The Big Splat Theory

Space can inspire the imagination like nothing else. If you have ever looked up into the night sky and seen a full moon, it seems like space goes on forever. On Earth, the most distinctive feature in the night sky is the moon. But how was the moon formed, and why does it look the way it does today?

The most prevalent theory is called The Big Splat Theory. Essentially, many scientists believe that the Earth once had two moons, until they collided, leaving the Earth with one larger, slightly lopsided moon. For a long time, scientists struggled to explain why the nearside of the moon appeared so different from the far side of the moon. The Big Splat Theory helps to explain that.

According to the theory, two moons were formed roughly 4.5 billion years ago, when the Earth collided with another space body, roughly the size of Mars. The ensuing debris formed two moons, which orbited Earth and usually rose in the sky together, the smaller one trailing the larger one. The larger moon is estimated to have been three times larger and 25 times heavier than the smaller moon. Because of the size difference in the moons, it was only a matter of time before they would collide, as the larger moon’s gravitational pull was just too strong for the smaller moon. 

Scientists believe the collision occurred at a (relatively) slow speed of 5,000 mph (slow only when you are talking about speed in space!), meaning the rocks did not melt when the moons hit, which would have happened at a higher speed.. The rocks and crust from the smaller moon settled around the larger moon, which did not cause a crater. The side of the moon that does not face the Earth is much more hilly than the side that does face the Earth, which is dominated by low-lying lava plains. Some scientists believe that The Big Splat Theory helps to explain that variance.

Erik Asphaug from the University of California-Santa Cruz was one of the co-creators of the theory, and described the physics as being “surprisingly similar to a pie in the face.” The collision would have smoothed out one side of the mountain, which is what causes the two sides to be so distinct from each other. According to the computer model, the collision “squished” the KREEP (a layer on the moon high in potassium, rare earth elements, and phosporus) to one side of the moon, almost like a pancake. This would be the side of the moon visible to the earth, which is much more flat. All of the rocks and debris from the collision were pushed to the far side, which caused it to take on the highlands terrain that it has today.

H. Jay Melosh of Purdue University said about the theory, “We can’t find anything wrong with it. It may or may not be right.” Even among the most brilliant scientists in the world, the moon still inspires the imagination.