Excavation and Melting of the Hadean Continental Crust by Late Heavy Bombardment

TL;DR

This study models the impact effects of the Late Heavy Bombardment on Earth's early crust, suggesting it could explain the absence of Hadean rocks by causing widespread melting and excavation.

Contribution

It provides a semi-analytical framework to estimate impact-induced melting and excavation of early continental crust during the Hadean eon.

Findings

LHB could cover over 70% of Earth's surface with melts.

LHB impacts likely did not directly excavate all Hadean crust.

Impact effects depend on the impact size-frequency distribution parameter lpha.

Abstract

No Hadean rocks have ever been found on Earth's surface except for zircons---evidence of continental crust, suggesting that Hadean continental crust existed but later disappeared. One hypothesis for the disappearance of the continental crust is excavation/melting by the Late Heavy Bombardment (LHB), a concentration of impacts in the last phase of the Hadean eon. In this paper, we calculate the effects of LHB on Hadean continental crust in order to investigate this hypothesis. Approximating the size-frequency distribution of the impacts by a power-law scaling with an exponent {\alpha} as a parameter, we have derived semi-analytical expressions for the effects of LHB impacts. We calculated the total excavation/melting volume and area affected by the LHB from two constraints of LHB on the moon, the size of the largest basin during LHB, and the density of craters larger than 20 km. We also investigated the effects of the value of {\alpha}. Our results show that LHB does not excavate/melt all of Hadean continental crust directly, but over 70% of the Earth's surface area can be covered by subsequent melts in a broad range of {\alpha}. If there have been no overturns of the continental crust until today, LHB could be responsible for the absence of Hadean rocks because most of Hadean continental crust is not be exposed on the Earth's surface in this case.