The Moon is Earth's most reliable partner in space, having orbited our world for around 4.5 billion yearsâ€"nearly as long as our planet has existed. But new analysis of crystals from the Moon's surface indicates the satellite may be even older than previously thought.
The Moon is thought to have formed when the early Earth collided with a Mars-sized protoplanet, an event dated to about 4.35 billion years ago based on rocks on the lunar surface. Pinning down the timeline of the Moon's evolution doesn't just clue us into the history of that rocky sphereâ€"it helps planetary scientists understand the evolution of our world and the greater solar system.
Now, a team of researchers posits that while estimates for the Moon's age range between 4.35 billion and 4.51 billion years old, the younger date is indicative of a remelting event distinct from the "original crystallization of the lunar magma ocean," as they wrote in a paper published today in Nature.
The sheer number of 4.35 billion year old rocks on the surface suggested to the team that they were due to a widespread remelting event, and the true age of the Moon was somewhat older. The researchers drew an older age from zircon crystals from the lunar surface, recovered by the Apollo missions. Though the rest of the lunar surface went through the remelt, some of the crystals near the surface did not, thus keeping a truer record of the Moon's age.
The team notes that the Moon is almost certainly not older than 4.53 billion years old, "the earliest time at which core formation can have ceased." The earliest time the Moon could have formed, the researchers said, is about 180 million years before the later tidal heating event on the satellite. In other words, if the lunar surface we know and love is largely the result of a remelting event, and the Moon is older than generally thought, it's not superlatively older than thought.
In the paper, the team stated that "current models do not support the idea of impacts being responsible for the resetting event," though the jury remains out on what may have caused such a widespread melting of the lunar surface. The researchers said the remelting could have been "driven by the Moon's orbital evolution"â€"in other words, the stress of gravity's pull on the Moon by bodies like Earth and the Sun.
Earlier this year, research published in Nature Geoscience concluded that the Moon likely turned itself inside out a few million years into its formation. The new paper further complicates the origin story of our longtime partner in solar orbit.
We still have a long way to go with space exploration, though the Artemis programâ€"which will put humankind back on the Moon for the first time in decadesâ€"will be an important step in understanding the origins of our rocky satellite.