An important new dynamical effect has made it easier for scientists to simulate the Moon's formation following a giant collision with primordial Earth
One of the solar system's most nagging problems literally stares into the collective faces of planetary scientists on many clear nights every month. It's the Moon — or, specifically, how it came to exist.
Prior to the Space Age, scientists kicked around three models of lunar origin. In the first, the Moon and Earth simply formed together, existing as paired worlds from the outset. A second concept envisioned that the Moon was a maverick world that strayed too near Earth and became trapped in orbit around it. In the third model, Earth somehow began to spin so fast that a huge blob of its crust and mantle tore away to create the Moon. However, this "fission hypothesis" would have left the Earth-Moon system with far more angular momentum than it has today.
Once researchers got their hands on actual samples of the Moon, it became clear that none of these models was a good fit. For example, lunar rocks lack the water, iron, and certain other metals that are common on Earth, so forget about side-by-side formation. Yet the relative abundances of oxygen's three isotopes in lunar and terrestrial rocks are identical, so the Moon and Earth must share some common genetic link — and that rules out the capture hypothesis.
About 30 years ago a radical new idea emerged. If a body roughly the size of Mars had struck the very young Earth, then superheated vapor and debris from this "big splat" could have formed an orbiting ring of matter that quickly coalesced into a single large object. Making the Moon this way would satisfy a host of physical and geochemical constraints — but not all of them.
All along researchers realized that this scenario only works if the impactor dealt Earth a glancing blow, in order to eject a Moon's worth of debris into orbit. But decades of computer simulations of a glancing blow have all yielded the same result: a lunar composition with too much impactor and too little proto-Earth.
Now there's a breakthrough that sidesteps this geochemical dilemma. The new results were published and also presented at a big meeting of planetary scientists last month, and we noted them at the time, but I want to delve into the story a little more deeply here.
The key is what happened after the impact. Initially, the Moon would have been very close to Earth. Strong tidal forces would have transferred energy to the Moon's orbit, pushing it farther outward, at the expense of Earth's rotation, which gradually slowed. (This process continues slightly even today.) Soon the two bodies became coupled in what's called an evection resonance. Dynamicists Jihad Touma and Jack Wisdom had probed this effect back in 1998. They found that it could pump up the Moon's orbital eccentricity and inclination — neatly explaining why the lunar orbit is tipped so much with respect to Earth's equator — but leaving the system's total angular momentum virtually unchanged.
However, Matija Ćuk (SETI Institute) and Sarah Stewart (Harvard University) have taken another look at the evection resonance and discovered it that could have lasted long enough to transfer much or even most of the post-impact angular momentum to the Sun. As they report in Science Express for October 17th, this means Earth could have been spinning so rapidly when it was hit, rotating once in just 2½ hours, that it was close to flying apart on its own.
With angular momentum no longer a constraint, it's much easier to fashion a Moon with the right mass and with a composition identical to that of Earth's mantle. Ćuk and Stewart's simulations envision relatively small impactors with only 3% to 5% of Earth's mass. They get the best fit to lunar-sample constraints with head-on and slightly retrograde impacts at velocities from 10 to 30 km (6 to 20 miles) per second.
Robin Canup (Southwest Research Institute) uses the relaxed angular-momentum constraints to try a very different approach. In her computer simulations, the impactor and primordial Earth have essentially the same mass. The two strike obliquely, though more nearly head-on than in her prior runs. The wildly gyrating two-lobed blob that results eventually settles into a molten, iron-rich central mass surrounded by a white-hot silicate-rich disk. The planet and disk end up with identical isotopic ratios, Canup explains, because the impactor adds its substantial mass to the planet and the two bodies become compositionally blended.
Historical footnote: The fission model for the Moon's origin was originally proposed by George Darwin (Charles's son) in 1879. He calculated that if the primordial Earth had spun faster and faster, it would have assumed a bowling-pin shape just before a large chunk broke away to form the Moon. (He calculated correctly that the spin rate would have to be about 2½ hours for this to occur.)
With a few notable exceptions, latter-day scientists dismissed the idea because there was no obvious way to slow down Earth's spin enough after the Moon calved away. Now they realize it's quite doable.
There might still be a "gotcha" in all this. The Earth-Moon system ends up with the right amount of angular momentum only if the two bodies remain locked in the evection resonance long enough after the impact, and if each body has just the right tide-inducing characteristics during that time. But at least the isotopic mismatches have been resolved, and it's more certain than ever that the Moon had a very dramatic birth.