When in 1971 Apollo 14 astronauts brought moon rocks back to Earth, they may have also brought back a little piece of home.
One of the rocks appears to contain a small chunk of Earth dating back some 4 billion years. Incredibly, it’s now amongst the oldest terrestrial rocks known to exist.

Rocks get tossed into space from the moon and end up on Earth as meteorites all the time, so the same should be true in the other direction. “Earth’s been hit by a lot of very big things, and it’s conceivable that some of those impacts have ejected material far enough away that it’s been able to escape the clutches of Earth and make it to the moon,” says William Bottke at the Southwest Research Institute in Colorado.

The researchers evaluated the chemical makeup and physical properties of zircon crystals in this piece of granite to figure out how they formed. They found that the crystals formed in an environment much richer in oxygen than the moon, and at unusually low temperatures and high pressures for lunar rocks.

“If it formed on the moon, it must have formed 167 kilometres deep,” says Bellucci. Even a massive impact on the moon would not be able to dig up rocks from that far down, he says.

An image of the Earth rock found by the Apollo 14 crew.

Technically, it’s possible that this rock could have formed on the moon, but it would have been one heck of a coincidence.

But how the heck did it get on the Moon, then?

According to the scientists, it was launched off Earth about 4 billion years ago when an asteroid or comet slammed into our young, roughly 540-million-year-old planet, sending rock fragments flying off into space.

Because the Moon was much closer to Earth at that point – about three times closer than it is now – it was in a better position for pieces of this debris to end up there.