Since the first images from the Soviet Luna 3 spacecraft were returned to Earth in 1959, scientists have wondered about the differences between the two halves of the Moon. The far side, the side that we do not see, has mountains, valleys and highlands. The side that we do see is much more flat with large “seas” of basalt which produce the darker areas. It is these darker areas which create the “man in the Moon.”

In science this is referred to as the “Lunar Farside Highlands Problem” and a paper published in the June 9 edition of Astrophysical Journal Letters proposes a solution to the riddle. Jason Wright, assistant professor of astrophysics at Penn State University and his colleagues Steinn Sigurdsson, professor of astrophysics and Arpita Roy, graduate student in astronomy and astrophysics, believe the difference dates back to the formation of the Moon.

The Giant Impact Hypothesis suggests that the Moon was formed shortly after the Earth. It is believed that a collision, or at least a glancing blow, with a Mars size object threw debris from the Earth and the other object known as Theia into space. It is this material that is believed to have formed the  Moon.

After the event the Moon became tidally locked with the Earth, which means that the side facing us is always facing us and does not rotate away. The impact would have caused intense heat and it is likely that the Earth and Moon remained molten for some time afterward. Because it is smaller, the Moon would have cooled more quickly than the Earth. The side facing the Earth, which was 10 to 20 times closer at the time,  would have absorbed additional heat from the Earth.

This uneven cooling would have lead to different concentrations of elements on the two sides of the Moon.

“When rock vapor starts to cool, the very first elements that snow out are aluminum and calcium,” said Sigurdsson in a statement.

Heavy concentrations of aluminum and calcium are known to exist on the Lunar surface. These elements would have been present in larger amounts on the far side because of the cooler temperatures. Hundreds or possibly thousands of years later the difference in the composition and temperature of the two sides would have caused them to react differently to meteor impacts.

On the far side of the Moon, which would have been cooler and thicker, the impacts would have simply left a crater. On the near side, larger meteorites would have punched through the thin surface, releasing basaltic lava forming maria that make up the man in the Moon.

While the Giant Impact Hypothesis itself hasn’t been proven yet, it gained new credibility earlier this week after an analysis of Moon rocks by German researchers.