The Origin of the Moon

Formation of the Moon.
What are the theories of how the Moon was formed?

How was the Moon created? It is not known with certainty how the moon originated, but there are numerous theories that account for its appearance in orbit. The most widely accepted theory today says that the moon was formed by a collision between the planet Earth and a Mars-sized body. The collision happend approximately 4.6 billion years ago.

The collision between the two bodies is believed to have occurred late in the Earth’s formation process, when part of its core was lost. A dust cloud formed over the Earth as a result of the collision.

The missing part of the core underwent a condensation process and approached the plane along the ecliptic plane, which caused this condensed core to go into orbit. Its temperature after condensation explains the absence of volatile compounds in lunar rocks.

The Formation of the Moon

The origin of the Moon.
The Origin of the Moon.

Since ancient times, the Moon has aroused the curiosity of men. For the Guaraní, one of the most representative indigenous ethnic groups of the Americas, the Moon was a goddess called Jaci, protector of:

  • Plants
  • Lovers
  • Reproduction

Mythologically, Jaci is identified with:

  • Diana of the Romans
  • Xochiquetzal of the Aztecs
  • Chandra of the Hindus
  • Isis of the ancient Egyptians

The Mayan civilization, a pre-Columbian people from Central America who had their heyday during the period from 250 AD to 900 AD, also related the Moon to femininity and fertility. They had advanced knowledge in astronomy and mathematics and mapped the Moon’s motion with extreme accuracy.

In our recent history, we have sent astronauts to the Moon to study it further. With all this admiration throughout human history, we are led to wonder: how did the Moon come about? Several theories have emerged about it.

Co-Accretion Theory

Proposes that the Moon formed around the Earth early in the formation of our solar system. After the formation of the Earth, there would have been material left over. The left over matterial gravitated around it and, over time, eventually clumped together to form the Moon. The justification to validate this idea is that the satellite and our planet have some characteristics in common, such as the age of the rocks on both surfaces. However, there are significant differences that remain unexplained, such as the lower amount of iron found in lunar soil.

Capture Theory

It proposes that the Moon would have formed far from Earth. But would have been captured by the gravitational pull of our planet as it passed close to it. However, calculations show that such capture by Earth’s gravitational pull is practically impossible for an object as large as the Moon.

Fission Theory of The Formation of the Moon

According to this theory, the Moon and the Earth were initially one body. Our satellite would have become detached from the Earth while still red-hot. Also, the Earth would have begun to spin around itself, much faster than it does today. The theory claims that the Pacific Ocean region corresponds to the piece that formed the Moon. The idea is not widely accepted because the oceanic space would not be enough to make up a large rock the size of the Moon. And if the Moon had really separated from the Earth, its orbit would have been different.

Giant-impact hypothesis

The main theory of the Moon’s formation dates back to 1975. This theory considers that a celestial body the size of Mars, called Theia, collided with the Earth about 4.5 billion years ago, when our planet was still forming and was a big hot gooey ball. A piece of the globe would have broken off and formed the Moon.

There are also variations of the theory, which consider that various debris from this collision accumulated around the Earth and, over time, due to gravitational forces, came together and eventually formed the Moon.

This primordial Moon was very different from our current satellite, as it was still in a state of fusion. The moon formed of unsolidified magma, which, over time, was crystallizing. The Moon also rotated much faster than it does today. However, due to the gravitational interaction of the Earth-Moon system, its rotation speed decreased to the point that they became equal. Maintaining the same side of the Moon towards our planet. This is why we always see the same side of our natural satellite.

Multiple collisions theory

Proposes that instead of a single colossal collision, a series of impacts created miniature moons largely from Earth material. These mini moons merged over time to form one large moon.

That’s the latest hypothesis, from a study published in 2017 in the scientific journal Nature Geoscience.

Rufu, R., Aharonson, O. & Perets, H. A multiple-impact origin for the Moon. Nature Geosci 10, 89–94 (2017). https://doi.org/10.1038/ngeo2866

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