The Giant Impact Hypothesis

Overview

The Giant Impact Hypothesis is the most widely accepted theory explaining the creation of the Moon.
It suggests that a Mars-sized body, often referred to as Theia, collided with the early Earth about 4.5 billion years ago.
This catastrophic event would have resulted in a large proportion of Earth’s outer layer, or mantle, being ejected into space.

Theia is thought to have hit Earth at an angle, not a direct collision.
The resulting impact would have created a cloud of debris around the Earth, largely made up of fragments of both Theia and Earth’s mantle.
All this happened while the Earth was still in its formative phase, recognised as the proto-earth.

Over time, the ejected debris would have coalesced through the process of accretion and formed into the Moon.
Evidence to support this includes the moon’s similar oxygen isotope composition to the Earth, suggesting a common origin.
However, the Moon is deficient in volatile substances and iron compared to Earth, which fits the idea that it’s mainly composed of the Earth’s mantle material.

After its formation, the Moon was likely covered in a magma ocean.
As the Moon cooled down, this magma ocean solidified, forming different types of rocks – anorthite, olivine, and pyroxene.
The heating generated by decay of radioactive isotopes, and further impacts, kept the interior molten, which allowed dense minerals to sink and lighter minerals to rise, leading to the lunar differentiation process - the formation of crust, mantle, and core.
The lunar maria are the result of much later volcanic activity, adding the final touches to the Moon’s surface we see today.

The Giant Impact Hypothesis presents a powerful explanation as to how our Moon came into existence.
It also explains many of the differences and similarities observed when comparing Earth and the Moon’s composition and geological history.