Discovering a Forgotten Planet: Apollo-Era Moon Rocks Reveal Earth’s Celestial Neighbor

The story of how the moon came to be is fascinating, and new research suggests it might be even more intriguing than we thought. Instead of coming from a faraway place, the moon’s origin may be linked to a lost sibling planet that formed close to Earth long ago.

About 4.5 billion years ago, a Mars-sized body, called Theia, collided with the young Earth. This massive impact melted parts of Earth and sent debris into orbit, which eventually formed the moon. For a long time, scientists have accepted this “giant impact” theory, but they were unsure where Theia originated or what it was made of.

Recent analysis of lunar samples from the Apollo missions, alongside studies of terrestrial rocks and meteorites, has led researchers to propose that Theia was also a rocky planet, likely forming in the inner solar system. According to Timo Hopp, a geoscientist leading the study at the Max Planck Institute, both Theia and proto-Earth came from similar regions within the solar system. This finding aligns with traditional theories about how rocky planets formed billions of years ago.

The early solar system was chaotic. For the first 100 million years, it was filled with planetary embryos that frequently collided. Hopp explains, “Theia was just one of many embryonic bodies.” The challenge scientists face is that Earth and the moon share such a similar chemical makeup that pinpointing Theia’s origin has been tough.

Hopp and his team looked for tiny chemical clues in Earth’s mantle, focusing on elements like iron and molybdenum. These elements should have sunk into Earth’s core if they were present during the planet’s early formation. Their presence in mantle rocks suggests they arrived later—likely from Theia during the giant impact—providing insight into Theia’s composition.

To study this further, the researchers examined six lunar samples and 15 terrestrial rocks, including materials from Hawaii’s Kīlauea volcano and meteorites from Antarctica. They focused on tiny differences in iron isotopes, which can inform us about where materials originated in the solar system. By analyzing these isotopes alongside others, they modeled various scenarios to deduce what Theia was like.

The study concluded that Theia was a rocky, metal-rich planet, containing about 5 to 10% of Earth’s mass. They identified evidence of unique materials from a region near the sun that is absent from known meteorite collections, which adds a layer of mystery. Hopp noted that future samples from planets like Venus or Mercury may help unravel more about this missing material.

While this research sheds light on Theia’s origin, questions remain on how the collision mixed the two planets so thoroughly that their chemical identities became almost indistinguishable. Understanding this could complete the epic story of the moon’s formation and reveal deeper insights into the early history of Earth and its celestial neighborhood.



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