Current theories of the Moon’s formation hold that both Earth and the Moon formed from debris created by the collision of a Mars-sized world with the proto-Earth about 4.5 billion years ago. New research, however, suggests that the Moon subsequently underwent a second merger event that reset the apparent age of many geological samples. Credit: NASA
The Earth and the Moon are forever linked by a gravitational embrace that played a fundamental role in determining the fate of both worlds. Although they have drifted further apart than in their formative years, new research published today in Nature shows how powerful their pull was in their youth: According to the study, Earth’s gravity caused tidal effects that melted the surface of the early Moon, effectively giving it a global facelift. As a result, lunar samples may appear about 160 million years younger than the actual age of the Moon.
Understanding the nature of this lunar-face melting event could help refine measurements of the Moon’s age, which vary depending on the sample and method. The new work, led by planetary scientist Francis Nimmo of the University of California at Santa Cruz, could also help clarify the timeline of the Moon’s evolution after its formation.
Resolve an age discrepancy
Scientists widely believe that the Moon formed in a giant impact in which a Mars-sized world, nicknamed Theia, dealt the proto-Earth a glancing blow soon after the solar system formed. Both the Earth and the Moon today are thought to have formed from debris created by that impact.
The primordial lunar surface existed in a molten state, forming what is called a lunar magma ocean. Inside, iron and heavy elements sank to the core. Lighter silicate elements floated to the surface, forming the Moon’s current anorthosite crust. Ample evidence of this magma ocean exists today in the global presence of anorthosite on the Moon. Samples have been recovered from the Apollo, Luna and Chang’e missions – from sites hundreds of miles away – and India’s Pragyan rover conducted on-site analyzes of similar rocks in 2023.
The researchers measured the age of these samples to determine when they solidified. But this produced a wide range of dates. Examination of samples recovered from NASA’s Apollo missions and the Russian Luna and Chinese Chang’e robotic missions indicates an age of about 4.35 billion years. But analysis of individual zircon grains in lunar samples reveals that some grains are 4.51 billion years old, a difference of 160 million years.
Deepening the mystery, the thermal modeling of the Moon as it cooled is more consistent with the older age. But the Moon also appears to have too few large impact craters, which would suggest a younger age.
To explain these discrepancies, Nimmo and his colleagues suggest that the Moon underwent a second fusion event approximately 200 million years after its formation. During this event, part of the lunar crust melted again, “resetting” the age of formation of many minerals. This would explain the more recent geological formation date of these minerals, around 4.35 billion years, while allowing for the Moon to have formed earlier, around 4.5 billion years ago.
The question is: what was the heat source that remelted the early Moon? Heat-releasing radioactive rocks deep within the Moon may have contributed additional heat, but a body as small as the Moon would have to cool faster than Earth, not reheat and remelt.
Nimmo and his colleagues focused on an aspect of the early Moon that receives little public attention: It actually formed much closer to Earth than it is today. Current hypotheses place the newly formed Moon about five Earth radii in altitude, or 20,000 miles (32,000 kilometers) above Earth. This is about as close to Earth as geosynchronous communications and weather satellites are today, and about 10 times closer than the Moon is today. Because the force of gravity is stronger at shorter distances, the early Earth-Moon system experienced enormous gravitational tidal forces compared to the more distant current pairing.
Nimmo and his colleagues calculate that these tidal forces could have significantly heated the Moon when its orbit was between 16 and 22 Earth radii away. This region is called the Laplace plane transition and marks the point at which the Moon’s orbit, in its gradual migration away from the Earth, went from being influenced primarily by Earth’s gravity to also being influenced by the Sun. At this distance , the contributions of the Sun and Earth to the Moon’s orbital precession were equal, creating an orbital resonance that enhanced tidal effects.
Over a period of 3 to 5 million years, tidal heating should have been sufficient to remelt part of the moon’s crust and parts of its mantle. While this remelt did not form a global magma ocean or significantly alter the Moon’s overall shape, the resulting volcanic activity would have obliterated previous impact craters and basins, smoothing their surfaces and providing a clean slate for the creation of new ones. lunar characteristics.
A quick orbital push
One aspect of this scenario that required further explanation is that in it the Moon would have raised its orbit very rapidly from about 5 Earth radii to the transition region of the Laplace plane. Today, the tidal swelling of Earth’s oceans exerts a gravitational pull that slowly accelerates the Moon, raising its orbit by 3.8 centimeters per year. But Earth’s oceans only formed about 3.8 billion years ago, when the early atmosphere cooled enough to allow water vapor to condense.
Since Earth’s oceans did not exist at the time of the second lunar melting, which occurred 4.35 billion years ago, another mechanism was needed. Nimmo notes that in this time period the Earth was still molten, not a smooth sphere, but a lumpy one. Not only did the Moon’s gravity induce bulges, but the Earth also had a faster rotation rate, with days as short as four hours.
Earth’s soft bulges, coupled with a faster rotation rate, could have replaced modern ocean tides, Nimmo says, increasing the Moon’s orbit from five to 16 Earth radii in 200 million years. Once it reached that altitude – still much closer than the Moon’s current distance of about 60 Earth radii – Earth’s gravity would pull and squeeze the Moon in a constantly changing direction, like a baker kneading a ball of dough. Over time, the constant stretching and compression would raise the Moon’s temperature enough to melt it again.
Another implication of a lunar recast is that it places an age limit on two of the most important impact basins on the Moon: the Procellarum basin on the present-day near side and the South Pole-Aitkin basin. The latter is the largest impact basin in the solar system, covering much of the southern hemisphere. These events must have occurred after the lunar crust solidified a second time about 4.35 billion years ago.
Questions like the true age of the Moon are what drive our curiosity about the universe. The answer to one question inspires another question, perpetuating our reach for cosmic truths. The more we know about the Moon, the more we will seek answers to new questions and discover how fascinating the Moon really is.
