Four-and-a-half billion years ago, something humongous sideswiped our planet. The debris from the collision was tossed into the sky and eventually coalesced into a brand-new body that began following Earth around its orbit, mimicking Earth's spin as it circled tightly around it. Gradually, like a tiring partner in a very long dance, the smaller body began to slow down and drift away until it reached its current spot in space.
For years, that has been the best explanation scientists have come up with for the existence of the moon. The only problem, said Sarah Stewart, a planetary physicist at the University of California at Davis, is that this explanation isn't all that great.
“The whole giant impact model had been put into crisis several years ago,” she said, “to the point where people thought it might be completely wrong because we couldn't make it work in its details.”
The Giant Impact Hypothesis explained some odd attributes of Earth's only satellite: the fact that its orbit matches the Earth's spin, the moon's relative lack of iron (which is a very dense element and wouldn't have blasted up well), rock samples that indicate that the lunar surface was once molten, and more. But it couldn't resolve other strange facts, chief among them that the moon and the Earth are made of almost exactly the same stuff — the hypothesis predicts that the moon was composed mostly of material from the colliding body. Another problem for the theory is the fact the moon doesn't orbit around Earth's equator, but instead spins around our planet at a five-degree angle.
Various studies have put forward possible explanations for these complications. For example, maybe another mysterious body moved through the solar system and its gravitational pull tilted the moon after it was formed. But many of those ideas required one already unlikely event — a dramatic impact with another giant object — to be followed by another, and as Stewart explained, “If you do that too many times, everyone in the room starts squirming.”
“For those of us who live and breathe planetary science,” Stewart said, “the chemistry of the moon and the inclination [tilt] of the moon are major unsolved problems. Any theory that is going to stand the test of time has to get us there.”
This week in the journal Nature, she, postdoctoral fellow Matija Ćuk (now a researcher at the SETI Institute) and two other colleagues propose a new theory, one that Stewart argues offers a solid explanation for the chemistry and tilt of the moon based on a single, high-energy collision.
“You have the one event and then you stand back and just watch and everything will happen on its own,” she said of the new scenario.
But just because it's set off by one event doesn't mean it's not a complicated process, so buckle up.
Stewart and Ćuk have been working on their model — call it Giant Impact With a Twist — for a few years. It starts with a crash. But instead of striking Earth at an angle, they argue that the collider, which scientists call Theia, smashed into our planet head on. The high-energy impact would have melted the collider and the Earth and mixed them together, explaining how the planet and the moon could condense from the same swirl of material and take on the same chemical signatures.
“A more violent impact … that vaporized a good part of the Earth as well as the projectile would homogenize the isotope ratios,” Jay Melosh, a planetary scientist at Purdue University, said in an interview.
He noted that this model solves the moon's chemistry problem, but it creates a physics problem: “Those impacts would leave the moon and the Earth spinning too fast with too much angular momentum.”
Angular momentum — a measurement of the intensity of an object's spin — doesn't change unless some other force affects it. So in their new paper, Stewart and Ćuk needed to explain how a single impact could set up a scenario in which the moon and the Earth's angular momentum evolved to what we see today.
They theorize that the impact could have caused Earth to wobble dramatically, tilting the planet (and the moon) at a 60- to 80-degree angle from their normal plane. It also sent the Earth and moon spinning at an incredibly rapid pace: in the immediate aftermath of the crash, the Earth rotated so quickly that days lasted no more than two hours.
Over time, the gravitational interaction between the Earth and the moon, particularly the way the moon influenced the Earth's tides, caused both bodies to slow down and the moon to start drifting away (Cornell's Ask an Astronomer has a good explanation of why this happens). Eventually, the moon reached the “LaPlace plane transition,” a point where the influence of the Earth on the moon became less important than the influence of the sun on the moon. As a result, the Earth-moon system lost some its angular momentum to the growing gravitational relationship between the moon and the sun. It's as if the moon, while dancing around the Earth, suddenly noticed there was something bigger and brighter at the center of the solar system. The loss of angular momentum was enough to flip the jilted Earth back upright, so that it was tilted at more or less the same 23-degree angle we experience today.
But the moon — which, it has already been established, is a terrible dance partner — didn't follow suit. It continued to orbit at the ridiculous 60- to 80-degree angle of early Earth. The oblique angle changed the way the moon affected the Earth's tides. Not only did tides slow down the Earth and the moon and push the moon away, they also slowly dragged the moon's inclination from its crazy angle closer to Earth's equator. But because the moon started off so tilted to begin with, it couldn't quite reach the same plane that the Earth rotates on. By the time the moon reached a second transition point, the Cassini state, where its orbit stabilized, it was orbiting at a five-degree angle to the Earth.
In a previous study, published in Science in 2012, Stewart and Ćuk showed how this model can partly explain the moon's chemistry. Stewart says that their new research demonstrates mathematically how the high-energy impact can explain its particular orbital dynamics, using principles of astronomy (the effects of tides, the LaPlace plane transition, etc.) that are already well known.
“The importance of all of this … is that one giant impact sets up everything about the Earth-moon system. It makes the moon, gives the moon the right chemistry, and because the Earth starts off tilted over in the right way, we can remove angular momentum,” Stewart said. “The only thing we did was a step at the beginning. We didn’t have to ask for a step to come later.”
Melosh is definitely intrigued by the new idea: “Here is a scheme that seems to solve both problems at once,” he said, referring to the chemistry and inclination issues that have plagued the Giant Impact Hypothesis. “It does require things to be set up just so” — lowering the likelihood that it could have happened — “but then again, we do only have one example [of a moon like ours]. You can say, well it took something really special to make what we have, and maybe that’s right.”