I am sure that, like me, at least a small fraction of people have spent long stretches staring in awe at the Moon on a luminous night while listening to the distant hum of highway traffic or the quiet rhythm of the evening. Just thinking about it creates an irresistible sense of wonder: beyond Earth, a beautiful object sits proudly in the darkness as a source of light. But how did this graceful Moon come into being? Many people say it formed after a collision between Earth and another celestial body. But is that really the full story?
There are many theories about the Moon's origin, both impact-based and non-impact. The capture theory says Earth's strong gravity pulled in a rocky body that formed elsewhere in the Solar System and trapped it in orbit, creating what we now know as Luna, Earth's moon. However, bodies captured this way are often irregular in shape rather than round like the Moon.
Figure: Capture Theory
According to the fission theory, Earth was once spinning so fast that some of its material broke away, became the Moon, and began orbiting the planet. But if the Moon had formed elsewhere and was later captured by Earth's gravity, its composition should be very different from Earth's. In reality, the Earth and Moon are quite similar in composition. Likewise, if the Moon formed at the same time as Earth, or split away from it, its mineral types and ratios should closely match Earth's. They are indeed very similar, but not perfectly identical. That is why the most widely accepted explanation today is the Giant Impact theory.
Figure: Fission Theory
Before the Earth-Moon system existed, there may have been a proto-Earth and a roughly Mars-sized planet called Theia. According to the Giant Impact theory, Theia collided with Earth and blasted vaporized material from Earth's crust into space. Gravity then pulled the debris together to form a moon that is proportionally one of the largest in the Solar System relative to its host planet.
Figure: Giant Impact
This model explains why the Moon is made mostly of lighter material and is less dense than Earth. The material that formed it likely came from Earth's crust, while the rocky core remained largely untouched. Because the debris collected around the remains of Theia's core, it would have concentrated near Earth's orbital plane. That is one reason the Moon travels along a path similar to the one the Sun appears to follow across our sky.
Although this is the most popular theory, it also has challenges. Most models suggest that more than 60 percent of the Moon should be made of material from Theia. But rock samples brought back by the Apollo missions suggest otherwise.
Figure: Lunar Rock
According to astrophysicist Alessandra Mastrobuono-Battisti of the Israel Institute of Technology in Haifa, apart from a few compositional differences, the Earth and Moon are almost twins. This finding casts a long shadow over simple Giant Impact models. In 2017, Israeli researchers proposed an alternative impact scenario suggesting that the Moon could be the result of many small collisions. In this picture, each collision creates a debris disk around the proto-Earth, which then forms a moonlet. Those moonlets migrate outward tidally and later merge into the Moon. Such sub-lunar moonlets could have been a common result of impacts on the proto-Earth in the early Solar System. Their efficient merger after multiple impacts may explain why Earth and the Moon ended up with such similar compositions.
Figure: Multiple Impact Theory
There is another possibility called the co-formation theory. In 2012, Robin Canup of the Southwest Research Institute in Texas proposed that Earth and the Moon formed at the same time, when two enormous bodies about five times the size of Mars crashed into each other. As NASA explained, after the collision the two similar-sized bodies collided again and formed a proto-Earth surrounded by a disk from which the Moon emerged.
Figure: Co-formation (Before Collision, During the Formation of Earth)
However, research published in Nature Geoscience in 2020 offered another explanation for why Earth and the Moon have such similar compositions. After studying oxygen isotopes in lunar rocks brought back by Apollo astronauts, researchers found a small but important difference compared with Earth's rocks. Samples collected from the deep lunar mantle were much heavier than comparable Earth samples, and their isotopic signatures seemed more representative of Theia, the proto-lunar impactor. This evidence again supports the Giant Impact theory, though it still cannot be considered fully proven because the samples came from only one region of the Moon.
Many of these theories assume that the Moon formed gradually over months or years as material merged in orbit. But a newer simulation suggests a different possibility: the Moon may have formed almost immediately, within just a few hours, when material from Earth and Theia was launched directly into orbit after the impact. This rapid, single-stage formation model may also help answer other unresolved questions. It could place the Moon in a wide orbit with an interior that was not completely molten, potentially explaining features such as the Moon's tilted orbit and thin crust. It is one of the most intriguing explanations for the Moon's origin.
So it is clear that the mystery of the Moon's origin is far from settled. Many secrets about our round companion are still hidden. Yet with the rapid advance of science and technology, there is good reason to hope that we will uncover many more answers soon. NASA's upcoming Artemis missions are moving in exactly that direction. To get closer to knowing which theory is correct, scientists will need future lunar samples returned to Earth for study. As researchers gain access to material from other parts of the Moon and from deeper beneath its surface, they will be able to compare real-world data with these simulated scenarios. Whatever the final answer is, the Moon once again reminds us that the universe is a workshop of endless mysteries.