Previous age-dating evidence from Earth and lunar samples and the interpreted age of craters on the Moon suggest that the inner solar system experienced a period of intense meteorite impacts approximately 3.9 billion years ago, known as the “late heavy bombardment.” One hypothesis proposed to explain this observation is that migration of the giant planets in the outer solar system triggered a series of dynamical processes that resulted in a spike of intense impacts in the inner solar system. However, studies of asteroid-derived meteorites report older ages that conflict with, and cast doubt on, the late heavy bombardment hypothesis. A recent study led by Mizuho Koike of Hiroshima University examined a suite of samples believed to come from asteroid 4 Vesta in order to better assess our current knowledge of early solar system processes.
In this study, five Vesta samples were divided into two categories: samples that are heavily fractured, and samples that are relatively pristine. This ensured that the samples represented the early history of Vesta, from cooling and solidification (pristine samples), to being bombarded by meteorites (fractured samples). The age of each sample was determined through uranium-lead dating, and the fractured samples were found to be more than 4.15 billion years old, while the pristine samples were found to be as old as 4.5 billion years. Koike and colleagues propose that the oldest samples represent the formation of Vesta, while the younger, fractured samples represent meteorite collisions with Vesta. Since these ages are significantly older than the age associated with the late heavy bombardment hypothesis (approximately 3.9 billion years), the team proposes that collisions in the inner solar system began much earlier and slowly decreased over time. Furthermore, they suggest that the events recorded by lunar samples and craters represent a tail of weaker impacts. This study demonstrates the importance of studying asteroid samples because they record and preserve older information about the solar system that is destroyed by plate tectonics on Earth and heavy bombardment on the lunar surface. READ MORE