The origin of the Moon: 5 theories to explain the formation of Earth’s natural satellite

• The Moon was most likely formed after a Mars-sized protoplanet hit Earth around 4.5 billion years ago.
• The main evidence is derived from the chemical analysis of lunar rock samples retrieved during the Apollo 11 mission.
• Other theories suggest that the Moon was captured by the Earth's gravitational field or that it comes from the same protoplanetary disk as Earth.

The Moon is the Earth’s only natural satellite and the fifth largest satellite in the Solar System. It circles our planet at a mean distance of 238,900 miles (384,400 km), completing an orbit in about 27.3 days and returning to the same position relative to the Sun in around 29.5 days.

During this period, we can see the Moon in four different shapes according to what percentage of its surface is sunlit in the course of its orbit around the Earth. These “shapes” are the lunar phases: new moon, first quarter, full moon, and last quarter.

The Moon is known to be the cause of ocean tides due to the gravitational attraction it exerts on Earth. 

This force also helps stabilize the Earth’s axial tilt. If the Moon didn't exist, the Earth’s axial tilt could vary so much that we could experience much stronger climatic effects, such as no seasons, and more extreme weather, such as more frequent ice ages.

But how did the Moon get there in the first place?

When did the Moon form?

The formation of the Moon took place about 4.5 billion years ago, about 30-50 million years after the formation of the Solar System. 

Scientists were able to calculate this thanks to lunar rock samples taken by NASA astronauts Neil Armstrong and Buzz Aldrin during the Apollo 11 mission in July 1969.

How did the Moon form?

There are several Moon formation theories.

The most accepted theory states that a Mars-sized protoplanet, sometimes known as Theia, collided with the Earth. The debris and vapor that derived from the impact collected in orbit around Earth through strong gravitational forces and eventually formed the Moon. 

This is called the giant-impact hypothesis or Theia impact. It was developed in the 1970s, and it is mostly supported by chemical evidence.

An alternative impact theory from researchers at the Weizmann Institute of Science in Israel posits that the Moon was created from multiple, smaller impacts between the Earth and other bodies (most likely asteroids). 

This theory states that, as collisions were very common in the early stages of the Solar System, it is likely that they formed a debris disk around the proto-Earth, which later evolved into a small moonlet. More collisions created more debris, which created other moonlets. Eventually, these moonlets could have merged together into one big moon. 

Capture theory of the Moon

The capture theory was proposed by American astronomer Thomas Jefferson Jackson See in 1909. It postulates that the Moon was a rocky wandering body that was captured by the Earth’s gravitational field as it flew by.

According to this theory, the Moon allegedly formed elsewhere in the Solar System and just happened to become “trapped” by the Earth’s gravitational force as it passed near it. The Moon has been forced to orbit our planet since then.

This is actually how many scientists believe that some other planets got their moons, such as Saturn and its moon Phoebe, but they don’t usually think that this is true for our Moon. First, capture is almost impossible for objects the size of the Earth and our Moon.

Second, Phoebe and other captured moons describe eccentric, non-circular orbits, while our Moon’s orbit around Earth is circular. And third, Phoebe has an irregular shape, like an asteroid, while our Moon is perfectly round. 

Fission theory of the Moon

British Astronomer Sir George Howard Darwin, Charles Darwin’s fifth child, proposed the fission theory in 1878. 

The fission theory states that the Moon broke away from the Earth at the beginning of the Solar System. At that time, the Earth hadn’t fully solidified and spun very fast, describing a full rotation in three or four hours.

But the math didn’t add up. In the 20th century, scientists discarded this theory because they calculated that the Earth’s spin couldn’t have been fast enough to expel a portion of the planet into space. 

Plus, if that was the case, the debris would have fallen back on the Earth’s surface or gone into orbit around the Sun, but it is unlikely that it would have stayed in Earth’s orbit, as proposed in the theory.

Co-accretion theory of the Moon

According to this theory, the Moon formed at the same time as the Earth —independently of each other but within the same protoplanetary disk (the rotating cloud of gas and dust where all the planets of our Solar System developed).

There is some evidence for this theory, but it does not explain the observed angular momentum of the Earth and the Moon. In addition, the Moon is much less dense than the Earth, which would likely not be the case if both started with the same heavy elements at their core. Studies of the lunar mare basalts and rocks of the lunar highlands reveal that there is almost no iron in the Moon (less than one percent), whereas iron makes up around 35 percent of the mass of the Earth.

What did the early Moon look like?

If we follow the giant impact theory, the early Moon was probably a hot, liquid sphere made of gravity-bounded debris. It was covered in magma until the molten rock cooled and solidified, forming the lunar crust. 

After it became a solid rock, the Moon itself was impacted by a huge wandering body at the South Pole, forming the South Pole–Aitken basin, a crater on the far side of the Moon that has approximately 1,600 miles (2,500 km) in diameter and between 3.9-5.1 miles (6.2 to 8.2 km) in depth. 

Other impacts followed in the alleged Late Heavy Bombardment period when multiple asteroids and comets probably collided with the Moon and other planets in the Solar System. These impacts help explain the composition and surface volcanic deposits found on the Moon.

What are the dark spots on the Moon?

The dark spots of the Moon are called lunar maria, a Latin word meaning “seas.”

Early astronomers thought they were seas, but now we know that they are basaltic plains created by ancient asteroid impacts combined with volcanic activity between 3.1 and 3.9 billion years ago.

These dark spots, which are visible from Earth, cover up to 17 percent of the Moon’s surface.