Earth and Moon created from head on collision with Theia

The Earth and Moon were created after a massive head on collision with the ‘planetary embryo’ known as Theia, approximately 100 million years after our planet was born, say researchers from the USA, Germany and France. Both the Moon and Earth have identical rock compositions – they are both made of the same stuff – this is only possible if a colossal head on collision occurred.

Scientists have always known that there was a mega-huge collision nearly 4.5 billion years ago, which formed the Moon. However, most experts thought that Theia crashed into the young Earth at an angle of at least 45 degrees – they thought it had been an ultra-powerful side swipe.

In this latest study, which the researchers wrote about in the journal Science, new evidence points to a much greater likelihood that Theia crashed into a young Earth head on, like two trains running on the same track that crash into each other head-to-head.

Earth and Moon were created after a head on collision with TheiaThe scientists say that Theia crashed head-on into a young Earth. It is the only scenario that explains why the Earth and Moon have identical oxygen fingerprints. (Image adapted from sott.net)

Lead author, Professor Edward Young, who teaches geochemistry and cosmochemistry at UCLA’s Earth Planetary and Space Sciences Faculty, and colleagues analysed seven Moon rocks that three Apollo missions (12, 15 and 17) had brought back, as well as six volcanic rocks from the Earth’s mantle, five from Hawaii and one from Arizona.

The chemical signatures of Moon and Earth rocks

The chemical signatures found in the oxygen atoms of both the Earth and Moon rocks were key to reconstructing the massive impact. Ninety percent of the volume of the rocks and half their weight consisted of oxygen.



More than 99.9% of the oxygen on Earth is O-16 (16O) – each atom’s nucleus has eight neutrons and eight protons.

However, there are also minute quantities of O-17 and O-18, which have additional neutrons – one and two extra neutrons respectively.

All the planets that orbit the Sun have their own O-16 and O-17 ratios – each one has its own oxygen ‘fingerprint’.

A team of German scientists in 2014 wrote in the journal Science that the Moon, which also has its unique oxygen isotope fingerprint, is different from that of Earth’s. Evidence gathered in this latest study says this is not the case.

Moon & Earth have identical oxygen isotope fingerprints

Prof. Young said:

“We don’t see any difference between the Earth’s and the Moon’s oxygen isotopes; they’re indistinguishable.”

Head on collision created the Earth and Moon todayTheia crashed head-on into early Earth. The two planets broke into tiny bits, which mixed so that both their parts were equally dispersed. Then the Earth and Moon were formed – that is why they are made of the same stuff.

Prof. Young and team used state-of-the-art technology and techniques to make incredibly precise and careful measurements. They then confirmed their measurements with UCLA’s new mass spectrometer – a device for separating molecules, isotopes, and molecular fragments according to their mass.

For the Moon and Earth rocks to have the same chemical signatures – the two celestial beings are made of the same stuff – is very telling, Prof. Young explained.

Theia and Earth could not have collided in a glancing side blow he said. If that had occurred, most of the Moon would have consisted of Theia material, and the Earth and Moon would have different oxygen isotopes.

Space scientists holding a sample of Moon rockEdward Young holding a sample of Moon rock, together with colleagues Paul Warren (left) and Issaku Kohl (right). (Image: ucla.edu)

Only head-on collision explains identical isotope fingerprints

For Earth and Moon rocks to be of the same material, there must have been a collision that smashed both celestial objects – the young Earth and Theia – into tiny pieces, which came back together again with all their components equally dispersed. This could only have happened with a head-on collision.

Professor Young said:

“Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them. This explains why we don’t see a different signature of Theia in the moon versus the Earth.”

Theia, which did not survive the collision – in the sense that it was no longer a separate celestial object after the impact – had been growing and would probably have become a proper planet had it not crashed into a young Earth, Prof. Young said.

Oxygen atomsThree types of oxygen atoms. Their ratios are identical in both Moon and Earth rocks.

Several scientists, including Prof. Young, believe that Theia was about Earth’s size, while others think it was smaller, perhaps the size of Mars, which has a radius of 3,390 km (2,106 miles) compared to Earth’s 6,371 km (3,958 miles).

Did the impact get rid of Earth’s water?

There are still many unanswered questions about our early Earth – before the impact. Did our planet have water then? Did the impact rid the Earth of any water it might have had?

After the collision – over a period of many tens and hundreds of millions of years – small asteroids laden with water probably crashed into our planet, Prof. Young explained. During that period, collisions were much more common than they are today. However, Mars appears to have avoided super-massive impacts.

Matija Cuk, who currently works as a research scientist at the SETI Institute, which looks for the existence of life in the Universe, and Prof. Sarah T. Stewart, from UCDavis’ Earth and Planetary Sciences faculty, initially proposed the head-on collision hypothesis in 2012. Also in 2012, Robin C. Canup, Associate Vice President R&D at the Space Science & Engineering Division, Southwest Research Institute, suggested a head-on collision separately.

Reference: Alessandro Morbidelli, Edward D. Young, Seth A. Jacobson, Issaku E. Kohl, David C. Rubie and Paul H. Warren. Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact.” Published in the journal Science. 29 Jan 2016: Vol. 351, Issue 6272, pp. 493-496. DOI: 10.1126/science.aad0525.

Video – Where did the Moon come from?

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