Astronomers Discover Why the Dark Side of the Moon Doesn't Have a Face

Astronomers have solved one of the mysteries of the dark side of the moon. They've found out why there's a "face" on the Earth-facing side of the moon and why it doesn't exist on the dark side of the moon.

The "Man in the Moon" that we see in the night sky first appeared when meteoroids struck the Earth-facing side of the moon. This created large flat seas of basalt that we see as dark areas called maria. Yet no such phenomenon exists on the dark side of the moon. That's why scientists decided to take a closer look.

Most scientists believe the moon was first created shortly after the Earth formed. It was probably the result of a Mars-sized object hitting Earth with a glancing, but devastating impact. The debris that resulted from this impact eventually formed the moon. Yet in this distant past, the moon was 10 to 20 times closer to Earth than it is now. This caused it to assume a tidally locked position with the rotation time of the moon equal to the orbital period of the moon around the Earth.

The researchers believe that the moon cooled quickly after the heated blast of the impact. Yet the Earth was still hot-more than 2,500 degrees Celsius. This heat radiated toward the near side of the moon, which cause it to continue to be molten. The far side of the moon, though, slowly cooled. The moon's crust had high concentrations of aluminum and calcium, which are elements that are very hard to vaporize. These elements would have condensed in the atmosphere of the cold side of the moon. Then, they would have combined with silicates in the moon's mantle to form plagioclase feldspars, which eventually moved to the surface and formed the moon's crust. The farside crust had more of these minerals and is thicker.

That wasn't the end of the moon's formation, though. Large meteoroids struck the nearside of the moon and punched through the crust. This released the vast lakes of basaltic lava that formed the nearside maria. Yet when these meteoroids impacted the farside of the moon, the crust was too thick to punch through, which explains why the farside has no "Man in the Moon."

The findings are published in the journal Astrophysical Journal Letters.