Study: Why the Moon's Far Side Looks So Different

StartsWithABang writes 55 years ago, the Soviet probe Luna 3 imaged the side of the Moon that faces away from us for the first time. Surprisingly, there were only two very small maria (dark regions) and large amounts of mountainous terrain, in stark contrast to the side that faces us. This remained a mystery for a very long time, even after we developed the giant impact hypothesis to explain the origin of the Moon. But a new study finally appears to solve the mystery, crediting the heat generated on the near side from a hot, young Earth with creating the differences between the two hemispheres.

Rotation

[SimonInOz]

The moon became tidally locked within a few million years after its formation (around 4.5 billion years ago), so it's been tidally locked for over 4 billion years.

But really, did the earth stay hot enough for "a few million years" - hot enough to affect the locked side of the moon more than the other?

The moon would have cooled somewhat faster, being smaller, but this theory requires the earth to stay hot enough to affect the "earth side" of the for a very long time after the moon has cooled enough to solidify.

"Very Long Time?"

[Guppy06]

This remained a mystery for a very long time

Martians notwithstanding, nobody had any idea what the far side of the moon looked like before 1959 [wikipedia.org]. Sure, 55 years may be "a very long time" for some people, but we're not exactly talking about something that puzzled Hipparchus here.

Re:Rotation

[Dan East]

But really, did the earth stay hot enough for "a few million years" - hot enough to affect the locked side of the moon more than the other?

The moon has no atmosphere, thus radiation from the earth cannot affect the far side of the moon at all. So obviously, even to this day, the earth still affects "the locked side of the moon more than the other". The question is simply how much. The moon and earth were both molten after the collision, so it was not a matter of the earth being hot enough to melt the moon, but merely the earth imparting energy to prolong the cooling of the near side. No matter what, the near side must have cooled slower than the far side - it's a straightforward matter of thermodynamics. One side of the moon was receiving energy from the earth while the other side was not. The near side didn't need to stay so hot it was incandescent, but merely "softer" so that small impacts would heal more on the near side than the far side, and the duration only needed to be long enough to result in some degree of visible difference, which is what we still see today.

The whole thing sounds plausible to me.

Re:Rotation

[jfdavis668]

At the time, the Moon did have an atmosphere. It was hot enough to vaporize silicon and aluminum, components of today's crust. What the paper is proposing is that the hot Earth keep the Moon hot enough to keep these elements vaporized on the Earth facing side, while the far side was cooler and the elements precipitated out. This would cause a migration of crustal components to the far side, thickening the crust. This way it was not as susceptible to puncture by falling objects.