Among the mysteries lying on the far side of the Moon are why it has a thick mountainous crust, while the near side which we see has low-lying lava plains and a much thinner crust than the far side. Scientists believe they may be able to finally explain the moon's asymmetry.

The side of the Moon which faces Earth is relatively flat and covered with plains of volcanic rock called "maria", which is Latin for seas, while its far side is made up of highlands and mountainous with an average of 1.2 miles higher elevation than the near side.

But the contrast goes deeper, as the crust on the farside is 50 kilometers thicker than the crust on the near side. Scientists have been trying to figure out what could have caused the asymmetrical shape. Various theories have been proposed, but no final conclusion has been made.

A new study published in the journal Nature has suggested that Earth originally had two moons in orbit around it until a second moon flattened itself to the bigger moon more than 4 billion years ago.

This theory could explain why the two sides of the moon are so different from each other. Scientists are theorizing that this second moon, which would have been tiny about 750 miles wide, could have formed from the same collision between Earth and an object the size of Mars that they believe helped create the moon.

"Traces of this 'other' moon linger in a mysterious dichotomy between the Moon's visible side and its remote far side," says Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, who co-authored the study with Martin Jutzi, now of the University of Berne in Switzerland.

Asphaug and Jutzi created a computer model showing that a collision with a sister moon about one-thirtieth the moon's mass, or around 1,000 kilometres in diameter, can explain the current moon's state.

It is proposed that tidal forces from Earth would have caused both moons to migrate outward. The balance of forces became unstable as the two moons moved further away from the Earth and the two moons collided slowly, preventing a big crater from forming in the Earth's moon.

Asphaug said this slow crash happened at more than 5,000 miles per hour, and that the smaller moon was more than 600 miles wide. It took hours for the rocks and crust from the smaller moon to spread over and around the bigger moon without creating a crater, and a faster crash would've caused the opposite.

"It's not a typical cratering event, where you fire a 'bullet' and excavate a crater much larger than the bullet. Here, you make a crater only about one-fifth the volume of the impactor, and the impactor just kind of splats into the cavity,” Asphaug says in Nature.
“By definition, a big collision occurs only on one side and unless it globally melts the planet, it creates an asymmetry,” says Asphaug.

“All these collisions tend to make holes as opposed to mountains. It took, in the simulations, a fairly specialized set of conditions to make a mountain rather than a crater,” said Maria Zuber, a planetary scientist at the Massachusetts Institute of Technology in Cambridge

"I think this idea is going to get a lot of attention because it's very novel, it's very clever, and people are going to be interested in testing to see whether it's right or wrong," says Zub.

Many theories on how the asymmetry happened have been offered, but no solid conclusion has been made.

Planetary scientist Alan Stern, former NASA associate administrator for science, told CBS that the theory is a "very clever new idea," but one that is not easily tested to learn whether it is right.

The theory remains just a theory until it can be tested but there is no easy way to test the existence of the Moon's twin just yet.