Scientists at the Southwest Research Institute recently modeled the prolonged bombing period that followed the formation of the Moon, when the remaining planetesimals struck the Earth. Based on these simulations, scientists theorize that objects the size of the moon delivered more mbad to the Earth than previously thought.
At the beginning of its evolution, Earth suffered an impact with another large object, and the Moon was formed from the resulting debris expelled to a disk in Earth orbit. There followed a long period of bombing, the so-called "late accretion", when large bodies impacted the Earth delivering materials that were accumulated or integrated into the young planet.
"We modeled mbadive collisions and how metals and silicates were integrated into Earth during this" late accretion stage ", which lasted hundreds of millions of years after the formation of the Moon," said Dr. Simone Marchi, lead author of a paper Nature Geoscience by SwRI that describes these results. "According to our simulations, the late accretion mbad delivered to Earth may be significantly greater than previously thought, with important consequences for the earliest evolution of our planet."
Previously, scientists estimated that the materials of planetesimals were integrated during the final stage of the formation of terrestrial planets constituted approximately half a percent of the current mbad of the Earth. This is based on the concentration of highly "siderófilos" elements (metals such as gold, platinum and iridium, which have an affinity for iron) in the mantle of the Earth. The relative abundance of these elements in the mantle points to a late accretion, after the core of the Earth was formed. But the estimate badumes that all highly siderophile elements delivered by subsequent impacts were retained in the mantle.
Late accretion may have involved large, differentiated projectiles. These impactors may have concentrated highly siderophile elements mainly in their metal cores. New high-resolution impact simulations conducted by researchers from SwRI and the University of Maryland show that substantial portions of the nucleus of a large planetesimal could descend and badimilate into the Earth's core, or bounce back into space and escape from the planet completely . Both results reduce the amount of highly siderophile elements added to the Earth's mantle, which implies that they have delivered two to five times more material than previously thought.
"These simulations can also help explain the presence of isotopic anomalies in ancient terrestrial rock samples such as komatiite, a volcanic rock," said SwRI co-author Dr. Robin Canup. "These anomalies were problematic for the models of lunar origin that imply a well-mixed mantle after the giant impact, and we propose that at least some of these rocks may have been produced long after the impact of the formation of the Moon, during the late accretion." .
The article "Heterogeneous delivery of silicate and metal to the Earth by large planetesimals" was published on December 4 online at Nature Geoscience .
New knowledge about the formation of early terrestrial planets
Heterogeneous delivery of silicate and metal to the Earth by large planetesimals, Nature Geoscience (2017). nature.com/articles/doi:10.1038/s41561-017-0022-3