Collisions With Moon-Sized Objects Helped Earth Bulk Up In its Early Years

Metals such as gold and platinum were integrated into Earth's mass during our planet's so-called "late accretion" period.

Metals such as gold and platinum were integrated into Earth's mass during our planet's so-called "late accretion" period.

New research suggests that collisions with moon-sized planetary objects helped Earth gain a substantial amount of mass in its early years, with the amount of mass greater than what was previously theorized.

As noted by, our planet collided with another large object early on in its existence, with the moon forming when debris from that collision was deposited into an “Earth-orbiting disk.” This was followed by what scientists call the “late accretion” period, or hundreds of millions of years of bombardment where large bodies of similar size to our moon had collided with the Earth, integrating materials that helped our planet gain more mass as it continued to evolve.

Lead researcher Dr. Simone Marchi of the Southwestern Research Institute explained in a statement that his team used modeling techniques to simulate the collisions that took place during Earth’s early years, and how metals and silicates became a part of our planet’s makeup as part of the late accretion stage.

“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.”

Earlier research suggested that materials from planetesimals, or large, moon-sized pieces of debris that weren’t able to become full-fledged planets, only took up about half a percent of Earth’s present-day mass. According to, these theories were based on the concentration of gold, platinum, iridium, and other “highly siderophile” elements in our planet’s mantle. All these elements have an affinity for iron, and their high concentration in Earth’s mantle traced their origin to the late accretion stage. But these previous studies were based on the assumption that all highly siderophile elements that came from later impacts remained in the mantle.

With their study shining a new light on what took place during Earth’s early years, researchers at the Southwest Research Institute and the University of Maryland used high-resolution impact simulations to prove that good chunks of a large planetesimal’s core could be blended into the Earth’s core or bounce back into outer space and not be assimilated into our planet’s mass. In both possible scenarios detailed in the study published earlier this week in the journal Nature Geoscience, the amount of highly siderophile elements incorporated into Earth’s mantle is reduced, meaning that there might have been two to five times more material delivered than originally believed.

Explaining the outcome of her team’s modeling simulations, co-author Dr. Robin Canup, also from SwRI, said that the study could also explain certain peculiarities in ancient terrestrial rock samples.

“These anomalies were problematic for lunar origin models that imply a well-mixed mantle following the giant impact,” said Canup.

“We propose that at least some of these rocks may have been produced long after the moon-forming impact, during late accretion.”

The new study is not the first of its kind to suggest that some sort of celestial event helped bring precious metals to Earth during its early years. A 2011 study cited by National Geographic suggested that Earth was bombarded by a “cataclysmic” meteor shower about 3.9 billion years ago. This study lent credence to the belief that our planet’s crust and mantle became rich in gold and other precious metals as a result of meteor showers.