For 55 years, scientists puzzled over why the so-called “dark side,” or far side, of the moon remains relatively free of maria, or those dark spots that characterize the side facing Earth, forming images like the “man in the moon.”

Astrophysicists from Penn State University now think they’ve solved the mystery of why the two sides look so different.

The far side of the moon does not have as many dark maria compared to the near side. (Image source: NASA)

The far side of the moon does not have as many dark maria compared to the near side. (Image source: NASA)

“I remember the first time I saw a globe of the moon as a boy, being struck by how different the far side looks,” Jason Wright, assistant professor of astrophysics at the university, said.

Though the dark side of the moon, named so not because it was dark but long unknown, has mountains and craters, Wright wondered where its maria were.

“It turns out it’s been a mystery since the fifties,” he added.

The dark spots on this near side of the moon are known as maria. (Image source: NASA)

The dark spots on this near side of the moon are known as maria. (Image source: NASA)

The mystery, described as the Lunar Far Side Highlands Problem, was first identified in 1959 when the Soviet’s Luna 3 took the first images of that side of the moon.

Graduate student Arpita Roy, Wright and other colleagues, explained in the latest issue of Astrophysical Journal Letters the absence of maria on this side, which has thicker crust, is likely a consequence of the moon’s formation.

The difference in crusts, the authors wrote, probably resulted from heat coming off Earth and impacting the near side of the moon, compared to the far side.

“Since the accreting Moon rapidly achieved synchronous rotation, a surface and atmospheric thermal gradient was imposed by the proximity of the hot, post-giant impact Earth. This gradient guided condensation of atmospheric and accreting material, preferentially depositing crust-forming refractories on the cooler far side, resulting in a primordial bulk chemical inhomogeneity that seeded the crustal asymmetry,” the authors wrote.

When meteoroids hit the moon on the near side, its thinner crust caved releasing basaltic lava, which made the maria. On the far side, however, the authors said, the crust was too thick for meteoroids to break through and allow the magma to rise up and create dark maria.

(H/T: Science Daily)