Have distal impact ejecta changed through geologic time?

Abstract

Sand-size spherules of former silicate melt have been identified in ∼10 thin Early Archean to Paleoproterozoic layers that appear to be reworked ejecta from large impacts. Melt spherules are common in Phanerozoic layers of distal impact ejecta, but the aggregate thickness of spherules in early Precambrian layers is generally 10 to 100 times greater, and they display textures rarely seen in Phanerozoic spherules. These contrasts suggest secular variations in the nature of both impactors and target materials, such as a progressive decrease in the size of impactors and a progressive increase in the fraction of Earth's surface covered by continental crust.