2.1 Just What is the Moon?

The study of the moon, Selenology, has been a focal point of interest for scientists throughout history. Then in 1973 the Apollo mission provided rock evidence that the moon was formed from an object colliding with Earth, proving that the moon was once part of Earth.

Formation of the Moon

The Giant Impact Theory suggests that the Moon was formed around 4.5 billion years ago during a cataclysmic event whereby a Mars-sized body, referred to as the Theia by Halliday (2000), collided with Earth. The sheer impact ejected massive amounts of debris into orbit around Earth, each of varying shapes and sizes, which means the debris would orbit at varying speeds. Over time, the debris coalesced through a process known as accretion to form what is now known as the moon.

Lunar samples brought back from the Moon during the Apollo mission share isotopic similarities with Earth's mantle. This suggests a common origin (Mitrofanov, 2023). Further research has shown that the orbital dynamics of the Moon's size and distance align with computer-generated models of a large-impact scenario. This evidence has led to the Giant Impact Theory becoming the leading explanation for the Moon's existence.

However, because this event may have occurred 4.5 billion years ago, long before observation equipment, we use proxy variables to build confidence in a theory, such as chemical composition and computer-generated models. Therefore, other hypotheses have been proposed, although much less widely accepted due to the limiting supporting evidence.