New evidence that the Moon's explosive birth was weirder than we thought

Scientists have long believed that Earth's moon was born with a bang, in a colossal planetary collision commonly dubbed "The Big Splat." A study published in the latest issue of Nature all but confirms this violent hypothesis — but two additional studies published today in Science make the story of the Moon's origins far weirder and more mysterious than previously believed.


Yes, it Started With A Bang

The Big Splat story goes like this: 4.6-billion years ago, a Mars-sized body cannoned into our home planet, ejecting vast quantities of debris into Earth's orbit. Over time, this disk of debris came together to form the Moon we know today.

The Big Splat hypothesis has been widely accepted for decades, partly because of elemental evidence. Moon rocks tend to be lacking in elements like sodium, potassium, zinc and lead. These so-called "volatile elements" evaporate faster and dissipate more easily from vaporized rock (the vaporized rock that comes from a cataclysmic planetary collision, for example) than other members of the periodic table, so their absence from lunar samples would make sense in a Big Splat timeline.


But there's one hangup. Scientists investigating lunar rocks expected to find evidence of something known as "isotopic fractionation." Lighter isotopes enter the vapor phase faster than their heavier counterparts, so a Big Splat should have left the Moon with a higher proportion of heavy elemental isotopes. But when researchers probed lunar samples for signs of isotopic fractionation, they came up empty handed. [Artist's conception by Ron Miller]

Finally, in today's issue of Nature, planetary scientists present the first-ever example of this long-sought evidence, demonstrating that moon rocks collected over the course of numerous Apollo missions are all richer in heavy isotopes of Zinc than lighter ones. "This is very exciting," UC San Diego geochemist and study co-author James Day told io9. According to Day, the discovery of isotopic fractionation represents the first major evidence for a global vaporization event since the detection of volatile depletion in moon rocks way back in the 1970s.


"How do you remove all of the volatiles from a planet, or in this case a planetary body?" inquires Day. Then he answers his own question: "You require some kind of wholesale melting event of the moon to provide the heat necessary to evaporate the zinc. This strongly suggests a catastrophic origin for the Earth and Moon."

Just How Big Was The Big Splat?

But, as is so often the case in science, there's much more to the story. Sure, Earth's moon was likely born out of a collision between two objects, but a pair of studies published in today's issue of Science take two very different stances on the relative sizes of those objects.


The first study, conducted by planetary scientist Robin Canup, maintains the body that collided with Earth was not Mars-sized, as presumed by the longstanding Big Splat model, but something significantly larger.


"Mars is only about ten percent the mass of the Earth," Canup tells io9. "What I've modeled is a collision between two like-sized things."

In Canup's simulation, the impactor and the target each contain about half the mass of late-date Earth. These bodies first meet in a low-velocity impact, only to re-collide a short time later. Within 27 hours of first contact, the two orbs have merged into one, Earth-mass entity, encompassed by vaporized bits of protolunar rock.


In a symmetrical impact such as this, "the disk that forms around the Earth has about half its mass coming from the impactor, and half its mass coming from the target," explains Canup. The same goes for the newly formed planet. "Even if the impactor and the target originally had very different compositions — which we think is likely — they mix equally, so the final planet and the disk have the same composition." In Canup's simulations, screenshots of which are featured here, the composition of the disk and the outer layers of the planet differ by less than 1 percent.

The second study, conducted by Harvard planetary scientists Matija Ćuk and Sarah T. Stewart, presents a very different kind of impact. For one thing, Ćuk and Stewart envision a much smaller impactor — less massive, even, than the Mars-sized object in the conventional Big Splat model. But they also model it colliding with Earth at very high velocity:


"Both models involve impactors that are dramatically different from the Mars-sized object [commonly associated with the Big Splat]," explains Ćuk in an interview with io9. Both generate a disk-planet pair with an almost identical geochemical composition. And both models present compelling, new hypotheses for the creation of our present Moon-Earth system. And yet, both the large, slow-impactor and the small, fast-impactor model would have been labeled impossible as recently as two years ago. Why? Because both of these models incorporate what was once thought to be an impossibly fast-spinning Earth.

Making "Impossible" Collisions Possible

What makes the impact models from Canup, Ćuk and Stewart so different from past simulations is that they each incorporate an Earth that rotates 2 to 2.5 times faster than was previously considered possible.


For the last 4.5 billion years, the distance between the Earth and the Moon has been increasing. At the same time, the velocity at which Earth rotates on its axis has steadily slowed. Our days were once 5 hours long, and gradually increased to today's 24 hours. However, the angular momentum of the Earth-Moon system is thought to have remained more-or-less constant since shortly after the Big Splat.

But some months back, Ćuk and Stewart presented compelling evidence that a phenomenon known as "evection resonance" could have decreased the angular momentum of the Earth-Moon system by 2 to 2.5 times shortly following the Moon-forming impact, by way of a complicated gravitational interplay between the Earth, the Sun, and the newly formed Moon.


"It was after Ćuk and Stewart presented this evidence that I started looking at different types of impacts that would leave the Earth with that faster rotation rate," explains Canup, "something that would leave the Earth with a shorter, 2.5-hour day." So early Earth days might have been only 2.5 hours long.


At the same time, Ćuk and Stewart were busy simulating their own new brand of collision event, with three main differences:

1. They were simulating high-velocity collisions, not low-velocity ones like Canup.
2. They were considering small impactors, rather than large ones
3. They wanted to see what happened when an impactor collided with an Earth that was already spinning about its axis every 2—2.5 days.


The result is two very different models — not only from the original Big Splat hypothesis, but from one another. And in both cases, the "key ingredient," as Ćuk calls it, was the concept of evection resonance — that by way of this newly formed Moon, the Sun could rob the Earth-Moon system of its angular momentum, slowing the Earth's rotation in the process.

The researchers' findings are published in the latest issues of Nature and Science. Links will be provided when the papers go live.


Top image via NASA; second collision image by Ron Miller; Simulation image and video via Canup and Ćuk, respectively

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