The Moon is the byproduct of Earth’s planetary mashup
Growing evidence suggests that the Moon was born from an object colliding with the Earth, sending a hunk of mass into our young planet’s orbit some 4.5 billion years ago. The third party in this crash has been named Theia, which was was previously thought to have become the core of what the Moon is now, having grazed the Earth at a 45 degree angle, stealing some material and settling into place as our only natural satellite. As dramatic as all that sounds, it pales in comparison to the picture painted by new geological data, which suggests that Theia and a young Earth collided head on, with Earth basically enveloping the smaller body, and sort of burping out the Moon in the process.
Tracking origins with oxygen
While Theia’s assimilation and immediate redistribution was likely the most epic event in the history of our planet, this hypothesis was tested on a very small scale. Six volcanic rocks from the Earth’s mantle were compared to rocks gathered on the Moon during the Apollo 12, 15 and 17 missions in the 1970s. They were all analyzed for their oxygen content, specifically looking for ratios of heavier oxygen atoms. The variations would be due to the number of neutrons packed on the atom— most oxygen has eight, but a few have nine or ten. The prevalence of each variation, known as an isotope, has been found to be tied to the formation of planets. Mars’ ratio of oxygen isotopes is different than what you find in rocks on Earth, letting us trace the point of origin for a given mineral.
Theia, as a body separate from Earth, should have arrived with its own oxygen isotope cocktail. If it mostly careened off the Earth, becoming the basis for our Moon, rocks from the Moon should bear Theia’s isotope signature. However, analysis from the University of California, Los Angeles found that the Moon rocks were basically a match for those found on Earth. Rather than cast doubt on the concept of Theia hitting the Earth, the UCLA team suggests that it just hit directly enough to churn and mix with the materials already present, and that that a piece of that newly composed isotope mixture was then partially expelled into space as the Moon. It means that the collision was severe enough to basically remix the Earth’s mantle before the whole mess settled down again.
This degree of upheaval would have been within the range of possibility, as Theia was thought to have been a sizeable object on its own, somewhere between the size of Mars and the Earth. It was likely on track to become a planet in it’s own right before the collision occurred, as the solar system used to have many more “loose” objects crossing each other’s paths while settling into stable orbits.
My first grader asked: So was the Earth bigger or smaller when this happened?
My best guess is that since Theia is estimated to have been the size of Mars, and the Moon obviously isn’t, that the missing mass either spun off into space, or was absorbed back into our planet. So the Earth maybe saw a net gain in mass once things were settled.
Source: Moon was produced by a head-on collision between Earth and a forming planet by Stuart Wolpert, UCLA Newsroom