After the Moon’s formation, Earth experienced a protracted bombardment by large (over 1,500 km in diameter) planetesimals. Dr. Simone Marchi, a researcher at the Southwest Research Institute, and co-authors recently modeled this process. Based on their simulations, the researchers say that the large planetesimals delivered more mass to our planet than previously thought.
Marchi et al modeled the protracted period of bombardment after the Moon formed, determining that impactor metals may have descended into Earth’s core. This artistic rendering illustrates a large impactor crashing into the young Earth. Light brown and gray particles indicate the projectile’s mantle (silicate) and core (metal) material, respectively. Image credit: Southwest Research Institute.
“We modeled the massive collisions and how metals and silicates were integrated into Earth during the so-called ‘late accretion stage,’ which lasted for hundreds of millions of years after the Moon formed,” Dr. Marchi said.
“Based on our simulations, the late accretion mass delivered to Earth may be significantly greater than previously thought, with important consequences for the earliest evolution of our planet.”
Previously, planetary researchers estimated that materials from planetesimals integrated during the final stage of terrestrial planet formation made up about 0.5% of the Earth’s present mass.
This is based on the concentration of highly ‘siderophile’ elements — metals such as gold, platinum and iridium, which have an affinity for iron — in the Earth’s mantle.
The relative abundance of these elements in the mantle points to late accretion, after Earth’s core had formed.
But the estimate assumes that all highly siderophile elements delivered by the later impacts were retained in the mantle.
Late accretion may have involved large differentiated projectiles. These impactors may have concentrated the highly siderophile elements primarily in their metallic cores.
New high-resolution impact simulations by Dr. Marchi and colleagues show that substantial portions of a large planetesimal’s core could descend to, and be assimilated into, the Earth’s core — or ricochet back into space and escape the planet entirely.
Both outcomes reduce the amount of highly siderophile elements added to Earth’s mantle, which implies that 2-5 times as much material may have been delivered than previously thought.
“These simulations also may help explain the presence of isotopic anomalies in ancient terrestrial rock samples such as komatiite, a volcanic rock,” said Dr. Robin Canup, also from the Southwest Research Institute.
“These anomalies were problematic for lunar origin models that imply a well-mixed mantle following the giant impact.”
“We propose that at least some of these rocks may have been produced long after the Moon-forming impact, during late accretion.”
The research appears in the journal Nature Geoscience.
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S. Marchi et al. Heterogeneous delivery of silicate and metal to the Earth by large planetesimals. Nature Geoscience, published online December 4, 2017; doi: 10.1038/s41561-017-0022-3
