Between 363,570 and 405,410 kilometres (around 220,000-250,000 miles) from Earth is our only natural satellite, the Moon. It’s very different from its mother planet: it has around one-eighth of the Earth’s mass, a very thin, almost negligible atmosphere and it’s completely devoid of life. Yet at the same time, it’s very similar, with a distinct crust, mantle and solid iron core plus a mineral composition almost identical to Earth’s volcanic geology. A coincidence? Scientists think not.
There are four theories of the Moon’s creation, three of which have generally been discredited. One suggests the Moon is the result of dust coalescing during the formation of the Solar System, another that the Moon was an asteroid captured by Earth’s gravity. Finally, ‘fission theory’ states that Earth was spinning so rapidly when it formed the molten mass split in two.
The generally accepted idea though is that around 4.5 billion years ago, when the Earth was forming, a Mars-sized planet – dubbed Theia – collided with it. It was such a cataclysmic event that the smaller planet was completely destroyed, sending its own iron into the Earth’s core while throwing trillions of tons of debris from the surface into orbit. Eventually this coalesced to form the Moon.
This ‘giant impact’ theory ties into the creation of our Solar System about 50 million years before the Earth formed, during a period of intense asteroid bombardment that lasted for about 80 million years. This bombardment eventually settled, either colliding with early planets like Earth, being ejected from the Solar System or falling under the influence of the Sun’s gravity and becoming part of the Asteroid Belt located between Mars and Jupiter.
Recent studies of the Moon’s chemistry by the University of Chicago revealed the surface is virtually identical to Earth’s, which raised questions over the giant impact theory: if a Mars-sized planet struck our world, why is there no apparent trace of it? The impact is supposed to have been so colossal it nearly tore the Earth apart, causing it to warp into an oblong shape before rebounding under the force of its own gravity. Explanations include the possibility that Theia was a comet made of ice that could provide the required energy for the giant impact theory, but it subsequently evaporated. Scientists have also suggested that the Earth could have been spinning so fast at the time that Theia was thoroughly mixed into the Earth and the debris that eventually became the Moon. In either case, giant impact is still our most viable theory as to the creation of our lunar neighbour.
The Mars-sized body that hit Earth was named after the Greek Titan who gave birth to the Moon goddess Selene. One theory is that it formed in the same orbit as Earth and, overtime, coalesced with the abundance of debris in our early Solar System until it was big enough to come under the influence of its own gravity. Then, like the Earth, it began to attract asteroids that added to its bulk over time. When it reached the size of the Red Planet its orbit became unstable, its orbital angle to Earth began to increase until, eventually-just over 4.5 billion years ago, it collided with our world. Originally it was thought that the Moon was mostly the remains of Theia mixed with debris from Earth, but recent research shows that the core of the Moon and the Earth have a very similar iron composition, suggesting that they both originated from the same source.