‘Giant Impact Theory’ of Moon’s Origin –Nixed by New Research



New research from geophysical scientist Junjun Zhang and colleagues at Origins Lab at the University of Chicago, suggests that the giant impact hypothes of the creation of the Moon might be wrong. The team found   that in comparing titanium isotopes from both the moon and the Earth, that the match is too close to support the theory that the moon could have been made partly of material from another planet.

According to the giant impact hypothesis, there was once a Mars-sized body referred to as Theia orbiting in our solar system. Theia would have formed in about the same orbit as Earth, but about 60° ahead or behind.  When the protoplanet had grown to be about the size of Mars, its size made it too heavy for its orbit to remain stable. As a result, its angular distance from Earth varied increasingly, until it finally it crashed into the Earth.

The collision would have occurred 4.533 billion years ago when Theia would have hit the Earth at an oblique angle, and destroyed itself in the process. Theia's mantle and a significant portion of the Earth's silicate mantle were thrust into space. The left over materials from Theia mixed with the materials from the Earth and eventually formed the Moon.

Earlier research validating this hypothesis, was conducted by cientists from Oxford University, University of California, and Swiss Federal Institute of Technology, who  compared silicon isotopes from Earth rocks, as well as other materials from our solar system such as rocky materials from meteorites showing that the Earth’s core and the Moon’s core contain the same silicon isotopic material, which would support that the two were once a single body until a large impact separated them.

Up to about 2,900 kilometers (1,800 miles) into the Earth (not quite half way to the center), is what we know as the mantle and crust. They are predominantly formed of silicate, a compound mad of silicon, oxygen, and other elements. Past the halfway mark is a dense metallic iron material that makes up the Earth’s core.

The multinational team found that the heavier isotopes from silicate samples taken from the Earth consisted of increased amounts of the heavier isotopes of silicon. They found that Mars, the asteroid Vesta, and various chrondites (primitive meteorites that never produced ainner cores) do not contain such an arrangement, even though they have an iron core. is much smaller than the Earth (about one-eighth the size), so did not have enough mass to generate the pressure necessary to form the same core as found in the Earth.

On the other hand, the researchers found that the Moon did show a similar composition of the silicon isotopic composition as the Earth. However, it, too, is much smaller than the Earth—about one-fiftieth as large as the Earth and about one percent of the Earth’s mass—making it even less likely to have been able to generate enough pressure to form an Earth-like iron core.

However, such a core does exist at the center of the Moon, but no one can explain how it got there.
The researchers contend that the Moon indeed must have been created during a giant impact by a planet-size object (Theia) that hit during the early development of Earth. The impact was large enough that the materials, which eventually formed the Moon, mixed with the materials from the Earth, which already had a heavy silicon isotopic composition.

“The similar isotopic composition of the bulk silicate Earth and the Moon is consistent with the recent proposal that there was large-scale isotopic equilibration during the giant impact,” they wrote in a paper published in the journal Nature.

This research was the first of its kind using isotopes in this manner and offers intriguing insights into the creation of Mars, the Earth, and the Moon. It may also help explain how life evolved on the Earth and whether or not it might have existed at some time on Mars.  

Now, Zhang's research at the University of Chicago shows that although scientists had already found that oxygen isotopes from the Earth’s mantle and the moon were nearly identical, that wasn’t enough to put a dent in the theory that a collision with Theia had created the moon because oxygen isotopes from the Earth could have mixed with isotopes from the mass of molten material circling the planet after impact. Because titanium isotopes are not nearly so easily exchanged, it’s difficult to theorize that the same sort of mixing could have occurred.

Most scientists agree that if a planet had smacked into Earth and the moon came about as a result, than the moon ought to be made of some of that other planet as well. Some say the laws of physics suggest it would be somewhere in the neighborhood of forty percent. If that’s the case, why don’t studies of rocks brought back by the Apollo missions show evidence of this other planet?

Image at the top of the page is an infrared camera image from the moon taken with the Lunar Crater Observation and Sensing Satellite (LCROSS) mid-infrared camera. 

More information: The proto-Earth as a significant source of lunar material, Nature Geoscience (2012) doi:10.1038/ngeo1429

The Daily Galaxy via physorg.com and Origins Lab

Image credit: NASA/JPL

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