SKY & TELESCOPE'S NEWS BULLETIN
AUGUST 1, 1997
HOW NOT TO MAKE THE MOON
At a meeting held this past week in Cambridge, Massachusetts, planetary scientists learned that making the Earth's Moon with a giant impact isn't getting any easier. The idea that something the size of Mars sideswiped the Earth in its infancy to form the Moon is not new; researchers William Hartmann and Donald R. Davis first offered it in the mid-1970s. The "Big Splat" theory has wide appeal among geochemists because it explains, among other things, the Moon's low iron content and its complete lack of water.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.
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