Apollo rocks hint at Moon’s violent birth after collision on Earth

Ever looked to the moon and felt a connection with it? Turns out the moon is partly made of Earth-stuff, borne out of a cataclysmic collision early in the planet’s history. But scientists haven’t been clear on the details of this dramatic birth, because many theories didn’t seem to fit with the chemical fingerprints left in lunar rock.

Now, in a study published in the journal Science, a team of German researchers say they’ve finally found clear signs in Apollo-era rock to support major theories about the moon’s creation story.

“Earth’s Moon is distinct among the >150 moons of our solar system,” the study authors point out. “Most other planets are either captured planetesimals, or they formed along with the planet in a common accretion disc.”


So most moons are either adopted or they’re their planet’s little siblings, born of essentially the same stuff. But Earth’s moon was sired after a brief and ill-fated encounter with a smaller, Mars-sized proto-planet named Theia, which came barreling through space and crashed into Earth’s surface. Chunks of Earth and Theia went flying, eventually coalescing into the moon.

According to many accepted models, the newly formed moon would be made mostly of Theia’s remains, with 70% to 90% being Theia debris and the rest (30% to 10%) being from Earth. This would explain a lot of things about the moon, the authors point out: why the lunar satellite seems to be so lacking in water and volatiles; why its lunar core is so small, and why the Moon and Earth move around each other in the particular way they do.

When scientists want to find out which rocks are related, they look at their isotope ratios – a sort of chemical fingerprint powered by radioactive decay. In this case, they examined the amount of oxygen-17 (an oxygen atom with one extra neutron) to oxygen-16 (your archetypal oxygen). Based on the theories of the moon’s formation – which say that the lunar satellite should mostly be made out of Theia-stuff – the moon’s fingerprint should look very different from Earth’s.

And yet when previous scientists looked at the isotopic fingerprint of lunar meteorites on Earth, they found it seemed to match very closely with Earth. Was the moon more a child of Earth than of Theia? Were the theories wrong?

The researchers knew that many of these isotopic fingerprints that closely matched Earth’s were taken from lunar meteorites – bits of the moon that have broken off and fallen to Earth. But meteorites have passed through the Earth’s atmosphere and been exposed to the environment – and their oxygen-isotope fingerprints may have been altered in the process.

The researchers went through old research data that matched the moon’s oxygen isotope ratios to Earth’s, and found slight discrepancies between the two that showed they might be different.
Then, instead of using meteorites, they went back to the source, obtaining samples of three lunar basaltic rocks collected by NASA astronauts during the Apollo 11, 12 and 16 missions more than four decades ago.

In those rocks, they found that the oxygen isotope ratios between the moon and Earth were markedly different, by about 12 parts per million. That may not sound like a lot, but it’s more than enough to show that the Earth and moon are very distinct on a chemical level – which means the moon must more closely resemble Theia.

Theorists may breathe a sigh of relief: The scientists believe they can now say they have clear evidence that the impact happened. And in doing so, by discovering Theia’s mark on the moon, they may be learning more about the long-deceased proto-planet, which was destroyed in the moon’s creation.