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From: Stig_Agermose@online.pol.dk (Stig Agermose)
Date: Wed, 30 Jul 1997 08:58:32 +0200
Fwd Date: Wed, 30 Jul 1997 09:25:10 -0400
Subject: Moon Might Be Huge Collision's Child

Found on StarNet at:
http://www.azstarnet.com/public/dnews/0729no3.html


Moon might be huge collision's child


By Robert S. Boyd

Knight-Ridder Newspapers


CAMBRIDGE, Mass. - Astronomers have come up with a new answer to an
age-old question: Where did the moon come from?

They now suspect that a wandering planet three times bigger than Mars
sideswiped the Earth 4.5 billion years ago, destroying itself but
blasting enough matter into space to form our lunar companion.

Until recently, the moon was believed to have been formed independently
by the slow buildup of particles of gas and dust until it reached its
present size and was captured by Earth's gravity.

If the new theory is correct, the moon that has inspired lovers, poets
and myth-makers throughout human history is the result of a massive
traffic accident in space - a much more dramatic birth than the
conventional view. And similar collisions could explain a lot about how
the solar system was shaped.

According to Robin Canup, an astrophysicist at the University of
Colorado, the heat created by such a tremendous collision would have
vaporized much of Earth's crust, which was then just forming.

The fiery material spread into a gaseous disk spinning around the
Earth, she suggested. The disk then broke apart into a handful of
extremely hot "moonlets'' which eventually coalesced into today's
single large moon.

The "giant impact theory'' is based on computer simulations performed
by Canup and colleagues at Colorado's Laboratory for Atmospheric and
Space Physics. Canup's paper was made available in advance of her
scheduled presentation here Thursday at a meeting of 600 planetary
scientists, sponsored by the American Astronomical Society.

The notion that two planet-size objects banged into each other to form
the moon was first proposed by Harvard University researchers 10 years
ago. They suggested that the "impactor'' was about the size of Mars, or
twice the size of the moon. But such an object would not have the
energy to produce enough material to form the moon, so doubts remained.

The new computer simulations have strengthened the impact theory, but
indicate that the planet would have to have been much bigger - 2 1/2 to
three times the size of Mars. Earth is roughly twice the size of Mars.

Canup estimated that the moon began its career close to home - about
15,000 miles from Earth - but slowly drifted to its present distance of
239,000 miles.

Collisions between heavenly bodies are not new, of course. The Earth,
the moon, Mars and Jupiter have frequently been battered by passing
comets or asteroids, which are much smaller than planets.

According to Canup, the lost planet was probably orbiting the sun
somewhere between the Earth and Mars when it got too close to Earth and
smashed into it at an oblique angle.

Canup said similar massive impacts may explain such puzzles as the
unusually large metal core of Mercury, the way Uranus tilts its north
and south poles toward the sun, and the peculiar "double-planet''
system composed of Pluto and Charon, a satellite half as big as Pluto.

In addition, the theory may encourage astronomers who are hunting for
planets around other stars. The disks formed by a planetary smashup
would be extremely hot and bright, and thus might be detectable by the
new generation of powerful ground-and space-based telescopes.


Astronomers at Harvard, the Southwest Research Institute in Boulder,
Colo., and the Tokyo Institute of Technology contributed to the work.


The Moon Shop Web space has links the Net's lunar sites.