Figuring out where a planet came from is a lot like tracing its DNA. Planets have elemental signatures, compositions scientists use to differentiate one from another and determine where a body came from. In the case of the Moon, new analysis by a team from the University of Chicago on soil samples returned by Apollo astronauts revealed that Earth and the Moon have identical traces of the element titanium. This suggests that the Moon’s “DNA” isn’t from two parents but Earth alone. It isn’t exactly the stuff of the Maury Povich show, but it does pose the interesting question of how Earth alone begat the Moon.
For years, the Moon was thought to be the progeny of the Earth and a hypothetical planet sometimes called Theia. Four and a half billion years ago, the two planets smashed into one another. Earth survived and Theia was blown to pieces. The material from the impact coalesced as it orbited around the Earth and formed the Moon.
If this were the story behind the Moon’s genesis, our satellite would have some “DNA” from Earth and some from Theia. But, at least as far as titanium is concerned, the Earth and Moon look to share this elemental signature completely, suggesting there was no other player in its creation.
Titanium, the element central to this study, was forged in stellar explosions long before the sun’s birth. Different isotopes were shot throughout the universe and were ingested by newly-forming bodies. Each isotope acts as a marker, essentially leaving a trail of clues for scientists to determine where bodies came from. As an element, titanium is refractory — it’s incredibly stable and tends to remain in a solid or molten state even at high temperatures. Since it rarely vaporizes, it’s unlikely to be found in equal amounts in the two bodies.
When the isotopic composition of the Earth, the Moon, and meteorites were compared, Earth and the Moon were identical. That’s even taking into account the effects of cosmic and solar rays, which hit the Moon full force since it has no atmosphere.
Interestingly, this result with titanium supports a previous result that compared oxygen isotopes and found the signatures to be the same between Earth and the Moon. But what does it actually mean? Well, it’s unlikely this titanium measurement will end the discussion about where the Moon came from. Even though it’s incredibly resilient, the possibility remains that the impact between Earth and Theia produced such high temperatures that titanium did become a gas and settle uniformly throughout the two bodies.
Models that explain Earth as the Moon’s sole creator make the common titanium signature demand situations that could change the meaning of the identical signature. One long abandoned theory says that the Moon arose from fission from a molten, rapidly rotating Earth following a devastating impact. This explains the similarity, but explaining how a body like the Earth could spin fast enough for it to cleve in two is a lot harder.
Another scenario suggests that the Earth collided with an entirely titanium-poor icy body, explaining the lack of planetary DNA from the Moon’s other parent. But this is incredibly unlikely as even ice worlds have some solid mass that would contain some small trace of titanium. The suggestion that Theia had the same composition as Earth is equally unlikely since our planet has been amassing elements through meteorite impacts for millions of years.
The other and more complicated problem is that the samples we have of the moon are limited, and come from a few select points. It’s like traveling to a new country, looking around from the airport, and deciding you know exactly how that culture relates to your own. It seems, to this non-scientist anyways, a little too early to call the Moon’s mystery solved.