A new moon formation theory, from ScienceDaily:
The moon was formed from a violent, head-on collision between the early Earth and a ‘planetary embryo’ called Theia approximately 100 million years after the Earth formed, almost 4.5 billion years ago.
Scientists had already known about this high-speed crash, which occurred almost 4.5 billion years ago, but many thought the Earth collided with Theia (pronounced THAY-eh) at an angle of 45 degrees or more — a powerful side-swipe (simulated in this 2012 YouTube video). New evidence reported Jan. 29 in the journal Science substantially strengthens the case for a head-on assault.
The fact that oxygen in rocks on the Earth and our moon share chemical signatures was very telling, Young said. Had Earth and Theia collided in a glancing side blow, the vast majority of the moon would have been made mainly of Theia, and the Earth and moon should have different oxygen isotopes. A head-on collision, however, likely would have resulted in similar chemical composition of both Earth and the moon.
“Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them,” Young said. “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 (except that it now makes up large parts of Earth and the moon) was growing and probably would have become a planet if the crash had not occurred, Young said. Young and some other scientists believe the planet was approximately the same size as the Earth; others believe it was smaller, perhaps more similar in size to Mars. More.
There’s no question Earth’s relationship with the Moon is unusual; it is somewhat like a double planet system and, as Privileged Planet explains, the arrangement favours life.
See also: Origin of the moon still shrouded in mystery
What if the Moon disappeared?
More on Theia
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Here’s the abstract:
Earth and the Moon are shown here to have indistinguishable oxygen isotope ratios, with a difference in ?’17O of -1 ± 5 parts per million (2 standard error). On the basis of these data and our new planet formation simulations that include a realistic model for primordial oxygen isotopic reservoirs, our results favor vigorous mixing during the giant impact and therefore a high-energy, high-angular-momentum impact. The results indicate that the late veneer impactors had an average ?’17O within approximately 1 per mil of the terrestrial value, limiting possible sources for this late addition of mass to the Earth-Moon system. (paywall) – E. D. Young, I. E. Kohl, P. H. Warren, D. C. Rubie, S. A. Jacobson, A. Morbidelli. Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact. Science, 2016; 351 (6272): 493 DOI: 10.1126/science.aad0525