The moon may be more than 100 million years older than some scientists previously thought, according to a new study.
The research, published Wednesday in the journal Nature, challenges the longstanding idea that the moon formed roughly 4.35 billion years ago, after a Mars-sized object smashed into the early Earth and created our natural satellite.
That timeline is based on analyses of lunar rock samples from NASA's Apollo missions. But the new study suggests that the moon formed earlier -- around 4.51 billion years ago -- and then experienced a dramatic "re-melting" event at the time other scientists had assumed it first formed.
The melting occurred as the moon was moving away from the Earth, the authors say, when the planet's constant gravitational tugs warped the moon in a way that caused it to super-heat. The process altered the lunar surface and thus hid the moon's real age, according to the study.
Francis Nimmo, the study's lead author and a professor in the Earth & Planetary Sciences Department at the University of California, Santa Cruz, said the extreme heating likely re-melted the moon's surface, effectively "resetting all the clocks" in lunar rocks.
"So the moon rocks are not telling us when the moon formed, but they are telling us when a later event happened that heated the moon," he said.
Within the scientific community, there have been disagreements about the moon's precise age for decades; Nimmo and his colleagues are not the first to put forward an older estimate. The new findings add to a growing consensus that there may be more to the moon's history than what the Apollo samples revealed.
Planetary scientists have, for instance, struggled to explain how a major collision created the moon 4.35 billion years ago, at a time in the solar system's history when most large celestial objects were thought to have already clumped together to form planets.
"The people who studied the Apollo samples had their reasonable guess for the age of the moon, but people who model how planets in the solar system formed always found it hard to explain how there was still so much big stuff flying around 200 million years after the solar system formed," Nimmo said. "That has been the state of affairs, with two camps wanting different ages."
The adjusted timeline from Nimmo's team may also help explain why minerals on the moon called zircon -- which were found in Apollo moon rocks -- were estimated to be roughly 4.5 billion years old. Lunar zircon, like other minerals on the moon, was thought to have crystallized from extreme temperatures when the moon was created, but its much older age has long confounded scientists.
In the new study, Nimmo and his colleagues suggest that the super-heating of the moon was the product of a process known as "tidal heating."
"As the moon gets pushed away, there are particular points where the orbit can temporarily go haywire," Nimmo said. "During that time, the moon can get squeezed and stretched by Earth's gravity, and that causes it to heat up."
Similar tidal heating is thought to occur between Jupiter and its moons. A 2020 study found that the gas giant's gravity may stretch and squeeze some of its icy moons enough to heat their interiors or even melt rock into magma. That's thought to be the case with the Jovian moon Io.
Recent and upcoming lunar missions could provide better insights into the moon's evolutionary history, according to Nimmo. That includes China's Chang'e 6 mission, which collected samples from the far side of the moon, and NASA's planned Artemis moon missions.
Carsten Münker, a scientist at the Institute of Geology and Mineralogy at the University of Cologne who was not involved with the study, said tidal heating is a plausible way to satisfy disagreements over the timing of the moon's formation.
Münker's own research has focused on dating lunar minerals as a way to narrow down the moon's precise age.
The new study, he said, "was written by people who were long in the camp that the moon was younger, but now all three [authors] agree with an older lunar age."
"This certainly moves our understanding closer," Münker said.
Although the difference between 4.35 billion years and 4.51 billion years may seem relatively small when it comes to timescales in the universe, nailing down what occurred in those chaotic early days of the solar system is key to understanding how the planets in our celestial neighborhood came to be.
"The evolution of the solar system was quite rapid. Within just a few tens of millions of years, the whole array of celestial bodies as we know them today was formed," Münker said. "That's why we need a really good time resolution of these very early events, and that's why it's important to understand how the Earth-moon system formed."