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August 16, 2001 - Issue #469
NEW SIMULATION FOR OUR MOON'S ORIGINS
A new computer simulation of the Moon's formation predicts that it might be
much younger than scientists originally believed. This new simulation,
developed at universities in the United States, shows how a Mars-sized
object struck the Earth 4.5 billion years ago, and ejected a hail of
material which eventually eventually formed the Moon. This story was
published in the journal Nature.
More to read:
SKY & TELESCOPE'S NEWS BULLETIN
AUGUST 1, 1997
But the devil is in the details, and for 20 years dynamicists have struggled to find the right combination of impactor size, collision point, and speed to validate the theory. New computer simulations by Shigeru Ida, Robin Canup, and Glen Stewart show that after the collision, a disk of vaporized material forms. Unfortunately, roughly two-thirds or more of it lies within what's termed the Roche limit (about 12,000 km from Earth's surface) and quickly falls inward. Canup reports that only impacts involving something roughly three times the size of Mars creates a disk massive enough to deposit a Moon's worth of matter outside the Roche limit. The catch, she says, is that the Earth-Moon system is then left with about twice the angular momentum it has today -- a dynamical excess that's hard to eliminate. Details will appear in a forthcoming issue of SCIENCE.