Multiple studies address riddles of the Moon's origin

By Jonathan Webb
Science reporter, BBC News

image copyrightHagai Perets
image captionThe planet that collided with the early Earth might have been made of similar stuff

The Moon may have been formed by a collision between Earth and an object that was strikingly similar in composition to our own planet.

This possibility, described in a new computer modelling study, could explain why rocks from Earth and the Moon are much more alike than we would expect from this "giant impact hypothesis".

The research forms one of three papers published in the journal Nature.

Two others report subtle, previously unseen differences in lunar rocks.

Scientists say the new findings paint a consistent - and much clearer - picture of our satellite's history.

The modelling study, done by researchers from Israel and France, precisely simulated the turmoil of the early, inner Solar System and quantifies the variety of collisions that might have occurred.

In its early stages, the proto-Earth would have been subjected to a string of brutal collisions with other wannabe planets.

According to our best understanding, the last of these was a cataclysmic tangle with a planetary body just ten times lighter than Earth - and the resulting debris eventually clumped together to make the Moon.

Insufficiently alien

The problem is that most of what became the Moon should have come from the imposter - and based on our existing knowledge of what was flying where at that time, that imposter was thought to be a very different type of planet.

"So if the impactor had a different composition from the Earth, we should expect the Moon to have a different composition," Dr Hagai Perets, one of the study's authors, told the Nature podcast.

But this is not the case.

"They are almost identical. This is one of the major challenges for this really beautiful giant impact hypothesis," said Dr Perets, from the Technion-Israel Institute of Technology.

This is why a study made headlines in 2014 when it pinpointed some tiny differences between Earth and Moon rocks.

What Dr Perets and his colleagues found in their new simulations was that the impactor planet might, in fact, have been made of surprisingly similar stuff to the Earth - leaving only the sort of subtle differences that we do see in lunar material.

image copyrightNASA/SPL
image captionSisters in space - how similar are the Earth and the Moon?

"What we found is that many of these impactors on a planet have very similar composition to that of the planets they impact - as similar as what we measure between the Earth and the Moon," Dr Perets explained.

Specifically, the models indicated a 20% chance that the impact could have been between such similar proto-planets.

These odds give our prevailing "origins story" for the Moon a fighting chance, Dr Perets said.

"[Now] I am even more confident about the giant impact hypothesis."

Violent veneer

A different challenge to our current ideas about the Moon relates to what happened subsequently. To explain certain details in the Earth's make-up, scientists have proposed that both the Earth and Moon amassed a large amount of extra matter - a so-called "late veneer" - during a subsequent period when they were bombarded by huge numbers of meteorites.

And just like the original giant impact, this process should also have left a trail of evidence that had not been detected until now.

Even if, as the French-Israeli study now suggests, the Earth and Moon got started from very similar building blocks, this bombardment should have had a much bigger effect on the bulkier, heavier Earth with its much stronger gravity, shifting the balance of its ingredients away from that of the Moon.

The two new investigations of lunar rocks, one from the US and one from Germany, find support for that shift for the first time, by analysing samples from the Apollo missions with new levels of precision.

Both teams looked specifically at traces of tungsten within small chunks of the Moon that they borrowed from Nasa, and found a small but tell-tale difference compared to Earth rock. The ratio of the metal's different isotopes was altered.

image copyrightNASA/JSC
image captionTwo teams of researchers examined moon rocks collected by the Apollo missions

Importantly, the ratios measured by the two teams match - and the difference fits what might be expected if the Earth collected a bigger "veneer" from the bombardment.

"The small, but significant, difference in the tungsten isotopic composition between Earth and the Moon perfectly corresponds to the different amounts of material gathered by Earth and the moon post-impact," said Prof Richard Walker from the University of Maryland, one of the authors on the US study.

Considering the three studies in total, Dr Matthias Willbold from the University of Manchester told the BBC he was impressed and excited.

"They all tell the same story - it all falls into place," said Dr Willbold, a planetary scientist who has also investigated the late veneer idea using tungsten ratios.

"It's quite striking. [The lunar rock studies] mention that it is quite baffling that the Earth and Moon have the same starting composition, before the bombardment.

"And that links perfectly into the modelling paper, where they say look - we can resolve that. If you look at the models, the impactor and the Earth were similar, so we just solved your problem!"

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image copyrightNASA/JPL-Caltech
image captionHistory of violence: The "great impact hypothesis" looks more and more solid

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