From cosmic collision to moon-maker: Scientists reveal how inner solar system planet Theia impacted Earth

around 4.5 one million million years agone, a major planet called Theia slammed into the Earth. The impact melted parts of Earth’s mantle and sent a massive cloud of debris into space. Over time, this debris came together to form the Moon. Scientists have long wondered what Theia was composed of and where it originated. A new study now suggests that Theia formed in the inner Solar System, closer to the Sun than the Earth.

Early models of the Moon’s formation predicted that the Moon should mostly consist of Theia’s material. But scientists noticed a puzzle: Earth and the Moon are almost chemically identical, far more alike than two separate planets should be.

“To understand why Earth and the Moon are so similar, we needed to look at elements left behind by Theia, like iron and molybdenum,” said Thorsten Kleine, director of the Max Planck Institute for Solar System Research in Germany.

Kleine and his team analyzed 15 rocks from Earth and six lunar samples brought back by Apollo missions. They measured tiny differences in iron isotopes, variations of iron atoms that reveal a rock’s origin. They also studied molybdenum and zirconium isotopes to estimate Theia’s size and composition, as per Scientific American.

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 likely formed closer to the Sun than Earth.

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The study also explains why the Earth’s mantle contains more heavy elements than expected. “Earth gained some of its molybdenum and zirconium from Theia during the giant impact,” Kleine said.

Planetary scientist Sara Russell from the Natural History Museum in London added, “This work helps us understand the violent events that shaped our planet and Moon. It also gives insight into how the Earth-Moon system became a cradle for life.”

The team plans to run simulations of the giant impact using these new findings and hopes future lunar missions will bring back samples for even more detailed analysis. Even decades after the Apollo missions, scientists continue to discover new secrets about the Moon and Earth.

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Theia was a planet that collided with the Earth about 4.5 billion years ago. The debris from this giant impact eventually came together to form the Moon.

New research shows that Theia formed in the inner solar system, closer to the Sun than Earth.

The Moon and Earth share many chemical elements because Theia was a rocky planet with a metallic core that contributed heavy elements to the Earth during the collision.

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