Why is the far side of the moon so weird?

We had to wait until 1959 to get our first glimpse of the far side of the moon. Due to a quirk of celestial mechanics, our nearest neighbour rotates at the same speed it orbits the Earth, which meant that no matter how familiar we might have been with one side of the moon, the other was a near-total mystery.

It was, however, not hard to guess what the far side of the moon might look like. The lunar surface has not exactly escaped attention over the course of history: the Greeks noted that its markings could have been caused by varied topology, while the diamond ring effect witnessed during a solar eclipse is more than a little suggestive of rugged mountains and plunging valleys.

By the 20th century, telescopic study had left the near side of the moon charted down to the mile. The brightly-lit areas of the lunar surface were mountain ranges, while the great grey ‘maria’ were the result of lava flows filling in the lowlands. Great impact craters (and even their children) were named and catalogued in beautiful lunar atlases. We had a pretty good idea, in other words, of what was going on up there.

So when the Soviets launched Luna 3, the first spacecraft to examine the far side, they thought they knew what they’d find in the photographs. There was no reason to expect anything other than a landscape more or less like the near side, albeit with more severe cratering to account for its positioning relative to Earth.

When the results were published, there were surprises in store. Yes, the cratering was more intense. But there was barely a mare in sight. Instead, the far side was almost entirely highland. Weirder still, further studies showed that the moon’s crust is actually 15km thicker on the far side than the near. What could possibly explain a few hundred trillion tonnes of rock turning up where it’s not supposed to?

Our current best guess involves enough destructive power to wipe out life on earth about a billion times over.

The evidence suggests that the moon was formed shortly after the rest of the solar system, when a rogue planet roughly the size of Mars ploughed into the newborn Earth. The collision was horrific, producing enough energy to liquefy the entire surface of the planet. Sizable chunks of both the rogue and its victim were thrown into orbit — debris that coalesced and cooled to become the moon.

So far so good, although the story hasn’t yet done much to explain hemispheric differentiation. Fortunately, we’re not done just yet.

Remember that quirk of celestial mechanics that left the moon’s near side facing permanently towards earth? It’s called tidal locking, and essentially involves tidal friction in the interior of a moon (or, indeed, a planet*) slowing down its rotation, acting as a giant brake until its day synchronises with its orbital period. The young moon was much closer to the earth at the time, and would have experienced full tidal locking within a few tens of millions of years.

*You’re probably wondering if this mechanism operates on Earth, too. The answer is yes — the moon has lengthened Earth’s day by something like 30 percent since it formed.

A more obvious consequence of physics was also in operation shortly after the Earth-Moon system formed: small bodies cool faster than larger ones. The moon, with a mass barely one percent of Earth’s, would have solidified much faster than its partner.

And this seems like the key to understanding just what happened to differentiate the near and far sides: even after the moon had cooled, it was orbiting a hellish open furnace. The Earth was so hot and so close that it prevented the formation of solid rock on the near side. Instead, the rock vapour in the moon’s atmosphere (which was reasonably thick at the time) would have preferentially condensed on the far side.

The sheer scale of this mechanism — we’re talking a millions-year-long storm so intense that it rained fifteen kilometers of solid rock over an entire hemisphere — feels almost obscene. But it’s an important reminder that no matter how well we might think we know something (and we thought we knew the moon reasonably well, right up until we didn’t), big surprises can be lurking anywhere

The above is a good story. But is it actually true? The short answer is that we’ll never know for sure. The longer answer is, I think, rather a lot more profound: science is a mechanism by which we select the stories that best explain our universe.

With the known facts as they are, the Furnace Earth theory is a neat explanation for the moon’s weird structure. Tomorrow, however, we might get new data that would force us to revisit our current ideas. After all, Ptolemy ‘knew’ that planetary motion is governed by epicycles, just as Newton ‘knew’ space and time were nice, static substrates within which he could cleanly embed his mechanics.

But no matter what, the moon is going to stay up there, being extremely serene and extremely weird. And we’ll keep telling ourselves stories about why.