Around 4.5 billion years ago, a dramatic event changed Earth forever.
A massive, planet-sized body called Theia slammed into the young Earth in a violent collision.
This impact reshaped our planet, altered its orbit and structure, and led to the formation of the Moon.
Yet even today, scientists are still trying to answer basic questions about Theia: What was it made of? How big was it? And where did it come from?
Since Theia was almost completely destroyed in the crash, it cannot be studied directly. However, scientists believe its chemical fingerprints were left behind in the Earth and the Moon.
By carefully examining these clues, researchers can build a clearer picture of Theia’s identity.
In a new study published in the journal Science, a team led by scientists from the Max Planck Institute for Solar System Research and the University of Chicago used this approach to trace Theia’s possible origins.
The researchers focused on isotopes, which are slightly different versions of the same element. Isotopes share the same chemical properties but differ in weight because they contain different numbers of neutrons.
In the early solar system, isotopes of elements like iron, chromium, and zirconium were not evenly spread out. Different regions of the solar system had their own unique “isotope signatures.”
This means the mixture of isotopes in a planet or a rock can act like a cosmic fingerprint, revealing where its original building materials came from.
To look for traces of Theia, the scientists measured iron isotopes in 15 rocks from Earth and six samples of Moon rocks brought back by Apollo astronauts. They performed these measurements with extraordinary precision.
Their results showed that Earth and the Moon are nearly identical in their isotope ratios, confirming earlier results from studies of other elements. This strong similarity helped confirm that the Moon and Earth are closely linked by the same violent event.
But matching isotope patterns alone did not immediately reveal Theia’s exact makeup. So the researchers used a kind of “reverse engineering” approach.
Starting from the present-day composition of Earth and the Moon, they calculated what combinations of materials from early Earth and Theia could have produced what we see today.
They expanded their analysis to include not only iron, but also chromium, molybdenum, and zirconium, each of which provides clues about different stages of planet formation.
They considered how, long before the collision, Earth had already begun to form an iron core. Heavy elements like iron and molybdenum sank into the center, leaving the outer layers poorer in those elements. This meant that much of the iron now found in Earth’s mantle could have arrived later, possibly from Theia itself.
The most convincing result from their calculations is that Earth and Theia were likely formed from similar material in the inner solar system. In fact, they were probably neighbors, orbiting the Sun at nearby distances before their catastrophic encounter. Some of Theia’s building blocks may have come from a region even closer to the Sun than Earth, suggesting that Theia originally formed slightly inward from Earth’s orbit.
This discovery adds an important piece to the story of our planet’s early history. Instead of coming from a distant region of the solar system, the object that created the Moon may have been a close companion of Earth for millions of years.
By analyzing the tiniest variations in atoms, scientists are slowly uncovering the dramatic, violent, and fascinating story of how Earth and its Moon were born.
