Heights of Lunacy

"There is no dark side of the moon really.
Matter of fact it's all dark."

While the Moon has a nearside and a farside, it does not, actually, have a dark side or a light side, now. At least not a fixed dark side, just a slowly moving night side, and day side.

But it used to. Sorta.

"I remember the first time I saw a globe of the moon as a boy, being struck by how different the farside looks," said Jason Wright, assistant professor of astrophysics. "It was all mountains and craters. Where were the maria? It turns out it's been a mystery since the fifties."

Jason, Arpita Roy a Penn State grad student, and I, just published a paper on the "Lunar Farside Highlands Problem.

"EARTHSHINE ON A YOUNG MOON: EXPLAINING THE LUNAR FARSIDE HIGHLANDS" - Roy, Wright and Sigurdsson, ApJL 788, L42 (2014) (pdf) or here (subscription)

The basic idea, as explained in Jason's excellent series of "Hard Rain" blog post, is that given Lunar formation by impact, the young Moon forms molten and very close to the Earth, where it tidally locks very rapidly, while the Earth is still molten and looming very large in the sky.
Thus the young Moon's temperature as it cools is regulated not by insolation but by "Earthshine" or interration
- is that even a word? well, now it is! -
the nearside the side of the Moon facing the Earth, sees the thermal glow of the molten Earth for a long time, as the Earth cools more slowly than the Moon. In contrast the farside, the dark side, radiates to cold space, heated only by the (faint young) Sun, and cools more rapidly.
This sets up a "cold trap" for the hot lunar atmosphere, and, for a while, hard rocks snow out on preferentially on the farside.
This, and some other supporting mechanisms, can set up the peculiar hemispheric dimorphism we see.
Or so we conjecture.

This was a fun paper.

Jason has much more.

A Hard Rain's A-Gonna Fall on the Dark Side of the Moon III: The Crust is Made With Refractories

"...So, bottom line:

1) The Moon has probably always had the same face towards the Earth, even during formation

2) The Moon's formation was a messy business, with vapor phase likely to have been important

3) Earthshine was an important component of thermal energy budget of the post-giant-impact system, and should have produced a chemical gradient in the protolunar nebula and proto-lunar atmosphere

4) The present-day chemical dichotomy on the Moon looks an awful lot like the result of the condensation gradient one expects from Earthshine.

5) The lunar farside highlands are the result of a primordial chemical gradient caused by tidal locking and the temperature gradient caused by the hot post-impact Earth and, possibly, the shadow proto-Moon."

55-year old dark side of the moon mystery solved - Penn State Press Release