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

University of Colorado-Boulder

Contact: Robin Canup, 303-492-8918, canup@sargon,colorado.edu
Glen Stewart, 492-3737
Jim Scott, 492-3114

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."