Widespread mixing and burial of Earth’s Hadean crust by asteroid impacts

A new bombardment model of the early Earth, calibrated with existing lunar and terrestrial data, shows that the Earth’s surface would have been widely reprocessed by impacts through mixing and burial by impact-generated melt; the model may also explain the age distribution of ancient zircons and the absence of early terrestrial rocks. Extensive resurfacing masks Earth's Hadean history Little is known about the bombardment history of the early Earth following the Moon-forming giant impact because of the extreme paucity of terrestrial samples more than four billion years old. Simone Marchi et al . have developed a new model for Earth's collisional evolution, calibrated using existing lunar and terrestrial data. They show that the Earth's surface would have been widely reprocessed by impacts through mixing and melting during the Hadean period, between 4 and 4.5 billion years ago. Their model may also explain the age distribution of ancient zircons, as well as the absence of early terrestrial rocks. The history of the Hadean Earth (∼4.0–4.5 billion years ago) is poorly understood because few known rocks are older than ∼3.8 billion years old 1 . The main constraints from this era come from ancient submillimetre zircon grains 2 , 3 . Some of these zircons date back to ∼4.4 billion years ago when the Moon, and presumably the Earth, was being pummelled by an enormous flux of extraterrestrial bodies 4 . The magnitude and exact timing of these early terrestrial impacts, and their effects on crustal growth and evolution, are unknown. Here we provide a new bombardment model of the Hadean Earth that has been calibrated using existing lunar 4 and terrestrial data 5 . We find that the surface of the Hadean Earth was widely reprocessed by impacts through mixing and burial by impact-generated melt. This model may explain the age distribution of Hadean zircons and the absence of early terrestrial rocks. Existing oceans would have repeatedly boiled away into steam atmospheres as a result of large collisions as late as about 4 billion years ago.