Recipe for a Moon

Saturday, November 01, 1997
How did our moon form? Many astronomers believe that Earth and a Mars-size protoplanet crashed into each other 4.5 billion years ago, shortly after the birth of the solar system. The debris from this collision eventually coalesced into the moon. But Robin Canup, an astrophysicist at the University of Colorado, says the conventional model has a serious flaw: most of the debris from the collision would have rained down onto Earth instead of remaining in orbit.

Canup and Al Cameron, an astrophysicist at Harvard, have made a detailed model of the collision. They have found that while an off-center collision with a Mars-size body launches up to twice the mass of the moon into orbit, most of the debris stays too close to Earth to form a large body. This vaporized rock, spread out into a spectacular disk like the rings of Saturn, falls back to Earth in a matter of months.

To get a full-fledged moon, Canup and Cameron found that the object that smacked Earth must have been much larger than previously thought--about three times the mass of Mars. Several years after such a collision, a newly formed moon would hang just 14,000 miles above Earth, racing across the primordial sky 12 times every 24 hours, with its orbit gradually widening.

Right now the moon is 240,000 miles away, Canup says. Back then it would have been an enormous sight in the sky. But we may also have had a system of a couple of moons for thousands or tens of thousands of years, depending on the way the debris initially orbited Earth. Later these satellites would have collided to form a single moon.

But while such a massive crack-up would have launched sufficient rock far enough out to coalesce into an object the size of the moon, the energy of the impact would have left Earth spinning so fast on its axis that just after the impact a day would have been only two hours long. So far Canup hasn’t found an obvious way to slow things down. Until this problem is solved, the moon’s birth will remain a mystery.
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