Much about the Moon remains shrouded in mystery, including its age. Analyses of samples brought back from the lunar surface indicate our celestial sidekick could be about 4.35 billion years old, which means it came to be at about 200 million years after the formation of our solar system.
But that immense lag doesn't sit well with some scientists. During the solar system’s early days, debris and planetary bodies collided and coalesced to form planets. By 200 million years, most of this chaotic debris had been swept up into larger bodies. Thus, many scientists who simulate the solar system’s evolution see the idea of a massive collision forming the Moon this late as improbable.
In an "idea paper" published on December 18 in Nature, UC Santa Cruz Professor Francis Nimmo and his co-authors propose a possible explanation for this discrepancy: that the Moon underwent a "remelting" 4.35 billion years ago due to the tidal pull of Earth causing widespread geological upheaval and intense heating. They say this remelting would have "reset" the age of the lunar rocks —masking the Moon’s true age with what could be likened to a volcanic facelift.
“We predict that there shouldn't be any lunar rocks that are older than 4.35 billion years because they should have experienced the same resetting,” said Nimmo, a professor of Earth and planetary sciences. “Because this heating event was global, you shouldn't find rocks anywhere on the Moon that are significantly older than that.”
Moon as microcosm for the cosmos
The Moon has fascinated humanity for millenia, and in the last few centuries, people started to wonder how—and when—the Moon formed. One of the reasons for sending astronauts to the Moon was to answer this question. The Moon also serves as a luminous stepping stone to comprehending more distant objects. But, if we can't nail down the age of the Moon, how can we be confident about the precise age of anything beyond it?
The Moon is thought to have been born from a collision between the early Earth and a Mars-sized protoplanet—the last giant impact in our planet’s history. The timing of this event has been estimated by dating lunar-rock samples presumed to have crystallized from the lunar magma ocean that existed post-impact, placing the Moon’s age at about 4.35 billion years old.
However, this age fails to account for several discrepancies with thermal models and other pieces of evidence, such as the ages of some zircon minerals on the lunar surface, which suggest that the Moon could be up to 4.51 billion years old.
Nimmo and his colleagues hypothesize that a remelting event driven by the Moon’s orbital evolution could account for the frequent occurrence of approximately 4.35-billion-year-old rocks—such as those collected by the U.S. Apollo mission and others—rather than the first solidification of the lunar magma ocean.
For their paper, the authors used modelling to show that the Moon may have experienced sufficient tidal heating to cause this remelting approximately 4.35 billion years ago, which could "reset" the apparent formation age of these lunar samples.
A misleading magma mask
Tidal heating is a process where the gravitational forces between two celestial bodies cause internal friction that leads to intense heating. For the Moon, this effect was likely more pronounced in its early history when it was closer to Earth. According to the latest models, during certain periods in its early years, the Moon's orbit would have been unstable, causing it to experience intense tidal forces from Earth that could have led to significant heating events, dramatically altering the Moon’s geology.
The research team draws parallels between this hypothetical heating event on the Moon and the current volcanic activity observed on the Jupiter moon Io, which is known as the most volcanically active body in the solar system. Volcanic activity on Io is driven by tidal forces similar to what may have marked the Moon’s early history, with widespread volcanic activity and the surface being constantly reshaped by eruptions.
The researchers also say the remelting of the Moon would explain why there are fewer lunar impact basins from early bombardments than might be expected, as they would have been erased during a heating event. The authors posit that this explanation would suggest the formation of the Moon occurred between 4.43 and 4.53 billion years ago, at the upper limit of previous age estimates.
Nimmo said the next stage of research will involve more complex simulations that refine our understanding of how tidal heating might have reset the Moon’s geological clock. This, along with additional lunar samples from future missions, should shed more light on the Moon's true age.
That’s why excitement surrounds the recent return of lunar samples by China's Chang'e 6 mission. These samples, collected from the far side of the Moon, will provide invaluable data for understanding the processes that shaped its history. Researchers are particularly eager to see whether these new samples support the idea of a global resetting event caused by tidal heating.
Nimmo’s team also envisions more detailed modeling to further explore the effects of tidal heating on the Moon’s geology. Although the initial models are promising, more complex and realistic simulations will be necessary to fully understand the scope of these heating events.
A New Era of Lunar Research
This paper not only offers a fresh perspective on the Moon's past, but also opens the door for more nuanced investigations into its formation and evolution. The interplay between geochemistry and simulation modeling is helping scientists to fill in the gaps of lunar history, with tidal heating emerging as a crucial mechanism for understanding the Moon’s geological features.
“As more data becomes available—particularly from ongoing and future lunar missions—the understanding of the Moon’s past will continue to evolve,” Nimmo said. “We hope that our findings will spark further discussion and exploration, ultimately leading to a clearer picture of the Moon’s place in the broader history of our solar system.”
