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Giant Protoplanet Said To Have Sideswiped The Earth


Contact:                Robin Canup, (303) 492-8918
                        Jim Scott, 492-3114
July 25, 1997

Note to Editors:  Contents embargoed for use until Monday, July 28.

FORMATION OF EARTH'S MOON LIKELY
CAUSED BY HUGE PLANETARY COLLISION

        A rogue planet three times as massive as Mars probably sideswiped
Earth 4.5 million years ago, vaporizing enough material from Earth's upper
layers to form the moon, according to a University of Colorado at Boulder
study.

        While many theories on the moon's formation have been proposed,
detailed analyses of lunar rocks obtained during NASA's Apollo missions
were key in creating the "giant impact theory" in the 1970s that is now
widely accepted, said Robin Canup of CU's Laboratory for Atmospheric and
Space Physics.  But the Mars-sized "impactor" proposed by Harvard
University researchers in the late 1980s is not large enough to account for
the formation of our unusually large moon, according to calculations by
Canup and several other CU planetary scientists.

        The modeling work by the CU-Boulder group, which is collaborating
with the Harvard group, indicates the impactor must have been at least 2.5
to 3 times the mass of Mars to create the volume of debris required to
eventually coalesce into the moon, Canup said.  The "protoplanet" probably
was orbiting the sun somewhere between Earth and Mars when the collision
occurred.

        "This was a surprising result," said Canup.  "Our calculations
indicate a lot more impact energy than previously believed would have been
required to produce enough material to form the moon."

        Canup presented a talk on the subject at the American Astronomical
Society's annual Division of Planetary Sciences Meeting held July 28 to
Aug. 1 in Cambridge, Mass.

        The CU research indicates an "oblique impact" between Earth and the
ancient planet vaporized the upper portions Earth's crust and mantle,
spraying the material into Earth's orbit.  The material appears to have
spread into a gaseous disk around Earth, then formed a handful of small,
extremely hot moonlets that eventually coalesced into the single, large
moon we see today, said Canup.

        "Large-scale impacts like this one probably played a crucial role
in shaping the solar system," said Canup.  The puzzling size and
composition of Mercury, the extreme tilt in Uranus' axis and the peculiar,
"double-planet" system of Pluto and its large moon, Charon, indicate such
impacts may have been relatively common.

        "We believe this theory is a linchpin to understanding how planets
formed in our solar system and in solar systems that may exist around other
stars," she said.

        Questions regarding the formation of Earth's 2,160-mile-in-diameter
moon still remain, Canup said.  Although the size of the impactor proposed
by the CU team provides the correct amount of material required to form our
unusually large moon, the model also yields "an Earth that is spinning too
quickly."

        The angular momentum, or intensity of rotational momentum, in the
Earth-moon system depends on the spin of Earth and the moon and their
distance apart, said Canup.  Although the total amount of rotational spin
in planet-moon systems must remain constant over time according to Newton's
laws, the CU-Boulder model produces a system with roughly twice as much
rotational spin as the Earth-moon system exhibits today.

        "The Harvard model produced the right amount of initial spin for
the Earth, but not enough material to have formed the moon," she said.
"Our closest celestial neighbor remains a mystery in many ways."

        The ongoing moon-origin study may pave the way for searches of
newly forming solar systems, planets and moons elsewhere in the universe,
she said.  Because the disk material formed by planetary collisions would
be extremely hot, such impacts would likely be "very bright" and might be
detectable using new generations of sophisticated Earth- and space-based
telescopes.

        The moon, which orbits Earth at about 239,000 miles distant,
appears to have formed at roughly 15,000 miles from Earth, according to the
CU researchers.
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