific basins is critical for assessing the reality (or otherwise) of the late cataclysm. An excellent example is the Descartes Formation, a regional unit of fragmental impact breccia that was sampled by the Apollo 16 mission in 1972. Taking advantage of recent improvements in mass spectrometry we measured high-precision ³⁹Ar-⁴⁰Ar ages and trace element composition of anorthositic clasts from two of the Descartes breccias. These measurements show that the breccias were deposited 3.866 + 0.009 billion years, most likely as ejecta from the massive Imbrium basin, located some 1000 km to the NW of the Apollo 16 landing site (Fig. 2). The trace element compositions support this interpretation.

The significance of this discovery is that Imbrium is one of the youngest impact basins on the Moon.  Previous interpretations linked the Descartes breccias to the older Nectaris basin, strongly supporting the idea of a late cataclysm. Our study pulls the pin on the cataclysm hypothesis by showing that age of Nectaris is effectively unconstrained by the sample data. Absolute ages of older basins will be necessary to define the pre-3.9 Ga impact history of the Earth and Moon.

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Norman MD, Duncan RA, and Huard JJ (2006) Identifying impact events within the lunar cataclysm from 40Ar-39Ar ages and compositions of Apollo 16 impact melt rocks. Geochimica et Cosmochimica Acta 70: 6032-6049.