Research Indicates Earth's Moon May Have Formed In Year Or Less

September 24, 1997

New computer simulations by a team of scientists working at the University of Colorado at Boulder indicate a disk of debris orbiting Earth early in its history may have taken less than a year to coalesce into the moon we see today.

The researchers modeled a variety of conditions leading to the formation of the moon based on the widely held scientific assumption that a rogue "protoplanet" sideswiped Earth 4.5 billion years ago, vaporizing much of its crust and mantle into a swirling disk around the planet. The so-called "giant impactor theory" was first proposed in the 1970s following extensive research by NASA Apollo scientists.

Although "giant impactor" models created by a Harvard University group in the 1980s and early 1990s indicated the protoplanet was about the size of Mars, research presented at a July 1997 planetary science meeting in Cambridge, Mass., by CU-Boulder research associate Robin Canup indicated the object must have been at least three times more massive than Mars to create enough debris to form our moon.

The newest modeling results, which estimate the year-long time frame for the moon's formation, were published in the Sept. 25 issue of Nature. Calculations by the research team also indicate less than half the orbiting debris coalesced into the moon, while the rest eventually fell back to Earth.

The Nature paper was authored by Shigeru Ida of the Tokyo Institute of Technology and research associates Robin Canup and Glen Stewart of CU-Boulder's Laboratory for Atmospheric and Space Physics. Ida spent the 1996-97 academic year on sabbatical at CU collaborating with Canup and Stewart on the project.

A "ballpark figure" for the cooling of material blown off Earth by the violent collision with the impactor and its accretion into swarms of large, orbiting debris particles is thought to be somewhere between one and 100 years, speculated Canup.

At this point in the process the team began modeling a variety of scenarios that may have taken place, including the numbers of large debris particles in orbit and their distances from Earth. Twenty-seven different computer models produced by the team varied the number of particles from 1,000 to 2,700 and assumed sizes of up to 60 miles across for some of the larger debris particles, said Canup.

In each of the simulations, the particles invariably clumped together to form the moon in a year or less, always at a distance roughly 14,000 miles from Earth, she said. This is the equivalent to about 3.5 to 4 Earth radiuses from the planet.

In the outer regions of the disk, the debris particles apparently clumped together quite easily, she said. But in the inner regions of the disk "they probably bounced off each other" due to the effects of Earth's gravity.

The reason the particles in the inner portion of the disk failed to coalesce is due to their proximity to the "Roche limit," said Canup. The Roche limit is the distance from any planet or star inside of which tidal forces from the object pull orbiting particles apart rather than allowing gravity to hold them together.

For Earth, the Roche limit is about three Earth radiuses from the planet. "That's why the moon always forms just outside that region in our models," she said.

"Once the particles in the outer disk accreted to form the moon, its gravitational forces likely scattered the inner disk material back onto Earth," said Canup. In each of the computer simulations, only about 15 percent to 40 percent of the material from the initial debris disk wound up being incorporated into the moon. "This was a result we did not anticipate," Canup said.

The researchers calculated the debris particles were orbiting Earth every nine to 10 hours, and that it would have required about 1,000 orbits -- the equivalent of about one year -- for the large particles to coalesce into our single moon.

Interestingly, about one-third of the simulations formed two similarly-sized moons rather than one larger moon. "If this were the case, a two-moon system may have persisted for some time," she said. "That would have been quite a sight."

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University of Colorado at Boulder

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