Washington: The Man in the Moon appeared when meteoroids struck the Earth-facing side of the Moon creating large flat seas of basalt that we see as dark areas called maria.
But no "face" exists on farside of the Moon and now, Penn State astrophysicists think they know why.
This mystery is called the Lunar Farside Highlands Problem and dates back to 1959, when the Soviet spacecraft Luna 3 transmitted the first images of the "dark" side of the Moon back to Earth.
Jason Wright, assistant professor of astrophysics, Steinn Sigurdsson, professor of astrophysics and Arpita Roy , graduate student in astronomy and astrophysics, and lead author of the study, realized that the absence of maria, which is due to a difference in crustal thickness between the side of the Moon we see and the hidden side, is a consequence of how the Moon originally formed.
The general consensus on the Moon`s origin is that it probably formed shortly after the Earth and was the result of a Mars-sized object hitting Earth with a glancing, but devastating impact. This Giant Impact Hypothesis suggests that the outer layers of the Earth and the object were flung into space and eventually formed the Moon.
The Earth and the impact object did not just melt; parts of them vaporized, creating a disk of rock, magma and vapor around the Earth.
The geometry was similar to the rocky exoplanets recently discovered very close to their stars, said Wright. The Moon was 10 to 20 times closer to Earth than it is now, and the researchers found that it quickly assumed a tidally locked position with the rotation time of the Moon equal to the orbital period of the Moon around the Earth. The same real estate on the Moon has probably always faced the Earth ever since. Tidal locking is a product of the gravity of both objects.
The Moon, being much smaller than Earth cooled more quickly. Because the Earth and the Moon were tidally locked from the beginning, the still hot Earth - more than 2500 degrees Celsius - radiated towards the near side of the Moon. The far side, away from the boiling Earth, slowly cooled, while the Earth-facing side was kept molten creating a temperature gradient between the two halves.
This gradient was important for crustal formation on the Moon. The Moon`s crust has high concentrations of aluminum and calcium, elements that are very hard to vaporize.
The reports have been published in Astrophysical Journal Letters.