Astrobiologists have solved a 55-year-old Moon mystery known as the Lunar Farside Highlands Problem.

When looking at the Moon from Earth, one of the first things you notice are the large, dark areas of basalt seas known as maria. These dark spots are what give the Moon it’s familiar ‘face.’ For centuries this was the only view of the Moon that humankind knew because the nearside always faces planet Earth.

In 1959, the Soviet Luna 3 spacecraft returned the first images of the Moon’s farside. Scientists were surprised to find that very few maria existed on this side of the Moon, which permanently faces away from our planet. Nobody knew why the two sides of the Moon were so different. Now, a team of astrophysicists from Penn State may have solved the mystery.

The Moon’s crust is thin on the nearside and thick on the farside. Both sides of the Moon were pummeled by impacts in the past, but when objects struck the nearside of the Moon they punched through the thin crust and released basaltic lava that formed the maria. On the far side, the crust was thick enough that few meteorites actually made it through.

To help understand why the crust of the Moon is different on the nearside and farside, the scientists gathered clues from studies of extrasolar planets that orbit near their host stars. They found that the differences in crustal thickness could be a relic of the Moon’s formation circumstances.

The Moon was likely formed when a Mars-sized object crashed into the early Earth. The massive impact created a ring of dust and debris that eventually coalesced into the Moon. The young Moon orbited close to the Earth with its nearside locked toward the planet. As it cooled, the nearside was cooked by Earthshine from the still-hot Earth. This caused differences in how the lunar atmosphere condensed on both sides of the Moon, resulting in more crust-forming material snowing down onto the farside.

The study was published in the The Astrophysical Journal Letters, and supported by the NASA Astrobiology Institute and the Pennsylvania State Astrobiology Research Center.