Earth Had Two Moons That Collided to Form One, New Theory Suggests

The Earth once had two moons but they collided to form one moon, according to a new study by scientists looking at the Moon's landscape.

Findings by Dr. Martin Jutzi from the University of Bern in Switzerland and Professor Erik Asphaug from the University of California, Santa Cruz suggest that a smaller moon also once orbited the Earth, but fused with our known Moon in a low-speed collision 4.4. billion years ago.

“Impact modelers try to explain everything with collisions. In this case, it requires an odd collision: being slow, it does not from a crater, but splats material onto one side,” Asphaug said.

The scientists also point to major differences in the landscape of the Moon’s near and far sides as evidence that the Earth once had a second moon.

Ever since its exploration in 1959, scientists have noticed that the Moon’s dark side has an extremely different topography than its near side that is ever-facing the Earth.

The dark side consists of elevated grounds and mountainous regions while the near side is relatively flat and low. Among an endless list of differences, high levels of minerals such as potassium and phosphorus are found on the near side, but not on the far side.

The study, which has been published in Nature journal, builds on the theory that the Moon was created when a Mars-sized object collided with the Earth. The resulting debris eventually cooled and coalesced into our Moon, but it is possible that other bodies could have been created as well.

The second Moon was kept in orbit for tens of millions of years because it was located at a Trojan point, a point where the gravitational attraction between the Earth and Moon are balanced. Once the Moon’s orbit had moved far enough away from the Earth, the Trojan Moon destabilized and was unable to maintain its own orbit.

Computer simulations confirm that the tiny moon, just a third the diameter of our Moon, crashing at a sluggish 4,500 to 6,700 miles per hours, would have “pan-caked” itself onto the larger moon.

Consider, “a ball of Gruyere colliding into a ball of cheddar,” Asphaug told

A lunar crash could explain why the Moon’s near side is so mineral rich. The study details that while the rock and debris would have settled on the far side due to the Moon’s incongruity, the impact would have forced magma to the surface of the near side in a process called accretion.

“One of the elegant aspects of Erik’s article is that it links these two puzzles together: perhaps the giant collision that formed the Moon also spalled off some smaller bodies, one of which later fell back to the Moon to cause the dichotomy that we see today,” said Professor Francis Nimmo of UCSC, who has suggested an alternate theory of tidal waves creating the highlands of the Moon’s dark side.

“The fact that the near side of the Moon looks so different to the far side has been a puzzle since the dawn of the space age, perhaps second only to the origin of the Moon itself.”