📸 How the moon formed is a question that continues to be asked by astronomers.
📸 A visualization of the Theia impact.
So much of what we know about distant space is a mystery, but even our closest cosmic neighbor remains an enigma. The question of the Moon’s formation is one that still lacks a satisfying answer. The most commonly accepted model, the Theia-impact hypothesis, can account for many of the questions surrounding Earth’s satellite, but it has its own limits, and is only the lastest in a long line of theories. This question of how the Moon came about is one older than the scientific field that hopes to answer it, a question that goes beyond the origins of our satellite into the formation of the Earth itself.
Our moon is a singular entity. Earth's sky is not littered with dozens of orbiting satellites like Jupiter or Saturn and unlike those planets our moon is massive relative to Earth, clocking in at about a quarter of the planet. Given its size and unique singularity, it's clear that Luna is the odd one out in the solar system. Prior to the Apollo mission’s retrieval of lunar material and 1982’s identification of the first lunar meteorite, scientists were left to merely speculate on the Moon’s formation. However, before the exact beginnings of the Moon could be known, an even more basic question had to be answered: the Moon’s age.
📸 A drawing of the moon's craters by amateur astronomer James Nasmyth (circa 1856-1890)
The earliest studies into the Moon’s age were basic estimates that hoped to study the concentration of craters on its surface. These scientists correctly surmised that the lunar terrare were older as they have more craters than the lunar maria, but opinion diverged sharply as to how they formed. The (correct) impact hypothesis was the minority opinion in favor of the craters forming from volcanic activity. This latter theory was championed by astronomer William Herschel in 1787, celebrated for discovering Uranus six years before. On this question however, he was wrong, as well as his idea that “Selenite” aliens lived on the Moon’s surface.
Lacking lunar material and limited to relative age estimates based on crater density, the Moon’s age could be estimated one other way. By measuring the Moon’s movement away from Earth (caused by our gravitational field making our satellite orbit faster) one can reverse engineer the age of the Moon. With the aid of laser-reflecting mirrors left on the Moon during Apollo 11, we know the Moon moves about 1.5 inches away each year. From that, most modern estimates of the moon’s age place it around 4.5 billion years old. Importantly, this is near the age of the planet it orbits. However the Moon formed, it had something to do with the early Earth.
📸 A visualization of Darwin's theory. (Source: Hutchinson’s Splendour of the Heavens)
With the age of the Earth and Moon in mind, three main theories dominated to account for the latter’s formation. First, the Fission theory (supported by Charles Darwin’s astronomer son George) suggested a molten but cooling Earth ejected material as it rapidly spun, which then formed the Moon. Second, the Co-accretion theory suggested the Earth and the Moon formed alongside each other from the same part of the proto-planetary disk. Third, the Moon was simply captured by the Earth’s gravitational field, having formed elsewhere in the solar system. These theories had their merits, but with the examination of lunar material, they fell apart.
When the first lunar samples were examined, they were found to have a strikingly similar composition to Earth’s material, suggesting a massive impact that ejected material into space. This idea had been anticipated as a form of the Fission theory of formation, but only now did scientists have definitive proof. This model can also explain the Earth’s high angular momentum and its pronounced axial tilt. The catch is that while Earth-like material is in abundance on the Moon, there is a lack of matter that can be tied to Theia, the Mars-sized planet that is speculated to have crashed into Earth.
In such a collision, the Moon would primarily be made up of the impactor’s material, a problem as the Moon has an unusually low iron concentration. Where are the remnants of Theia to be found then on the Moon? It remains a controversy: for the physicists, the Giant-impact is the perfect model to account for the Earth and Moon’s movements relative to each other, while the chemists find a dearth of material one would expect from Theia. The Giant-impact hypothesis is the best current model for the Moon’s formation, but it does not satisfy all criteria. With NASA’s Artemis Program set to return to the Moon this decade, perhaps more evidence will be found to explain the origin of the Earth’s enigmatic satellite.
Aderin-Pocock, Maggie. The Book of the Moon : a Guide to Our Closest Neighbor / Maggie Aderin-Pocock. Abrams Image, 2019.
Cummings, W. D. Evolving Theories on the Origin of the Moon / Warren D. Cummings. Springer, 2019.
Galimov EM (Ėrik M, Krivtsov AM. Origin of the Moon, New Concept : Geochemistry and Dynamics. De Gruyter; 2012.