The moon — for all its silent loveliness — was born in violence. About 4.5 billion years ago, a planet-sized object rammed the Earth and reduced it to a bleeding molten gob of starstuff, leaking like a Junior Mint shot by an air rifle. As the Earth slowly reformed, the debris left over from the invading object coalesced into the small companion satellite that gives us our tides and illuminates our romantic nighttime strolls.
This “giant impact” theory accounts for the Moon’s mass and its relative lack of iron, since computer models indicate that the iron core of the impactor object, rather than joining the rest of its material in the Moon, would have merged with the Earth’s. It also explains the Earth’s current 24-hour day. The collision, so the theory goes, helped the planet start spinning, at first with a day of 5 hours. As the Moon has grown farther and farther away over the eons, pushed by tidal interaction between the two worlds, preservation of angular momentum has slowed the Earth’s days down to reach the current, languid 24-hr. length.
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But as scientists have scrutinized Moon rocks brought back by the Apollo missions, they’ve discovered there is one thing a giant impact does not explain well: when it comes to certain aspects of geochemistry, the Earth and the Moon are virtually identical. The impactor thought to have provided material for the Moon would likely have been very different from Earth. Modelers have been working to see whether the debris from both bodies could have mixed thoroughly enough to explain the similarity, but it has been slow going.
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However, scientists publishing in this week’s Science have found a surprising new way to fill this gap in the theory, using a fourth player: the Sun. They then present two possible scenarios that could explain why the Moon is so Earth-like.
Planetary scientist Matija Cuk, affiliated with both Harvard University and the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, Calif., came up with the crucial breakthrough. Cuk is an orbits man; his coauthor, Harvard astrophysics professor Sarah Stewart, focuses on giant impacts. While he was her post-doc, he pored over the Moon formation literature to see if he could apply his knowledge of orbital dynamics to the standstill in this area. One big problem was that to get enough Earth-stuff flung into orbit to make the Moon, the Earth would have had to have been spinning twice as fast as the 5-hr. rate post-impact that all of the existing calculations posited. But looking through papers about orbital interactions, Cuk saw a way that Earth could have been spinning that fast, just for a little while.
According to Cuk’s calculations, early in the Moon’s existence, it came under the influence of the Sun, with which it fell into a sort of synchrony known as orbital resonance. This meant that whenever the Earth’s tidal influence pushed the Moon away, the Sun tugged it back. This could result in a transfer of angular momentum from the fast-spinning Earth to the Moon to the Sun, enough to slow the Earth’s spin down to nearly the 5 hours required by current models. But until that happened, the Earth would have indeed been spinning fast enough to fling off the material that became the Moon.
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When Cuk and Stewart presented their orbital resonance findings at meetings last year and early this year, the news fell on fertile ground: in the audience was astrophysicist Robin Canup of the Southwest Research Institute, in Boulder, Colo., who has spent the last twenty years modeling Moon formation and authored many of the papers describing the giant impact theory.
After the meetings were over, she and Stewart independently devised separate models incorporating the cosmic braking idea, taking two different approaches: Stewart’s model involves an almost-fully formed Earth struck by a small object moving quickly; Canup’s model involves a half-formed Earth struck by a large object moving slowly. In Stewart’s model, the Earthly debris spins so fast that enough flies up to form the moon; in Canup’s model, the similar size of the Earth and the impactor means they both disintegrate and intermingle so completely they have similar chemistry. In both cases, resonance with the Sun gets the Earth’s spin down to the initial 5-yr. rate within 100,000 years — peanuts in cosmic time. Canup’s paper appears alongside Cuk and Stewart’s in Science.
The fact that two such different models are possible using the idea of resonance is a good sign, according to Stewart. “Since there are so many different scenarios that could work to produce a Moon and Earth with matching chemistry this way, the odds are high that [models incorporating resonance] could work,” she says. “This means there are more ways to make the Moon.”
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What’s not clear yet is which of the two impactor theories is likeliest: a small, fast object, probably from far away, or a slow, large one, probably from nearby? That’s a question Canup says will be answered by looking at what the early solar system was like: “Models of the inner planets forming should be able to tell us the relative likelihood of these two types of impacts,” she says. That further research aside, however, nobody — including Canup, Stewart and Cuk — pretends that the matter of the moon’s formation will be firmly settled soon. Cosmological forensics involve piecing together events that occurred billions of years ago. It may be a good while yet before the case can at last be closed.
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This article also appears on discovermagazine.com