Mystery of the Dark Side of the Moon Solved!
Everyone knows the man in the moon - the friendly face that guides you through the night.
That face is created from maria, or "seas," large flat areas of basalt on the moon's near side. But what about that famous "dark" side of the moon?
That's where The Lunar Farside Highlands Problem comes in. It poses the question: why are there dark maria on the near side but not the far side?
The mystery dates back to 1959 when the Soviet spacecraft Luna 3 transmitted the first images of the dark side of the moon and researchers noticed fewer maria than existed on the near side.
We know now that this absence of basalt is due to a difference in crustal thickness between the near and far side of the moon, and is a direct consequence of the way in which the moon was originally created.
Our moon was formed by a Mars-size object hitting Earth and flying back out into space, taking the outer layers of the Earth with it. Shortly after the impact, both the Earth and the moon were at least partially melted. The moon was 10-20 times closer to Earth than it is now, and it quickly established a tidal lock, meaning that the same side of the moon has faced towards the Earth since then.
Early in its history, large meteoroids struck the nearside of the moon, and punched through its crust, releasing the vast lakes of basaltic lava that formed maria. But when they struck the far side, the crust was too thick, and no magnetic basalt welled up, leaving the dark side with valleys, craters, and highlands, but no maria.