The Moon's far side, although not lacking for light, remained dark in the sense of hidden or obscured until the space race between the US and USSR took aim at the Moon. The Soviets' Luna 3 probe returned the first images of the far side in 1959, and the results were a bit of a surprise. The near side is covered with large, dark, basaltic flows that are called maria; these are rare on the far side, which is dominated by the rugged lunar highlands. A number of explanations have been offered for this difference, but today's issue of Nature contains what is certainly the most dramatic one yet: it suggests that the highlands are the remains of the Earth's missing moon, plastered across the far side of the one remaining Moon.
A consensus has formed around the theory that the Moon originated from a collision early in the history of the solar system, when a near-Mars sized body smacked into the Earth. The resulting debris coalesced into two bodies. Models of this process nicely account for some of the difference between the Earth and the Moon, including Earth's large, iron rich core. (Robin Canup, who does some of this modeling, has placed videos of the process on her website.)
Frequently, these simulations produce a three body system: the Earth, the Moon, and a smaller companion. In most of these cases, the smaller companion is quickly swallowed up by the Moon while it still primarily molten, erasing all traces of it. But the authors suggest a possible alternative: a small moon could end up in one of the Trojan points, where the gravity of the Earth and Moon cancel each other out, providing a semi-stable home. In this situation, the small companion would be stable for up to 70 million years before a resonance with the gravity of the Sun would pry it from the Trojan point. That would be enough time for the Moon to develop a crust, and for the smaller body (we'll call it Moon II) to solidify entirely.
The authors went on to model what would happen if Moon II, once pried out of the Trojan site, were to end up having its own collision with the Moon. Moon II was estimated to be about a third the size of the existing Moon, with a similar composition, except that it would have an entirely solid crust and core, since its small size would allow it to cool faster. The Moon itself was estimated to still have some molten material (a 50km deep magma ocean), with a 20km deep solid crust floating on top of it. The whole system was modeled as a set of blocks 5km on a side. All that computation was apparently quite expensive, as the authors only test two different collisions, one head-on, the other at a 45 degree angle.
Both of these runs assumed relatively low velocity, just over the two-body escape velocity: 2.4km/s. Because of the difference in masses involved, the authors estimate that the Moon/Moon II impact would carry only 2.5 percent of the kinetic energy that the Moon-forming impact did. It's also below the speed of sound in silicates, one of the primary components of the two bodies involved. And these factors, the authors say, is enough to make it a qualitatively different collision. "Our primary finding," they note, "is that a companion moon, 1/3 the diameter of the Moon, striking at subsonic velocity, does not form a crater." The volume of the impacting body ends up exceeding the volume the impact could possibly excavate. "The impact produces an accretionary pile rather than a crater."
But that doesn't mean that it has no effect on the Moon. For starters, their model suggests that the majority of the magma ocean would get pushed to the opposite side of the Moon, which would explain the preponderance of Maria on that side. In addition, most of the material from Moon II would stay near the point of impact, "pasting on a thickened crust and forming a mountainous region comparable in extent to the far side highlands," they conclude. In short, their model produces something that looks a lot like the actual Moon.
The problem is that, since Moon II probably looked a lot like the Moon in terms of its composition, there's no obvious way of telling which rocks came from which. Crustal rocks originating on the Moon have a wide spread of ages (about 200 million years), which is consistent with multiple origins, but could also be consistent with uneven cooling. And, as noted above, this isn't the first model proposed for the differences between the near and far sides of the Moon (alternatives include things like uneven tidal heating and a large impact near the Moon's South Pole).
Fortunately, a mission that may help resolve this (or at least eliminate the impact model) is already in progress. NASA's GRAIL (Gravity Recovery and Interior Laboratory) will produce the same sort of gravity maps that the GRACE mission is making for the Earth. GRAIL is scheduled to launch next month. If the authors are right, the magma that was pushed off the far side should have left some indications of its shift behind, and these should show up in the gravity analysis.