The reason why the near side of the Moon always faces the Earth may be the result of it being a 'loaded dice', according to a team of researchers.
A new study reported in the journal Icarus concludes the rate at which the Moon's rotation slowed since its creation, ultimately determined that the smoother side permanently faced the Earth.
The research led by Professor Oded Aharonson from the California Institute of Technology in Pasadena, dismisses previous views that the side of the Moon permanently facing Earth is just coincidence.
"We now know that's not the case," says Aharonson.
The lunar near side is largely covered by dense, topographically low, dark mare basalts, the pattern of which to some, resembles the image of a face: the so called man in the Moon.
The Lunar far side has a thicker crust, higher mountains and an elevated topography.
"Many people assume that because the highlands are a bit closer to Earth and have more mass they should have been more likely to face the Earth," says Aharonson.
Lunar rotation and orbit
Today the Moon is locked in a synchronous orbit, in which it completes a full rotation on its axis over the same length of time it takes to complete a full orbit of the Earth.
However four billions of years ago, the moon rotated much faster.
"All of the Moon was visible from different parts of the Earth at various times," says Aharonson.
"It also developed a slight bulge caused by tidal forces generated by the Earth. As the Moon rotated, the bulge remained facing Earth while the Moon rotated through causing its interior to flex and heat up due to tidal friction."
The internal friction from this flexing acted as a torque brake slowing the lunar spin rate until its rotation matched the length of its orbit around the Earth.
Aharonson and colleagues reached their conclusions by undertaking multiple numerical computer simulations, trying many different starting conditions and seeing how they worked out.
He says, unlike a coin that has a 50:50 chance of showing heads or tails when it is tossed into the air, the Moon "was loaded".
"We found the rate at which the Moon slowed down its spinning, dissipating its rotational energy, determined which side of the Moon we wound up with," says Aharonson.
"A faster slow down in spin rate would have resulted in an even chance of either side eventually facing Earth."
"But because the Moon slowed down more gradually, we wound up being twice as likely to have the lunar lowlands facing us".
"It's a lot like a train going up a couple of hills and down a couple of valleys," says Aharonson.
"It uses up the last of its energy going through the deepest valley and up the tallest hill, so when it reaches the next valley it's stuck there. And in this case, the valley is the lunar near side."
"I think it's beautiful that such elegant physics can explain why the Moon faces the way it does," says Aharonson.