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About 4.5 billion years ago, a planet called Theia collided with the Earth. Due to this impact, parts of the Earth’s mantle melted and a huge cloud of debris was sent into space. Over time, this debris came together to form the Moon. Scientists have long wondered what Theia is made of and where it originated. Now a new study suggests that Theia formed in the inner solar system, much closer to the Sun than Earth.
The mystery of the similarity of the Earth and the Moon
Early models of the Moon’s formation predicted that the Moon should consist mostly of Theia material. But scientists noticed a puzzle: Earth and the Moon are almost identical chemically, more alike than two separate planets.
“To understand why the Earth and the Moon are so similar, we need to look at elements like iron and molybdenum left behind by Theia,” said Thorsten Klein, director of the Max Planck Institute for Solar System Research in Germany.
studying rocks for clues
Klein and his team analyzed 15 rocks from Earth and six lunar samples brought back by the Apollo missions. They measured tiny differences in iron isotopes, variations of iron atoms that reveal the rock’s origin. According to Scientific American, they also studied molybdenum and zirconium isotopes to estimate Theia’s size and composition.
By comparing these measurements with meteorites from the inner and outer Solar System, the team concluded that Theia was a rocky planet with a metallic core, weighing about 5–10% of Earth’s mass, and that it probably formed closer to the Sun than Earth.
Heavy elements tell the story
The study also explains why Earth’s mantle contains more heavy elements than expected. “Earth received some molybdenum and zirconium from Theia during the giant impact,” Klein said.
“This work helps us understand the violent events that shaped our planet and moon. It also provides insight into how the Earth-Moon system became the cradle of life,” said planetary scientist Sarah Russell of the Natural History Museum in London.
The team plans to run simulations of the giant impact using these new findings and hope that future lunar missions will bring back samples for even more detailed analysis. Decades after the Apollo missions, scientists are still discovering new secrets about the Moon and Earth.
questions to ask
1. What is Theia and how did the Moon form from it?
Theia was a planet that collided with Earth about 4.5 billion years ago. The debris from this giant impact eventually came together to form the Moon.
2. Where in the solar system did Theia originate?
New research suggests that Theia formed in the inner Solar System, much closer to the Sun than Earth.
3. Why are the compositions of the Earth and the Moon so similar?
The Moon and Earth share many chemical elements because Theia was a rocky planet with a metallic core that contributed heavy elements to Earth during the collision.
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