Why the Moon Only Has One Face

Astrophysicists believe that they have finally determined why the Moon's Earth-facing nearside, with its "Man in the Moon," looks remarkably different than its unseen farside.

The "Man in the Moon" is made up of massive flat "seas" of basalt that appear dark on the lunar surface. Astronomers have long theorized that these flat regions, called maria, formed when asteroids impacted a young Moon, breaking through its crust to release vast lakes of basaltic lava which then welled and cooled on the its surface.

All sides of the Moon boast an impressive number of craters from asteroid impact and early volcanic activity. However, this then begs the question: why does only the nearside have a face?

"I remember the first time I saw a globe of the Moon as a boy, being struck by how different the farside looks," Jason Wright, an assistant professor of astrophysics, said in a Penn State press release. "It was all mountains and craters. Where were the maria? It turns out it's been a mystery since the fifties."

This mystery - called the Lunar Farside Highlands Problem - dates as far back as 1959, when the Russian space program's Luna 3 obtained the first images of the Moon's farside - commonly known as the "dark side."

Now, according to a study co-authored by Wright and recently published in the Astrophysical Journal Letters, the Lunar Farside Highlands Problem may be solved.

Wright, Steinn Sigurdsson and lead author Arpita Roy believe that the manner in which each side of the Moon cooled determined how vulnerable it was to asteroid impact.

"Shortly after the Giant impact, the Earth and the Moon were very hot," Sigurdsson said, referring to when the Moon formed after a massive astronomical body known as Theia collided with the Earth.

Researchers say following this impact, the Moon - being smaller - cooled faster than the Earth. However, heat radiating from the still-hot Earth slowed the cooling rate of the Moon's nearside, whilst the farside continued to cool rapidly.

This difference in cooling rate would have caused elements alike aluminum and calcium to preferentially condense in the atmosphere of the Moon's cooler side, eventually forming a firm and thick crust. The crust on the nearside, however, would have remained relatively brittle in comparison, proving far more likely to collapse under the impact of asteroids.

This, the researcher claim, would then explain why maria are only found on the nearside, as the farside was far too thick for maria formation to occur.

The study was published in Astrophysical Journal Letters on June 9.