'Embryo' Planet Plowed Into Earth, Creating Moon —and Stayed

Scientists already know that roughly 4.5 billion years ago planet Earth was violently smacked into by another mass—Theia, a so-called "planetary embryo" that was still forming and may have been just as large as Earth. But now UCLA researchers report that the collision was not a 45-degree angle "powerful side-swipe" as thought, but rather a "head-on assault" that resulted in the formation of the moon as well as the Earth's mantle. Their evidence lies in the oxygen isotopes of rocks on both Earth and the moon. Reporting in the journal Science, they reveal that the oxygen isotope ratios, which differ from one planet to the next and thus act as fingerprints, are "indistinguishable" in rocks on the moon and Earth, suggesting a common origin.

"Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them," the lead researcher tells ScienceAlert. "This explains why we don’t see a different signature of Theia in the moon versus the Earth." A violent head-on collision also likely robbed Earth of its water, which was probably added tens of millions of years later when small asteroids collided with the Earth's surface. "While it's pretty sad to think about our planet wiping out another one in order to get where we are today, it's fascinating to think about all the accidents that had to come together to result in life on Earth," notes Fiona MacDonald at ScienceAlert. (Check out how tiny the difference in oxygen isotope ratios are between the Earth and moon.)