The moon has been Earth’s constant companion for approximately 4.5 billion years, escorting the planet during its myriad journeys around the sun. While its origins are a little hazy — it cannot be definitively said what created the moon — the leading theory is that it was born during a collision between proto-Earth and a Mars-sized planet named Theia.
How Theia Formed
Theia is a mysterious object. Astronomers do not know with 100 percent certainty that it ever existed, but compelling evidence buried in Earth’s mantle suggests it did. Using this evidence, researchers writing in Science have determined Theia’s “ingredients” and where it likely came from. The answer is closer to home than previously thought.
“Theia is a hypothetical planetary embryo that collided with the Earth at the end of its formation history,” says lead author Timo Hopp, a scientist at the Max Planck Institute for Solar System Research. “Some studies inferred it to be a water-rich body from the outer Solar System, others to be a rather water-poor body from the inner Solar System.”
“We provide new evidence that Theia likely formed in the inner Solar System. Moreover, we can show that Theia must have incorporated material that formed closer to the sun than the material that Earth was predominantly made of,” Hopp adds.
Read More: There Might Be Remnants of an Ancient Planet Buried Inside Earth? Yup
What Iron Can Tell Us About Earth and the Moon
Studying the concentrations of metals in Earth’s mantle offers insights into events that occurred billions of years ago, early in the planet’s history.
During its formation, certain elements (like iron and molybdenum) amassed in Earth’s metallic core, leaving the planet’s rocky mantle relatively devoid of these metals. This means that iron in the mantle today must have arrived at a later point — for example, on an extraterrestrial body like Theia.
Analyzing iron found in Earth’s mantle and elsewhere can provide further clues to where the metal originally came from. This is partly because of processes such as radioactive decay and partly because of the uneven distribution of materials in the Solar System’s protoplanetary disk, which can affect an element's isotopic composition.
This means that two planetary bodies from different parts of the Solar System may contain iron, but the iron on one body has a higher number of neutrons per atom than that on the other, and thus has a higher atomic weight.
Earth And Theia: Celestial Neighbors
For the study, researchers compared iron isotopes in 15 Earth samples and six moon samples. Twenty meteorite samples were also included for reference.
They found the isotopic composition of iron in the Earth samples was “indistinguishable,” while any differences in the moon samples could be attributed to galactic cosmic rays. Once these were accounted for, the isotopic composition of iron in the Earth and moon samples was also “indistinguishable” — a finding that supports previous research showing similarities in the Earth’s and moon’s isotopic makeup.
The similarity between Earth and the moon tells us little about Theia, but the team was able to use this information to determine the factors that could lead to this particular outcome — specifically, the size and composition of Theia and the composition of pre-collision Earth.
Based on their calculations, it is likely that Theia (and, indeed, most of the material that makes up the Earth) arose in the inner Solar System. Indeed, according to the calculations, Theia may have formed closer to the sun than planet Earth did.
“Earth and Theia are likely to have been neighbors,” Hopp said in a statement.
Read More: From a Pink Moon to Blue, the Moon's Colors Aren't Always Colorful
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a new study that was published in Science: The Moon-forming impactor Theia originated from the inner Solar System
