Maria Mystery: Why Moon's Far Side Has No 'Face'

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Heat radiating from the young Earth could help solve the more than 50-year-old mystery of why the far side of the moon, which faces away from Earth, lacks the dark, vast expanses of volcanic rock that define the face of the Man in the Moon as seen from Earth, researchers say.

A new map of lunar craters pop with color.
NASA/Goddard Space Flight Center/DLR/ASU

The Man in the Moon was born when cosmic impacts struck the near side of the moon, the side that faces Earth. These collisions punched holes in the moon's crust, which later filled with vast lakes of lava that formed the dark areas known as maria or "seas."

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In 1959, when the Soviet spacecraft Luna 3 transmitted the first images of the "dark" or far side of the moon, the side facing away from Earth, scientists immediately noticed fewer maria there. This mystery — why no Man in the Moon exists on the moon's far side — is called the Lunar Farside Highlands Problem. [How the Moon Evolved: A Photo Timeline]

"I remember the first time I saw a globe of the moon as a boy, being struck by how different the far side looks," study co-author Jason Wright, an astronomer at Pennsylvania State University, said in a statement. "It was all mountains and craters. Where were the maria?"

Now scientists may have solved the 55-year-old mystery; heat from the young Earth as the newborn moon was cooling caused the difference. The researchers came up with the solution during their work on exoplanets, which are worlds outside the solar system.

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"There are many exoplanets that are really close to their host stars,"lead study author Arpita Roy, also of Penn State, told Space.com. "That really affects the geology of those planets."

Similarly, the moon and Earth are generally thought to have orbited very close together after they formed. The leading idea explaining the moon's formation suggests that it arose shortly after the nascent Earth collided with a Mars-size planet about 4.5 billion years ago, with the resulting debris coalescing into the moon. Scientists say the newborn moon and Earth were 10 to 20 times closer to each other than they are now.

"The moon and Earth loomed large in each other's skies when they formed, " Roy said in a statement.

Since the moon was so close to Earth, the mutual pull of gravity was strong. The gravitational tidal forces the moon and Earth exerted on each other braked their rotations, resulting in the moon always showing the same face to Earth, a situation known as tidal lock.