Astronauts discovered what could be Earth's oldest rock — on the moon

Apollo 14 astronauts may have made a far bigger discovery than they expected nearly 50 years ago.

Some of the first moon walkers brought back extraterrestrial rocks that are still being studied today. And inside one of them, scientists have found what may be the oldest, best preserved chunk of Earth ever discovered, a study published in the journal Earth and Planetary Science Letters reveals.

A team of scientists recently performed a chemical analysis on one granite-like rock from the 1971 mission, finding it was made of quartz, feldspar, and zircon, The Guardian says. Those minerals are rare on the moon, but pretty common back on Earth. The analysis also shows the rock fragment formed "more than 12 miles underground in an oxidizing environment," The Guardian writes — another thing the Earth has but the moon doesn't. That evidence suggests the chunk was formed on Earth but hurtled to the moon when an asteroid or comet hit the ground. It then was likely melted and buried in a lunar impact, then slammed again with an asteroid.

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After dating the zircon in the rock, the team found the 2-gram fragment is probably about 4 billion years old. That would put it at about the same age as the oldest minerals found on Earth, though those are just disintegrated debris, per National Geographic.

It's not entirely certain the rock blasted from Earth to the moon, Space.com notes. But seeing as 4 billion years ago, the moon was about three times closer to Earth than it is now, it's the best explanation scientists have.