The rocks were gathered by astronauts on NASA’s Apollo missions. But newer scanning electron microscopes have now allowed scientists to detect in them the first chemical traces of the Mars-size planet thought to have blasted the proto-Earth around 4.5 billion years ago. (Related: “First Explorers on the Moon.”)
When the ancient planet, Theia, smashed into Earth, it blasted debris into space. The moon formed out of that debris. Planetary scientists first came up with this theory in the wake of the July 20, 1969, Apollo moon landing, offering an explanation for why our world has such a massive moon. (See: “It All Began in Chaos.”)
“If the moon formed predominantly from the fragments of Theia, as predicted by most numerical models, the Earth and Moon should differ,” says the study.
Earlier looks at moon rocks hadn’t been detailed enough to reveal any difference in the lunar chemistry between them and rocks from the Earth. But this team found a small but significant differenceâ€”about 12 parts per million more of a heavier kind of oxygen atom in the moon rocksâ€”that serves as a fingerprint of Theia.