New Take On an Ancient Mystery: How Earth Got its Moon

Lunar history has always been murky, and a new paper offers additional riddles

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Correction appended Dec. 4, 2013

It’s a general rule in science that the more you know about some aspect of the natural world, the better you understand it. But rules are sometimes broken, and the question of how Earth got its Moon is a very good example of that. Planetary scientists back in the early 1980s concluded that the Moon was born around 4.5 billion years ago, when a Mars-sized object, now deceased, struck Earth a glancing blow at a relatively slow speed, creating a disk of debris that congealed into the familiar, modern Moon. That scenario fit nicely with Earth’s rotation rate and the Moon’s orbit and made particular sense since 4.5 billion years ago, there was a lot more free-flying debris in the solar system than there is today. Plus, says Sarah Stewart, a planetary scientist at Harvard, “It’s something you could explain in a sentence to your grandmother.”

Unfortunately, a growing body of evidence suggests that what we’ve been telling grandma is probably wrong—and a report just published in Nature makes clear exactly how murky the story of the Moon’s origin has now become. The central problem with the original theory, says author Robin Canup, of the Southwest Research Institute in Boulder, Colorado, is geological: the Moon simply looks too much like the Earth. “The disk of debris that formed the Moon should have come mostly from the impacting body,” she says, “which we think should have had a chemically different composition from Earth.”

Geologists have known for a long time how closely moon rocks resemble Earth rocks—ever since the Apollo astronauts began bringing back lunar samples and NASA labs had a go at them. The first clue was the fact that isotopes (or chemical variants) of oxygen locked in lunar material and terrestrial material were identical. But that finding was  swept aside as mere coincidence. As analytical techniques got better, however, Stewart says, “the geochemists got more and more antsy,” because more and more isotopes—of tungsten, titanium, chromium, silicon—were coming in looking identical as well. “We could have bought one match,” she says, “but not all of these.”

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One way around the problem: assume that somehow that long-ago impact vaporized not just part of the Mars-size intruder, but significant parts of Earth’s upper layers as well. If all of that material got blended together, with some of it congealing into the Moon and the rest falling back to Earth, you wouldn’t be surprised that the surface of both Earth and Moon look geologically similar. Unfortunately, that doesn’t work: it would take 100 years for Earth and impactor debris to mix thoroughly, and in the deep freeze of space, the Moon would have started to congeal more quickly than that. Heavier elements like tungsten, moreover, would condense too quickly for the contributions of Earth and impactor ever to reach chemical equilibrium with each other.

Another fix to the theory involves ramping up the speed of the impact, or assuming that Earth was significantly smaller at the time and that the impacting body was bigger than Mars. That would have made the collision much more violent and accelerated the rate at which the debris from both bodies melted and mixed. But there’s a problem here too: based on the Earth’s current rotation rate and the moon’s orbital distance, the Earth had to be rotating about once every 5 hours just after the impact (it slowed gradually to 24 hours, and the Moon retreated to its current distance, through tidal interactions between the two bodies).

The old theory was consistent with this post-collision rotation rate. Speeding up the impact would make Earth rotate more like once every two hours after the crash. That’s too fast to explain today’s 24-hr. day—unless you invoke some extra factor, like a previous collision that slowed Earth’s rotation before the Moon-making collision sped it up again, or a tidal interaction between Earth, moon and Sun that slowed Earth down after the collision.

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Either is possible, but, says Canup, “Every time you add an extra complication, you reduce the overall probability of an event happening. Maybe we’re just missing something. There could be a scenario we haven’t thought of.”

It’s also possible that the key problem with the original theory isn’t a problem after all. Planetary scientists assume the Mars-size impactor they first cooked up in the 1980’s would have been geochemically different from Earth because Mars itself is different—something we know from the chunks of Mars that have made their way to Earth.

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But maybe that difference is a fluke. Maybe Venus, which is nearly identical to Earth in size, is nearly identical to Earth geochemically as well—something we can’t know without a sample return mission, since meteorites from Venus have never been found. If Venus and Earth are indeed made of the same stuff, that would raise the odds that the impactor was too and the collision theory would once again make sense.

“If the composition of Venus turned out to be very similar to both the Earth and the Moon, that would change everything,” says Canup. And by change everything, Canup means keeping everything more or less the same, with the old model—which explains so much so elegantly—back in place. Not for nothing, it would also restore the scientific rule of thumb that the more you know, the more you actually understand.

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Correction: The original version of this story misspelled Sarah Stewart’s name. It is Stewart, not Steward.