It crystallised about 12.4 miles below the surface between 4 billion and 4.1 billion years ago, when the Earth was young.
An global team of scientists found a tiny fragment - weighing less than an ounce - composed of quartz, feldspar and zircon in one of those moon rocks, according to a news release about the discovery. If one fragment could be found, there should be others, and studies of other lunar samples may locate them.
Publishing its findings to Earth and Planetary Science Letters, an global team of scientists associated with the Center for Lunar Science and Exploration revealed that Earth's oldest rock has been discovered in a lunar sample returned to our planet by Apollo 14 astronauts.
There, Apollo 14 crew member Alan Shepard collected the rock, designated it as 14321 and brought it back to Earth. If it were to be created on the moon, researchers would have to rethink all they know about the moon's interior and surface from the past.
Many researchers believe the moon formed after Earth was hit by a planet the size of Mars billions of years ago.
"What we did was use the composition of minerals in the fragment to show it formed under conditions that only occur on Earth", Robinson told Gizmodo.
"It's quite a violent process and chemistry changes as a result of that".
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The final impact that affected the rock happened about 26m years ago when an asteroid slammed into the moon and made the Cone crater, measuring 340 metres wide and 75 metres deep, near the Apollo 14 landing site.
The researchers said there is a possibility the sample crystallized on the Moon and isn't from Earth.
"However, a simpler explanation is that this piece was formed on the Earth and brought to the surface of the moon as a meteorite generated by an asteroid hitting Earth about four billion years ago, and throwing material into space and to the moon".
The new study was published online Thursday (Jan. 24) in the journal Earth and Planetary Science Letters. After colliding with the Moon (which at the time was three times closer to the Earth than it is now), it mixed with other lunar surface materials.
"For that reason it provides a neat achieve of impacts, as it is unlike the Earth, which is affected by erosion and plate tectonics that disturb impact craters".
Available evidence suggests that the fragment crystallised about 20 km beneath the Earth's surface, and was launched into space by a powerful impact shortly thereafter.
The sample was on loan from NASA to Curtin University, where it was investigated in cooperation with researchers from the Swedish Museum of Natural History, Australian National University and Lunar and Planetary Institute in Houston. But given that the Earth was subjected to impacts during the Hadean eon, when the planet was forming 4.6 billion years ago, bits of Earth on the moon don't seem surprising to Kring and his team.
Speaking of the lunar surface, it seems surprising that the Apollo astronauts were able to find this breccia so easily.