spaceSpace and Physics

Young Earth May Have Been Repeatedly Hit By Moon-Sized Objects


Jonathan O'Callaghan

Senior Staff Writer

Our planet was bombarded by objects early in its life. Southwest Research Institute

A study has suggested that the young Earth was repeatedly pounded by objects the size of the Moon, which may explain the composition of rocks on our planet.

Published in Nature Geoscience, scientists from the Southwest Research Institute in Texas looked at the period after a Mars-sized body hit Earth and formed the Moon about 4.5 billion years ago, known as the giant-impact hypothesis. That impactor was thought to be at least 6,000 kilometers (3,700 miles) across.


Some of the pieces of rock from that collision, known as planetesimals, coalesced into the Moon. Others, we had thought, stayed in Earth orbit for about 100 million years before breaking apart or being scattered by gravity.

However, this study suggests a much more dramatic process took place. The researchers say their model hints at “multiple subsequent impacts with the Earth by 1,500- to 3,000-km-diameter [930- to 1,860-mile] projectiles”, they write in their paper.

"This is more violent than thought," the study's lead author, Dr Simone Marchi, told IFLScience. "Some of these planetesimals may have exceeded 1,000 kilometers [620 miles] in diameter, some were perhaps as large as the Moon itself."

We’d previously thought about 0.5 percent of our planet’s mass was made up of material from these planetesimals. However, the researchers suggest this may be two to five times greater than previous calculations.


It all stems around something called siderophile elements. These are things that get absorbed into iron like gold, platinum, and iridium. Some of these were delivered to our planet after the Moon was formed, while others were either absorbed into our core or ejected into space.

In order to explain the amount we observe today, we need more collisions. Thus, this paper points to the period after the Moon's formation as the culprit, with more large planetesimals hitting Earth.

Animation of a Moon-sized object hitting our planet. Southwest Research Institute

“We modeled the massive collisions and how metals and silicates were integrated into Earth during this ‘late accretion stage,’ which lasted for hundreds of millions of years after the Moon formed,” Dr Marchi said in a statement. “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.”

This also helps solve another quandary. Namely, the presence of isotopic anomalies in some rocks on Earth had suggested that our mantle was mixed more than we thought after the Moon formed. This latest research could explain how that mixing occurred, as our planet was repeatedly hit by other impactors.


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