Lunar Origins Under New Scrutiny
March 27, 2012

Lunar Origins Under New Scrutiny

Lawrence LeBlond for

The Moon, Earth´s closest companion for more than 4 billion years, is the object of a new theory on how our planet´s own natural satellite was created, reports ScienceNOW.

It is scientifically believed that Earth collided with a hypothetical Mars-sized planet called Theia early in its existence producing a disc of magma that orbited Earth and eventually amalgamated to form the Moon. Under this giant impact hypothesis, models show that the theory could be proven if 40 percent of the magma came from Theia.

But a new analysis of isotopes found in lunar minerals challenges that longstanding view. These oxygen isotopic compositions have been found to be identical in both the lunar and terrestrial samples, which is inconsistent with the giant impact models.

Reporting in the journal Nature Geoscience, researchers from the University of Chicago say their findings contradict the theory that the moon was formed from an impact between Earth and planet Theia 4.5 billion years ago.

Lead researcher Junjun Zhang and colleagues compared the isotopic ratios of titanium in both lunar and earthen samples. They found the samples were identical to within four parts per million, suggesting the moon is made up entirely of material from Earth.

Some have theorized that Theia may have been made up of the same isotopic composites as Earth. But Zhang and his team explained that titanium isotopic ratios are extensively diverse throughout the solar system, and even bodies forming in the same region of the solar system show extensive diversity in their isotope ratios.

Based on their findings, Zhang and colleagues conclude that it is far more likely that the isotopes found on the Moon came entirely from Earth, rather than from another planet.

Zhang and colleagues explained how these isotopes can occur in slightly different forms with slightly different masses. Oxygen, for example, has three isotopes: 16O, 17O and 18O, indicating differences in the number of neutrons each nucleus contains. If you compare any two samples of oxygen found on Earth, you will find the proportions of their isotopes are virtually identical in both samples. But the proportions found in samples from meteorites and other planets, like Mars, are usually different. So if you find a sample with the same oxygen isotope composition as one from Earth, it is very likely the sample came from our world.

Previous research established that oxygen isotope composition from lunar samples is indistinguishable from that of Earth. And since 40 percent of the Moon is supposed to have derived from planet Theia, this could spell trouble for the giant impact hypothesis.

But that finding doesn´t sit well with many scientists who have long believed the giant impact hypothesis.

Planetary scientist Brad Carter from the University of Southern Queensland said while it is unlikely to have two planets with the same isotopic composition, it isn´t impossible either.

“Despite what the paper says, a planet forming very close to the early Earth could have a similar composition resulting in similar isotopic ratios,” Carter told ABC´s Stuart Gary. “It´s also possible that Theia was essentially made of ice, something from the Kuiper belt in the outer solar system.”

“This would have provided the energy of impact, but with the ice evaporating away leaving Earth material to eject into space and form the moon,” he said.

Zhang and colleagues suggested it was also possible that early Earth was spinning much faster than previously thought, so rapidly that it threw some of itself off into space like a shot put, forming the disk that coalesced into the moon. This could explain why the Moon seems to be entirely made up of Earth material.

“However we don't really think Earth had spun rapidly enough for that to happen,” said Carter. “And then you have the problem of slowing down the Earth´s rotation afterwards.”

“Zhang and colleagues suggest a gravitational resonance effect between the Earth, moon and sun may have provided the forces needed to slow down the Earth's spin rate,” Carter told Gary.

Carter said the paper does make a good point about the need for revision of the current theory. However, the idea of a Mars-like planet “slamming into Earth remains sound,” he added. We just need to “refine current theories that don´t explain the incredible similarities of isotopic ratios between the Earth and the moon.”