By Leah Crane
When the Apollo 14 astronauts brought moon rocks back to Earth, they may have also brought back a little piece of home. A bit of granite found on the moon could be the first evidence that rocks can be chipped off Earth and land elsewhere. It’s also one of the oldest Earth rocks we’ve ever found, here or elsewhere.
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.
Jeremy Bellucci at the Swedish Museum of Natural History and his colleagues have found one such object, a tiny piece of granite in a moon rock brought back by NASA’s Apollo 14 astronauts.
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.
While it is possible that the rock formed under unusual conditions on the moon, Bellucci says that the simplest explanation is that the rock actually came from Earth because it is similar to rocks that form in magma here.
Bottke says the jury is still out. “What they’ve pointed out is an interesting inconsistency and they’ve pointed out a possible hypothesis, and now we get to figure out whether it holds water or not,” he says. That could involve looking in other lunar samples for rocks that have chemicals not found on the moon, which would mean they are unambiguously from Earth.
Because this meteorite from Earth is so old – it was likely formed about four billion years ago, making it one of the oldest rocks from our planet that we’ve ever found – it could teach us about what the surface of Earth was like shortly after it formed about 4.5 billion years ago. The moon has changed much less since it formed than Earth has, so it’s a perfect spot to store a time capsule from the early years of our home planet.
Journal reference: Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2019.01.010
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