The leading theory has been that a Mars-sized protoplanet barreled into Earth 4.4 billion years ago and the two bodies merged - like a kid kneading yellow and blue Play-Doh together to make a green ball. The moon condensed quickly from the spillover ejecta. This same scenario has been applied to the birth of Pluto's moon Charon.
However, a comparative analysis of titanium from the moon, Earth and meteorites, indicates the moon's material came exclusively from Earth. Imagine, Mother Earth as a single parent!
The research, published by Junjun Zhang at the University of Chicago and co-authors, appears in the March 25 edition of Nature Geoscience.
If the moon was born out of a collision, the logic follows that the moon should have inherited half of its material from the Earth and the rest from the impactor. This is natural consequence, at least, in the world of biological reproduction, but should also apply to the basic physics of collisions.
It turns out that forms of titanium tended to remain in a solid or molten state rather than being vaporized in the moon's birth. This means it's unlikely titanium would become incorporated by the Earth and the moon in equal amounts. Titanium isotopes, forged in supernova explosions, also slightly vary in chemical signature and abundance across the solar system. They carry sort of an isotopic postage stamp that tells what parts of the solar system they came from.
The surprise is that the moon's titanium has the identical isotopic composition as Earth's, say the researchers. There is no forensic evidence for a third-party alien interloper that came from another part of the solar system.
The titanium analysis is consistent with previous work by other researchers that found identical oxygen isotopes in lunar and Earth samples.
These results threaten to send planetary scientists back to the drawing board in search of a plausible theory for the origin of the moon, but there are no satisfactory alternative scenarios for the moon's formation.
At first glance this would tend to resurrect an old idea that the moon spun off the Earth in a "mother-daughter" hypothesis. But it would take quite a wallop to spin something the mass of Earth fast enough to have a big lump of material fly off into space. We're not a ball of Play-Doh after all!
Another idea is that Earth collided with a "snowball planet" that didn't have any titanium. This would be circumstantial evidence for a lost population of gigantic flying ice cubes that no longer exist, as best as we can tell.
It's also very improbable that a colliding body happened to have that same composition as Earth. Earth is an agglomeration of all types of materials that were flying around developing solar system.
The moon mystified skywatchers since the dawn of human consciousness. Now, decades after we walked on the face of the moon, it still mystifies us.
God didn't leave us any file footage of the creation of the planets. Our imagination and careful forensic analysis are all that's available to sort out what actually happened here 4.4 billion years ago.
Image credit: NASA, JAXA/NHK