Planetary collision with Earth created Moon: study

The Moon was formed when a planet-sized object struck the infant Earth some 4.5 billion years ago, a new study has found, confirming the most prevalent hypothesis on the formation of the lunar body.

A layer of iron and other elements deep underground is the evidence scientists have long been seeking to support this hypothesis, researchers said.

The study used laboratory simulations of an Earth impact as evidence that a stratified layer beneath the rocky mantle - which appears in seismic data - was created when our planet was struck by a smaller object.

Scientists argue this was the same impact that sent a great mass of debris hurtling into space, creating the Moon.

the giant impact hypothesis," said lead author Maylis Landeau, who was a post-doctoral fellow at Johns Hopkins University at the time of the research.

"They demonstrate that the giant impact scenario also explains the stratification inferred by seismology at the top of the present-day Earth's core. This result ties the present-day structure of Earth's core to its formation," said Landeau, now at the University of Cambridge in the UK.

Peter Olson, research professor at Johns Hopkins said the giant impact argument for the formation of the Moon is the most prevalent scientific hypothesis on how Earth satellite was formed, but it is still considered unproven because there's been no "smoking gun" evidence.

"We're saying this stratified layer might be the smoking gun. Its properties are consistent with it being a vestige of that impact," said Olson.

Their argument is based on seismic evidence of the composition of the stratified layer - believed to be some 321 kilometres thick and lying 2,900 kilometres below Earth's surface - and on laboratory experiments simulating the turbulence of the impact.

The turbulence is believed to account for the stratification - meaning a mix of materials in layers rather than a homogeneous composition - at the top of the core.
The stratified layer is believed to consist of a mix of iron and lighter elements, including oxygen, sulphur and silicon.

The very existence of this layer is understood from seismic imaging, as it lies far too deep underground to be sampled directly, researchers said.

Till now, most simulations of the impact have been done numerically, and have not accounted for impact turbulence.

Turbulence is difficult to simulate mathematically and no computer model has yet done it successfully, Olson said.

The researchers in this experiment simulated the impact using liquids meant to approximate the turbulent mixing of materials that would have occurred when the planetary object struck the young Earth. The study appears in the journal Nature Geoscience.