October 25, 2021

I, Science

The science magazine of Imperial College

A new theory explaining the birth of the Moon could help complete our understanding.

The prevailing theory of the moon’s creation is the ‘giant impact theory’, in which a Mars-sized object collided with Earth around 4.5 billion years ago. The collision scattered debris from both the Earth and Mars-sized object into orbit, which eventually came together to form the moon. It also set the angular momentum of the Earth-Moon system.

But if this theory were correct, the debris – and, thus, the moon – would have been similar in composition to the Mars-sized object. The moon would also orbit the Earth’s equator. Neither is the case; the Moon shares Earth’s chemical signature, and orbits at a 5 degree angle to the equator.

The new theory, published in Nature, proposes that the impact was more energetic than originally thought. A higher proportion of the debris would have been from Earth, explaining the moon’s chemical similarity. It accounts for the “extra” angular momentum a higher-energy collision would cause, by hypothesising that some was transferred to the Earth-Sun system.

It suggests that Earth’s axis nearly pointed towards the Sun after the collision, with the moon on a very tilted orbit. The moon drifted away from Earth, and at a certain point the Sun’s influence on it overtook the Earth’s. This transition caused the angle of the moon’s orbit to change abruptly, and it began to oscillate about the Earth’s equator. This extreme change transferred angular momentum to the Earth-Sun system, and corrected the Earth’s tilt. Afterwards the moon’s orbit was at about 30 degrees, and over millions of years reduced it to the current 5.

The new theory offers a convenient explanation of two somewhat unexplained phenomena, with just one event. If confirmed, it could potentially shed light on to the secrets of our closest celestial neighbour.

Henry Alman is studying for a BSc in Physics

Banner photo from NASA