But the Kalahari meteorite is made of basalt, a common rock that forms from cooling magma that is a telltale sign of volcanic activity.
The researchers examined small grains of two phosphate-containing minerals—apatite and merrillite—inside the rock.
Once magma solidifies, it becomes closed like a time capsule, and its radioactive uranium begins to split into other elements. Measuring the ratios of these elements reveals the rock's age.
This showed that the Kalahari meteorite is made of magma that solidified 4.35 billion years ago, soon after the moon formed.
So "volcanism on the moon had started much earlier than we had thought," Anand said.
"This does not definitively pin down the age of the moon," Anand added. "But it does tell you that the moon must have formed and cooled down by 4.35 billion years ago."
Scientists believe volcanism on the moon ended about three billion years ago.
Planet Formation Clues
The new research also sheds rare light on the earliest stages of planet formation. That information has largely been lost on Earth, where plate tectonics continually recycle the crust, sucking continents into the planet's interior and melting the rocks.
Because of this, even tiny bits of rock from early Earth are extremely scarce.
But since the moon is much smaller, it cooled quickly and never developed plate tectonics. Many of its ancient rocks are still on or near the surface.
"The beauty of the moon is that it preserves almost the entire history of the solar system," Anand said.
Mars should also have very ancient rocks on or near its surface, Anand said, but there has never been a return mission from Mars that brought back rocks.
And of about 40 Mars meteorites found, only one is more than four billion years old.
"Snapshot" of Early Earth
Irene Antonenko is a geologist with Geosoft, a geology software company based in Toronto, Canada, who was not involved with the new study.
She said the work is important because "we are looking at the moon for a snapshot of what the early Earth was like."
Other studies had also suggested that the moon was volcanically active in its very early days, Antonenko said. But these were based on more indirect evidence—such as counting the impact craters on the surface in rocks from different eras.
The new study shows "one of the earliest physical samples" of volcanic activity on the moon, she added.
The Apollo rocks had "been pounded ... over billions of years," but the Kalahari meteorite is a large rock, she said.
"That's one of the really neat things about it," she said. "It's something that hasn't been battered to hell."
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