The Non-Cheese Hypothesis: Where did the moon actually come from?

The moon is a frustrating reminder of the insignificance of the human race. Most every night we can look up, glimpse this celestial body and see enough detail to be reminded that there is a whole lot of amazing stuff going on in the universe that we can only experience on the most superficial level. The moon appears so damn close, but in reality is near the limits of our ability to explore. It took the combined efforts of the world’s two superpowers over 2 decades in the mid-twentieth century using all their resources and know-how just to get a few guys onto to moon’s surface and bring back some rocks. Generally speaking, the moon confounds us.

Nowhere is our confounditude better represented than in the simplest question that pops into a person’s mind when they sit and stare up at the shape-shifting orb in the night sky: where the hell did that thing come from? When we are kids we get all kinds of hair-brained explanations from story books and our parents that amount to a collective shoulder-shrug via their implausibility. I mean, come on, made of cheese? Are there cows in space I don’t know about or have we been ejecting sub-par dairy into orbit for the past couple thousand years?

The truth is, we would walk on the the moon's surface, orbit it a bunch of times and listen to ‘Beat It’ before scientists had a generally agreed upon theory of where the moon actually came from. It wasn’t until a conference in Hawaii in 1984 that two papers written nearly ten years earlier became accepted as sound science. Each pronounced that the moon was the result of an impact between the Earth and some other object. But this impact is like nothing we can remotely imagine. It makes the impact that ended the age of the dinosaurs look like a planetary bonk on the head. The impact that gave us the moon, to follow the comparison through, would have been like crashing a motorcycle into a wood-chipper.

Around 4.6 billion years ago – or so the story goes – the Earth was minding its own business, spinning on its axis and orbiting the sun, as planets do. Then seemingly out of nowhere, a spunky upstart proto-planet called Theia – presumably doing the planetary equivalent of texting while driving – absent-mindedly smashed into the Earth.

The collision completely fractured and melted the surface of both planets and flung material into the surrounding space. Fortunately for us, Theia was only about 10% the size of the Earth (roughly the size of Mars) so the Earth was able to pull itself mostly back together. The leftover material came together and began drifting around our planet and eventually became the moon, but for a few thousand years an Earthbound view of the night sky would have been a decidedly more dramatic experience with rocks the size of mountains smashing into each other and recombining constantly.

As elegant and awesome as this theory is, it took until April 2015 for it to get the evidence really needed to support it. Indeed, earlier this month three papers were published that provided much-needed corroboration for the giant impact hypothesis.

The first explained why the Earth and the moon are so similar. See, if the moon was the result of another planet smashing into the Earth we would expect it to be made of profoundly different material… but it isn’t. Researchers now explain this with new evidence suggesting that objects that form in proximate parts of the solar system are made of strikingly similar stuff. It appears Theia may have been a chemical baby brother to the Earth rather than a real alien.

The other two papers, oddly enough, support the impact hypothesis by explaining why the moon is different from the Earth. The first, published in 2014, found that the proportion of the element Tungsten in the moon's crust is greater than in the Earth’s. The second paper, published this month, explains this difference by suggesting that the amount of Tungsten in the Earth and the moon was roughly equal immediately after the collision with Theia; but the Earth, which is much bigger than the moon, attracted more asteroid and comet impacts in the meantime and thus added more material to dilute the amount Tungsten relative to the rest of the planet.

So that is where we stand. We finally feel like we know a very basic thing about the space rock we have been staring at since the dawn of time. Fine work humanity. Better late than never.