Dark Side of the Moon

In 1959, the Soviet spacecraft Luna 3 took the first pictures of the mysterious dark side of the Moon. To widespread surprise, the previously unseen lunar surface contained almost none of the large dark areas, known as lunar maria or “seas,” that dominate the familiar side of the Moon. In fact, maria make up only around 2 percent of the dark side of the Moon. And a mystery was born.

Maria are actually large basaltic plains formed by volcanic activity. A consensus eventually formed that a thicker crust prevented maria from forming on the dark side. That didn’t really solve the central mystery. Why was the dark side of the Moon different from the side that faces us? Why was the crust so thick there? The question remained unanswered for over 50 years, but astrophysicist Jason Wright now claims to have cracked the case.

            There is an existing theory that the Moon was formed from the debris scattered when a Mars-sized object collided with the Earth. The collision would also have created an enormous amount of heat. Since it’s much smaller than the Earth, the Moon would have been faster to cool off, but the heat radiating out from the molten planet would have warmed the side of the Moon facing Earth. The faster cooling process on the Moon’s far side created the thick crust that kept lava from reaching the surface. Astronomers usually dislike the term “dark side of the Moon” since both sides receive the same amount of light from the Sun, but it seems as though the far side genuinely was once darker, thanks to being shielded from the smoldering Earth.