Current theories of the Moon’s formation favor both the Earth and Moon forming from the 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 later underwent a second melting event that reset the apparent age of many geological samples. Credit: NASA
Earth and the Moon are forever locked in a gravitational embrace that has played a critical role in determining the fate of both worlds. Although they have grown more distant since their formative years, new research published today in Nature shows just how powerful their attraction 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 can appear to be some 160 million years younger than the Moon’s actual age.
Understanding the nature of this lunar face-melting event could help pin down 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 it formed.
Resolving an age discrepancy
Scientists widely think that Moon was formed in a giant impact where a Mars-sized world, dubbed Theia, dealt the proto-Earth a glancing blow soon after the formation of the solar system. Both today’s Earth and the Moon are thought to have formed from the debris created by that impact.
The early lunar surface existed in a molten state, forming what’s referred to as the lunar magma ocean. Within it, iron and heavy elements sank to the core. Lighter silicate elements floated to the surface, forming the Moon’s present-day anorthosite crust. Ample evidence of this magma ocean exists today in the global presence of anorthosite across the lunar globe. Samples have been recovered by Apollo, Luna, and Chang’e missions — from sites hundreds of miles apart — and India’s Pragyan rover conducted on-site analysis of similar rocks in 2023.
Researchers have measured the ages of these samples to determine when they solidified. But this has yielded a wide range of dates. Examination of samples recovered by NASA’s Apollo missions and by the robotic Russian Luna and Chinese Chang’e missions indicates an age of about 4.35 billion years. But analysis of individual zircon grains in lunar samples reveals some grains with an age of 4.51 billion years, a 160 million year difference.
Deepening the mystery, thermal modeling of the Moon as it cooled is more consistent with the older age. But the Moon also seems to have too few large impact craters, which would suggest a younger age.
To explain these discrepancies, Nimmo and his colleagues suggest the Moon underwent a second melting event around 200 million years after its formation. During this event, some of the Moon’s crust again became molten, “resetting” the formation ages of many minerals. This would explain the younger geological formation date for these minerals of around 4.35 billion years, while still 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? Radioactive rocks releasing heat deep inside the Moon could have contributed some additional warmth, but a small body like the Moon should cool faster than Earth, not heat up again and remelt.
Nimmo and his colleagues focused on a facet of the early Moon that receives little public attention: It actually formed much closer to Earth than it is today. Current assumptions place the newly formed Moon at just around five Earth radii in altitude, or 20,000 miles (32,000 kilometers) above Earth. This is roughly as close to Earth as today’s geosynchronous communications and weather satellites, 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 today’s more distant pairing.
Nimmo and his colleagues calculate that these tidal forces could have significantly heated the Moon when its orbit was between 16 to 22 Earth radii distant. This region is called the Laplace plane transition, and it marks the location where the orbit of the Moon, in its gradual migration away from Earth, transitioned from being affected primarily by Earth’s gravity to also being affected 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 the effects of tides.
Over a period of 3 million to 5 million years, there should have been sufficient tidal heating to remelt some of the Moon’s crust and parts of its mantle. While this remelting did not form a global magma ocean nor significantly alter the Moon’s overall shape, the resulting volcanic activity would have erased previous impact craters and basins, smoothing out its surface and providing a blank slate for the creation of new lunar features.
A quick orbital boost
One aspect of this scenario that required further explanation is that in it, the Moon would have raised its orbit very quickly from around 5 Earth radii to the Laplace plane transition region. Today, the bulging of Earth’s oceans with the tides exerts a gravitational attraction that slowly accelerates the Moon, raising its orbit by 1.5 inches (3.8 centimeters) per year. But Earth’s oceans did not form until about 3.8 billion years ago, when the primordial atmosphere cooled sufficiently to allow water vapor to condense.
Since Earth’s oceans did not exist at the time of the postulated second lunar melting 4.35 billion years ago, another mechanism was needed. Nimmo notes that around this time period, Earth was still molten — not a smooth sphere, but lumpy. Not only did the Moon’s gravity induce bulges, Earth also had a higher rotation rate, with days as short as four hours.
The soft Earth’s bulges, coupled with a faster rotation rate, could have substituted for modern ocean tides, says Nimmo, raising 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 roughly 60 Earth radii — Earth’s gravity would have pulled and squeezed on the Moon in a constantly changing direction, like a baker kneading a ball of dough. Over time, the constant stretching and squeezing would have raised the Moon’s temperature to the point that it again melted.
Another implication of a lunar remelting is that it places an age limit on two of the most important basin impacts on the Moon: the Procellarum Basin on the current near side, and the South Pole-Aitkin Basin. The latter is the largest impact basin in the solar system, covering much of the farside southern hemisphere. These events must have occurred after the Moon’s crust solidified a second time about 4.35 billion years ago.
Questions such as 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 truly fascinating Luna really is.