How Did the Moon Form? We May Need a New Theory

There may be much more water on the moon than we thought. And that could change everything.

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The Genesis Rock, a sample of the moon gathered by Apollo 15 astronauts James Irwin and David Scott in 1971, and assumed to be about as old as the moon itself (Wikimedia Commons)

How did the moon become the moon? Where did it come from? How did it first form?

We don't know in any fully satisfying way, but we do have a compelling theory in the form of the giant impact hypothesis. Per the theory, early in Earth's history -- when Earth was still, technically, proto-Earth -- an object the size of Mars slammed into the planet. The impact of the collision generated a ring of debris -- and the planetary detritus coalesced slowly (very slowly: over the course of millions of years) into the gray, glowing spheroid that has since been a source of fascination to scientists and poets alike.

The giant impact hypothesis, as its name suggests, is just that: a hypothesis, a theory, an educated guess. Some of the evidence for it comes from computer simulations suggesting how Proto-Earth could become Proto-Earth Plus Moon. Some of the remaining evidence, though, comes from the unique geological properties of the moon: dusty, dry, aggressively barren. Which means that the rocks of the moon, per the giant impact theory, hold some of the answers to the mystery of its creation. One of the assumptions made by the theory is that the impact itself would have essentially de-gassed the proto-moon, dispersing, among other gases, hydrogen -- the element, along with oxygen, required to form water. 

A theory based on the expulsion of water-making elements plus an obviously arid moon: a hypothesis and a phenomenon that jive well together.

Here's the problem, though: A string of recent research has suggested that there's more water on the moon than we previously thought. (Though liquid water can't persist on the moon's surface, scientists can search for "water" in the form of ice, and also in the form of hydrogen and oxygen atoms extant in the lunar landscape.) In 2008, Space Ref notes, an analysis of Apollo lunar samples conducted by ion microprobe detected indigenous hydrogen in the moon rocks. And in 2009, NASA's Lunar Crater Observation and Sensing Satellite slammed into a permanently shadowed lunar crater, ejecting a plume of material that ended up being rich in, yep, water ice. Hydroxyls -- chemical functional groups consisting of one atom of hydrogen and one of oxygen -- have also been detected in other volcanic rocks and in the lunar regolith, the layer of fine powder that coats the moon's surface.

So we've already had a hunch that the moon is wetter than we realized. And a paper published this weekend gives further evidence to that idea. A team at the University of Michigan re-analyzed the Apollo samples -- specifically, a sample of rocks gathered from the lunar highlands, which are thought to represent a crystallized form of the magma that originally formed the molten moon. (The sample included the "Genesis Rock," so named for its estimated 4.5 billion-year age -- which would date it around the time that the moon itself is thought to have formed.) The researchers, knowing that gaseous elements have a way of binding to and becoming absorbed by moon rocks, used cutting-edge Fourier-transform infrared spectroscopy to analyze the gaseous components of the Apollo sample. And they found evidence of ... hydroxyl. Which might also be evidence of ... water

The team found about six parts per million of water -- quantities large enough to suggest that the water didn't just come from elements in errant comets or asteroids. "The surprise discovery of this work is that in lunar rocks, even in nominally water-free minerals such as plagioclase feldspar, the water content can be detected," researcher Youxue Zhang put it. And given the age of the sample and the amount of hydroxyl discovered, the scientists say, the water-forming elements must have been on the moon since its formation. "Because these are some of the oldest rocks from the moon, the water is inferred to have been in the moon when it formed," Zhang told TG Daily.

The team's findings -- "Water in lunar anorthosites and evidence for a wet early Moon" -- were published this weekend in the journal Nature Geoscience.

None of that means the impact formation theory is invalid -- that the moon theory is moot. It means, though, that the giant impact theory now has another layer of complexity, in the form of evidence that would seem to contradict it. We've long known of the presence of water -- and of water-forming elements -- on the moon; the question is how it got there. Previous research suggested that those elements might have been brought to the moon from outside sources like comets and meteorites after the moon's crust formed and cooled. But evidence of water-forming elements in lunar-native rocks complicates that theory. "I still think the impact scenario is the best formation scenario for the moon," study leader Hejiu Hui, an engineering researcher at the University of Notre Dame, noted, "but we need to reconcile the theory of hydrogen."

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Megan Garber is a staff writer at The Atlantic.

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