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From: Stig_Agermose@online.pol.dk (Stig Agermose) Date: Wed, 30 Jul 1997 09:49:00 -0800 |
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.
Index: Earth's Moon
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Created: Jul 30, 1997