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Tilted View of Our Moon's Formation
By Robert Roy Britt
Senior Science Writer
posted: 01:08 pm ET
17 February 2000
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moon_tilt_000216 If you hold a favorite someone's hand and stare up at the moon, you can't help but feel warmed by the timeless beauty of Earth's steadfast companion. For many moons now, Earth's only natural satellite has been making its love-inducing rounds, for slimmer or fuller, with tethered regularity. | The Moon's Formative Years | | A Carnival of Collisions: The early years in the inner solar system, according to theory, were a tumultuous carnival of collisions. Millions of free-roaming rocks, and a dozen or so mini-planets vying to emerge as an Earth or a Mars, all orbited recklessly in the gravitational grip of a new sun. | But the moon -- moving steadily away from us as you read this -- hasn't always been where it is. In fact, there was a time when it wasn't there at all. And when it was born, carved viciously out of a previous incarnation of the planet you stand on, the moon is thought to have traveled a path quite different from its current one. Problem is, for 30 years researchers have been unable to account for how this suspected path came to be. 
The Moon was initially inclined 10 to 12 degrees compared to the plane of Earth's equator. It was so inclined The Moon's initial inclination -- its distance above or below an imaginary plane extending outward from Earth's equator -- was once much greater than it is now, according to a theory based on backward-looking math calculations. The math considers how the two spheres continually push and pull each other. This process, along with the sun's gravity, has allowed the moon to change inclinations and to drift away from us (from an initial 14,000 miles (22,530 kilometers) away to the current distance of more than 280,000 miles, or 450,000 kilometers). Starting with the moon's current inclination -- about 5 degrees -- computer calculations back into an inclination of around 10 or 12 degrees at the time of formation, some 4.5 billion years ago.But this theorized inclination, way back when, is inconsistent with another leading theory that the moon was created when a Mars-sized object slammed into the early Earth. This supposed violent, glancing collision should have resulted in a disk of debris, which orbited in or near Earth's equatorial plane and, rather quickly, coalesced into a satellite that would have traveled near the same plane. The impact-creation theory holds that the moon's initial inclination should not have been more than 1 or 2 degrees. A new set of calculations by William Ward and Robin Canup of the Southwest Research Institute attempts to account for the difference. The researchers argue the moon did start out very near the equatorial plane. It's what happened next, they write in the February 17 issue of the journal Nature, which bumped the satellite to such a relatively high inclination.How satellites usually orbit One need only look at Saturn's rings for a vivid example of how debris orbiting a planet tends to flatten out into an equatorial disk, Ward told SPACE.com. And most satellites around other planets follow a similar path, very near the equatorial plane."Collisions between orbiting debris naturally act to flatten material out into an equatorial disk, which then naturally accumulates into a moon with a very low-inclination orbit," Ward explained. New idea about the debris disk Our moon, Ward says, gathered itself together by gravitationally combining the outer bits of debris after the glancing collision by the Mars-sized object. The moon-building process probably took a year or less, though other theories suggest it took much longer. Ward said that while studying models of lunar formation, he and Canup realized that the inner disk of debris would remain after the moon had formed, setting up a process of "gravitational resonance." "The moon causes ripple-like waves in the [inner debris] disk, which then interact with the moon's orbit to increase its orbital tilt to about 10 to 12 degrees from an initial value that was probably only about 1 degree or so," Ward said. "During this process, the last vestiges of the diskre-impact the Earth." Thereafter, the moon's orbit continued to slowly expand, Ward says. Gravitational interactions with Earth and the sun (the same things that create tides on our oceans) eventually changed the inclination of the moon's orbit to its present 5 degrees. Our natural satellite's initial inclination "has been discussed for eons," said Alan Binder of the Lunar Research Institute. Binder called the new study interesting and useful, and said that the results may well reflect what happened. He cautioned, however, that studies based on computer models -- instead of actual observations -- are not typically the main force that drives scientific understanding. So while the new view of the moon's past could be accurate, only more concrete evidence will provide a complete picture of how the moon came to be, Binder said.
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