When meteoroids struck the Earth-facing side of the moon, they created large flat seas of basalt that we see as dark areas called maria. These areas are what give the moon its “face.” But no “face” exists on the far side of the moon and until now, scientists have not known why.
“I remember the first time I saw a globe of the moon as a boy, being struck by how different the far side looks,” says Jason Wright, assistant professor of astrophysics at Penn State. “It was all mountains and craters. Where were the maria? It turns out it’s been a mystery since the fifties.”
The mystery—called the Lunar Farside Highlands Problem—dates back to 1959 when the Soviet spacecraft Luna 3 transmitted the first images of the dark side of the moon back to Earth. It was called the dark side because it was unknown, not because it never sees sunlight.
Researchers at the time immediately noticed that fewer “seas” or maria existed on that far side.
As reported in a new study in Astrophysical Journal Letters, the researchers 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.
How the Moon Formed
The 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.
The Giant Impact Hypothesis suggests that the outer layers of the Earth and the object were flung into space and eventually formed the moon. “Shortly after the giant impact, the Earth and the moon were very hot,” says Steinn Sigurdsson, professor of astrophysics.
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 moon and Earth loomed large in each others’ skies when they formed,” says lead author Arpita Roy, graduate student in astronomy and astrophysics.
The geometry was similar to the rocky exoplanets recently discovered very close to their stars, Wright says. The moon was 10 to 20 times closer to Earth than it is now. 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 2,500 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. “When rock vapor starts to cool, the very first elements that snow out are aluminum and calcium,” Sigurdsson says.
Aluminum and calcium would have preferentially condensed in the atmosphere of the cold side of the moon because the nearside was still too hot. Thousands to millions of years later, these elements combined with silicates in the moon’s mantle to form plagioclase feldspars, which eventually moved to the surface and formed the moon’s crust, Roy says. The far side crust had more of these minerals and is thicker.
The moon has now completely cooled and is not molten below the surface. Earlier in its history, large meteoroids struck the nearside of the moon and punched through the crust, releasing the vast lakes of basaltic lava that formed the nearside maria that make up the man in the moon.
When meteoroids struck the far side of the moon, in most cases the crust was too thick and no magmatic basalt welled up, creating the dark side of the moon with valleys, craters, and highlands, but almost no maria.
The NASA Astrobiology Institute and the Pennsylvania State Astrobiology Research Center supported this work.