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A Carnival of Collisions
By Robert Roy Britt
Senior Science Writer
posted: 01:08 pm ET
17 February 2000
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moon_sidebar_000216 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. As though picking sides in a battle for supremacy, small rocks and larger huddled masses of rubble would slam into one another, break apart, and recollect what they could via gravity to prepare for the next impact. And so it went in one catastrophic event, some 4.5 billion years ago, when one of these planetary competitors struck a glancing blow to the fledgling sphere we now call home. The renegade was about the size of Mars, and the devastation was grand, leaving a crippled Earth surrounded by a hot flash of vaporized material and molten rock. "Debris from both the impactor and the Earth's surface was ejected into orbit around the Earth," explains William Ward of the Southwest Research Institute. "The inner region of this disk near the Earth stayed hot, and was unable to coalesce due to the gravity of the Earth, in much the same way as Saturn's rings stay dispersed due to Saturn's gravity," Ward says. "However, the outer portion of the debris disk coalesced rapidly. Since disk particles orbited the Earth once every few hours, it only took months to a year for them to mutually collide and aggregate to yield a moon." Other theories suggest it took much longer to form the moon. Either way, the impact is thought to have sped up Earth's rotation and probably tipped the planet on its axis. An idea supported by moon dirt The first suggestion that the moon was carved during a violent impact came in the 1980s after long studies of soil samples brought back during the Apollo moon missions. Because minerals on Earth and the moon are similar, scientists are now pretty sure they share a common origin. 
The evidence does not insure that an impact resulted in lunar genesis, however. A competing theory of formation holds that Earth and the moon developed at the same time, but independently, out of the same group of rocks and dust. But in March of 1999, analysis of data from NASA's Lunar Prospector spacecraft showed that the moon's core is small -- probably between 2 percent and 4 percent of its mass. This is tiny compared with Earth, whose iron core makes up about 30 percent of the planet's mass. If they had formed out of the same batch of stuff, then the moon's core should be proportional to the Earth's, researchers say. The impact, which most researchers now believe created the Moon, must have come after the Earth's iron core had formed, ejecting only rocky, iron-poor material from the outer shell into orbit, says Alan Binder of the Lunar Research Institute in Tucson. "It was this material that collected to form the moon." Another theory holds that as Earth was forming, it spun so fast that a chunk simply flew off. Scientists have also suggested that the moon may have come from somewhere far away; as it wandered into the solar system, it was captured by Earth's gravity and swung into orbit.
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