An article published in the journal “Nature Geoscience” describes a research on the consequences for the Earth of the bombardment that followed the formation of the Moon. According to a team of the Southwest Research Institute (SwRI) led by Simone Marchi, the collisions that followed the one that led to the birth of the Moon kept on increasing the Earth’s mass for a longer time than previously thought.
According to the most accepted theory, about 150 million years after the birth of the solar system, the early Earth was hit by a planet the size of Mars that was called Theia. This caused the ejection of a huge amount of debris that formed the Moon. A part however fell back to Earth and at the time there were other small bodies wandering the solar system that hit the Earth, contributing to increase its mass but so far the estimate was 0.5% of the current mass of the planet while this new research offers estimates two to five times higher.
Simone Marchi, a researcher at SwRI in Boulder, Colorado, USA, explained that he and his team created models of the collisions and how metals and silicates were integrated into the Earth in this “late accretion stage”, which lasted for hundreds of millions of years after the Moon’s formation. Based on the simulations they created, the mass brought to Earth could be much higher than previously thought, with important consequences on the planet’s evolution.
For example, according to Robin Canup, another author of the article, the simulations can help to explain the presence of isotopic anomalies in samples of ancient terrestrial rocks such as komatiite, a volcanic rock. These anomalies were a problem for the models for the Moon’s origin that involved the formation of a well-mixed mantle after the giant impact.
According to this new research at least some of these rocks may have been produced during that late accretion stage, much later than the impact that led to the Moon’s formation. The new model explains those anomalies because those rocks could have arrived on Earth too late to mix well in the mantle. The high-density elements called siderophile in jargon such as tungsten could be present in the Earth’s mantle in much larger amounts than expected because they have not sunk to the center of the planet.
The image below (courtesy SwRI/Marchi. Komatiite image credit: Department of Earth & Atmospheric Sciences, University of Alberta) shows a representation of the situation after the impact. Theia’s particles are shown in dark brown, the Earth’s mantle particles in green. The red and gray half spheres indicate the core and the surface of the Earth respectively. The yellow cone defines the region where Theia’s materials were concentrated. In the box there’s an image of komatiite.
Our understanding of the events that led to the formation of the present Earth and the Moon may have been made more complex by an amount of impacts greater than what was predicted in previous models. As is often the case in various fields of science, the amount of data accumulated over the years and much more powerful computers that make it possible to produce much more sophisticated models can improve that understanding.