Illusory Late Heavy Bombardments

Patrick Boehnke and T. Mark Harrison

PNAS 2016
published ahead of print September 12, 2016


“The vast majority of evidence marshaled for the Late Heavy Bombardment comes from 40Ar/39Ar age spectra of Apollo samples, interpreted through “plateau” ages, which show an apparent cluster at ∼3.9 Ga. Whether such data can be uniquely inverted to constrain impact histories in the Earth−Moon system has never been tested. We show that diffusive loss of 40Ar from a monotonically declining impactor flux coupled with the early and episodic nature of lunar crust formation tends to create clustered distributions of apparent 40Ar/39Ar ages at ca. 3.9 Ga. Instead, these 40Ar/39Ar data can be reconciled with a continuously decreasing bollide flux. Thus, impacts may have played a minimal role in terrestrial habitability, early Earth dynamics, and the formation of Hadean zircons. ”

“The Late Heavy Bombardment (LHB), a hypothesized impact spike at ∼3.9 Ga, is one of the major scientific concepts to emerge from Apollo-era lunar exploration. A significant portion of the evidence for the existence of the LHB comes from histograms of 40Ar/39Ar “plateau” ages (i.e., regions selected on the basis of apparent isochroneity). However, due to lunar magmatism and overprinting from subsequent impact events, virtually all Apollo-era samples show evidence for 40Ar/39Ar age spectrum disturbances, leaving open the possibility that partial 40Ar* resetting could bias interpretation of bombardment histories due to plateaus yielding misleadingly young ages. We examine this possibility through a physical model of 40Ar* diffusion in Apollo samples and test the uniqueness of the impact histories obtained by inverting plateau age histograms. Our results show that plateau histograms tend to yield age peaks, even in those cases where the input impact curve did not contain such a spike, in part due to the episodic nature of lunar crust or parent body formation. Restated, monotonically declining impact histories yield apparent age peaks that could be misinterpreted as LHB-type events. We further conclude that the assignment of apparent 40Ar/39Ar plateau ages bears an undesirably high degree of subjectivity. When compounded by inappropriate interpretations of histograms constructed from plateau ages, interpretation of apparent, but illusory, impact spikes is likely. “