Far-Flung Space Crash May Help Solve Mystery of Moon's Formation

Richard A. Lovett
for National Geographic News
March 15, 2007
Scientists have found fragments they say are from a massive, far-flung collision that may help explain the formation of Earth's moon.

The spectacular smashup took place between two dwarf planets, one of them nearly as large as Pluto, near the dawn of the solar system. (Related story: "Pluto Not a Planet, Astronomers Rule" [August 24, 2006].)

The crash may also someday create the biggest and brightest comet of all time.

Astronomer Michael Brown of the California Institute of Technology found the five fragments while studying the Kuiper belt, located in the solar system's outermost reaches.

The fragments resemble 2003 EL61, the Kuiper belt's mysterious third largest object, suggesting all six bodies were formed in a single violent crash, Brown said.

The finding will appear in tomorrow's issue of the journal Nature.

Crash Landing

Measuring about 930 miles (1,500 kilometers) across, 2003 EL61 had puzzled scientists ever since its discovery, because it's oblong and spins end-over-end so fast that it completes a full revolution every four hours.

It's like a U.S. football "that's a bit deflated and stepped on," Brown said.

2003 EL61 is also unusually dense, suggesting it is the former core of a larger planet that got clobbered sometime in its history, Brown added.

The glancing blow made 2003 EL61 spin, knocked off its icy outer mantle, and created two moons that are almost pure chunks of ice, Brown theorizes.

Now, however, Brown says he has found more remnants of the crash. He and his team were studying the surface composition of Kuiper belt objects when they realized that five of them stood out, he said by email.

"While we were going through ideas of why these five could be so different, we suddenly realized that they all had essentially the same orbit, and that they had the same orbit as 2003 EL61."

The five newly found objects range in size from 6 miles (10 kilometers) to 250 miles (400 kilometers. They appear to be made of nearly pure ice—just right to be portions of EL61's missing mantle.

Their orbits indicate that they were blown away from 2003 EL61 about 4.5 billion years ago, just after the solar system formed.

"The impact made a tremendous fireball, and large icy chunks of the big object split off and went flying into space," Brown said.

These objects give scientists a chance to see what might be inside Pluto and other large Kuiper belt objects, he added.

Moon Mystery Solved?

The collision may have been similar to the impacts widely believed to have produced both Earth's moon and Pluto's moon Charon as well as a number of smaller objects in the asteroid belt, said French astronomer Alessandro Morbidelli of the Laboratoire Cassiopée in Nice.

Scientists don't fully understand how the moon and Charon collisions occurred, because they can't find enough of the remnants.

By spying more pieces of the smashup that produced 2003 EL61, Morbidelli said, Brown has allowed other scientists to create more accurate models of such impacts.

Morbidelli is also surprised that the 2003 EL61 collision occurred in the first place.

The objects involved, he said, must have been about 1,200 miles (2,000 kilometers) and 600 miles (1,000 kilometers) in diameter.

The probability that such a collision could occur in the Kuiper belt today is effectively zero, because "there are not enough [big] bodies," he said.

The discovery of the ancient collision means that the Kuiper belt once had much larger denizens.

But those other bodies must have been ejected from the belt on unstable orbits. Figuring out how that occurred, while 2003 EL61 and its siblings remained behind, is a daunting challenge for astronomers.

"This is hard for any model of solar system formation [to explain]," Morbidelli said.

Giant Comet

For astronomers of the far distant future, however, the most exciting result might be yet to come.

Orbits in the Kuiper belt aren't fully stable, Brown said, so objects occasionally get flung into space. When this sends them plunging toward the inner solar system, they become comets.

That is what likely happened to many other shards of the collision that produced 2003 EL61, Brown said. And it may be the ultimate fate of 2003 EL61 as well.

If so, in about a billion years it could become the biggest comet in eons.

"It will be something like 6,000 times brighter than Hale-Bopp, [which passed close by Earth] a few years ago," Brown said.

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