Moon Jr.: Once Upon a Time, the Earth Had Two

Earth must have felt pretty inadequate a week or two ago when astronomers announced that distant Pluto has yet another moon. Pluto! Recently demoted to a dwarf planet! And yet it boosted its satellite total to four, while we remain forever stuck at one. True, Earth is ahead of Mercury and Venus, which have no moons at all. But Mars, which is significantly smaller than Earth, has two; and don't even mention Jupiter, with more than 60; or Saturn, with 53. Even asteroids have multiple moons — including Sylvia, a 384-mile (617 km) space rock that boasts the twin satellites Romulus and Remus.

But Earth's rep may might gotten a bump, thanks to a paper just published in the journal Nature. Our home planet may have just one lonely moon now, but long ago, like Mars and Sylvia, we had two. "Whether it's right or not, I don't know," says Maria Zuber, a planetary scientist at MIT who wrote an opinion piece accompanying the new study. "But I think it's very plausible." (See TIME's special on the 40th anniversary of the moon landing.)

The idea, cooked up by astronomers Martin Jutzi and Erik Asphaug, of the University of California at Santa Cruz, started out as an attempt to explain why our moon has so asymmetrical a surface. The part that faces us is relatively smooth, with vast expanses of ancient lava forming flat, dark, low-lying plains that earlier astronomers mistook for oceans. But when space probes first circled the moon in the early 1960s, scientists learned that the far side is mostly covered with rugged mountains and craters.

Nobody has ever been able to explain with certainty why the moon should be so lopsided: maybe it had to do with some kind of massive impact that violently rearranged the surface, much like what happened to the asteroid Vesta. Maybe it was a slightly off-center core that caused the crust to be thinner on the Earth-facing half of the moon, which made that hemisphere more susceptible to lava bleeds.

But then Jutzi and Asphaug began thinking. "It looked to us a little bit as though the highlands on the far side accreted" — which is to say, they were added on top of the pre-existing surface. The astronomers thought a bit more, and realized that this idea was consistent with scientists' beliefs about how the moon formed in the first place. Thanks to the analysis of moon rocks that were brought back by the Apollo missions, planetary scientists are pretty sure that our satellite was born billions of years ago when a Mars-size planetoid smashed into the young Earth. (See pictures of a total lunar eclipse.)

The impact blasted off a cloud of debris from both of the objects and sent it spinning into space, where it eventually congealed into the moon. There could have been other, smaller pieces as well, says Zuber, but their orbits would have been unstable, causing them either to be flung away or to fall into Earth or the moon pretty much immediately.

Except, that is, if they happened to end up at a Trojan point — a place in the same orbit as the moon, but either well ahead of or well behind it, that's gravitationally stable — or relatively so anyway. Just last week, astronomers announced the discovery of a Trojan asteroid leading the planet Earth around the sun. There's no reason a Trojan moon couldn't lead or follow our moon around the Earth.

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After a few tens of millions of years, Moon Jr. would become unstable, almost certainly falling into Moon Sr. But Jutzi and Asphaug's computer simulations of how that would play out showed that the short-lived satellite would have fallen surprisingly gently, at only a few miles per second, making more of a splat than a bang. At a more typical impact speed, which would be at least 10 times faster, says Zuber, "you'd make a big hole and fling off ejecta" — in short form, a massive crater.
In this case, you'd form just a pile of extra stuff on one side, as though you slapped a handful of mud onto a basketball. And if the minimoon were about 750 miles (1,200 km) across, with about a third as much mass as its big brother, it could account for most of the extra material we now see on the far side. (See amazing photos of the sun.)
The scenario could also explain why the near side is paved over with so much lava. At the time of impact, the moon would have cooled from its original molten state to form a thin crust, with an ocean of magma underneath. But all the extra mass added to the far side could have squeezed most of the subsurface magma around to the side that faces us, providing an ample supply of molten rock for later eruptions. "Every once in a while I read a paper I really enjoy," says Zuber. "This is a genuinely new idea. That's what really struck me."
A new theory in science isn't worth much, however, unless you can test it somehow. The best way would be to look for the mineralogical signature of such an event in rocks brought back from the far side of the moon. Unfortunately, no such mission is in the works anytime soon. But the Lunar Reconnaissance Orbiter, or LRO, which is circling the moon even now, has detectors that can get at least a sense of the minerals below.
And in September, another moon probe will be on its way, with Zuber as principal investigator. It's called GRAIL, for Gravity Recovery and Interior Laboratory. Actually, it's a pair of probes that will orbit in tandem; changes in the distance between them will measure the local lunar gravity with extraordinary precision. That will give Zuber and her team a detailed look at the moon's geological (or technically, selenological) structure and history, and when combined with LRO's data, could make or break Jutzi and Asphaug's idea. (See photos of outer space from the Hubble telescope.)
Science is indifferent to what the answer will turn out to be, of course. But scientists and ordinary folks alike can't be indifferent to the romantic idea that two moons once shone together in the evening sky. If nothing else, it's a simple matter of planetary pride.

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