Scientists suggest that 4.35 billion years ago, the Moon’s surface may have “remelted” due to Earth’s gravitational effects, resulting in significant geological changes and extreme heating.
The Moon’s origins remain largely unknown, including its age. Studies of lunar samples returned to Earth suggest that the Moon is approximately 4.35 billion years old, which implies it formed about 200 million years following the creation of our solar system.
However, this significant delay raises questions for some scientists. In the early solar system, collisions among debris and celestial bodies led to the creation of planets. By 200 million years in, most of this chaotic material had coalesced into larger celestial bodies. Therefore, many researchers who model the solar system’s evolution find it implausible that a massive impact formed the Moon after this period.
In a paper published on December 18 in Nature, Francis Nimmo, a professor at UC Santa Cruz, along with his colleagues, presents an explanation for this inconsistency. They propose that the Moon experienced a “remelting” about 4.35 billion years ago, induced by Earth’s tidal forces, which caused extensive geological changes and intense heating. This remelting may have “reset” the age of the Moon’s rocks, akin to a volcanic makeover.
“We anticipate that there shouldn’t be any lunar rocks older than 4.35 billion years, as they should have undergone a similar resetting,” explained Nimmo, who specializes in Earth and planetary sciences. “Given the global nature of this heating event, it would be improbable to find rocks on the Moon that are significantly older.”
The Moon as a Gateway to Understanding the Universe
For many centuries, the Moon has intrigued humanity, and interest in its formation has surged. One reason astronauts were sent to the Moon was to explore this question. The Moon also acts as a bright platform for studying more distant celestial objects. However, if we struggle to determine the Moon’s age, how can we accurately assess the age of more remote bodies?
It is believed that the Moon was created from a colossal impact between the early Earth and a Mars-sized protoplanet, marking a critical point in Earth’s history. The estimated timing of this event comes from dating lunar rock samples thought to have solidified from the lunar magma ocean that formed after the impact, which places the Moon’s approximate age at 4.35 billion years.
However, this estimated age doesn’t align with several anomalies found in thermal models and other evidence, including the discovery of some zircon minerals on the Moon, which suggest it could be as old as 4.51 billion years.
Nimmo and his team conjecture that a remelting event caused by the Moon’s orbital changes could explain why rocks around 4.35 billion years old are commonly found, such as those obtained during the U.S. Apollo missions, instead of indicating the initial solidification of the lunar magma ocean.
For their study, the researchers employed modeling to demonstrate that the Moon might have undergone sufficient tidal heating around 4.35 billion years ago to cause this remelting, effectively altering the perceived age of these lunar samples.
A False Imprint from Magma
Tidal heating involves the gravitational interactions between two celestial bodies that generate internal friction and result in significant heating. This effect was likely more intense during the Moon’s early formation when it was closer to Earth. Current models suggest that during specific periods in its early development, the Moon’s orbit may have been unstable, subjecting it to intense tidal forces from Earth that could have triggered major heating events, fundamentally changing the Moon’s geological features.
The research group draws similarities between this hypothetical heating on the Moon and the ongoing volcanic activity found on Io, one of Jupiter’s moons, recognized as the most volcanically active body in the solar system. The eruptions on Io are fueled by tidal forces akin to those that may have influenced the Moon’s early history, leading to extensive volcanic activity and continuous reshaping of its surface.
The team also suggests that the Moon’s remelting would explain why there are not as many early impact basins as expected, which may have been erased during such heating events. They propose that this could indicate the Moon’s formation took place between 4.43 and 4.53 billion years ago, aligning with the upper limits of prior age estimations.
Nimmo stated that future research will delve into more sophisticated simulations to enhance our understanding of how tidal heating might have altered the Moon’s geological timeline. This, along with additional lunar samples collected from upcoming missions, aims to provide clearer insights into the Moon’s true age.
This is why the recent return of lunar samples by China’s Chang’e 6 mission has generated considerable excitement. These samples, taken from the Moon’s far side, promise to yield crucial data about the processes that shaped the Moon’s history. Researchers are particularly keen to determine whether these new samples support the theory of a global resetting event caused by tidal heating.
Nimmo’s group also plans to conduct more intricate modeling to further examine the effects of tidal heating on lunar geology. While initial models show promise, a deeper exploration through more advanced and realistic simulations will be necessary to comprehensively understand the extent of these heating events.
A Fresh Chapter in Lunar Research
This research not only presents a new viewpoint on the Moon’s history but also opens avenues for more detailed investigations into its formation and evolution. The combination of geochemistry and simulation modeling is assisting scientists in piecing together the history of the Moon, with tidal heating emerging as a vital mechanism for deciphering its geological features.
“As more data comes in — especially from current and upcoming lunar missions — our understanding of the Moon’s past will continue to develop,” Nimmo noted. “We hope our findings will encourage further discussion and exploration, ultimately leading to a clearer understanding of the Moon’s role in the broader history of our solar system.”
