24 Hours of Chaos: The Day The Moon Was Made
By Robert Roy Britt
Senior Science Writer
posted: 02:00 pm ET
15 August 2001 
For 25 years, scientists have pondered a theory that the Moon was created when an object the size of Mars crashed into Earth less than 100 million years after the Sun was born, some 4.6 billion years ago. The general idea has been run through the paces and massaged into shape and is now the favored explanation.
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But attempts to model cousins of that theory on computers generate inexplicable side effects.
In one of two leading computer models, Earth was creamed while it was still gathering mass, during a brief time when it was only half its current size. All the rocky inner planets are thought to have formed this way, a method rapid accumulation of matter called runaway growth.
But if the Moon was carved out during Earth's growth phase, then it would have been around when Earth continued bulking up by swallowing tremendous numbers of large asteroids. Some of these iron-rich rocks would have hit the Moon, too. Yet the iron is not there.
In the other model, the aggressor was three times as massive as Mars and created an excess of rotation in the Earth-Moon system that simply doesn't exist today.
Now researchers have harnessed the latest in computing power to provide the most detailed model ever made of the cosmic scene that supposedly created the Moon. The result, a 3-D animation of the blast and subsequent chaos, is comforting. It shows that the Moon could have formed when a Mars-sized object hit a fully formed Earth.
The collision would have given Earth its spin, defined what we now call an equator, and put enough material into orbit at the right distance from Earth to allow the formation of a satellite that generations would later swoon over.
24 hours of chaos
Robin Canup of the Southwest Research Institute has been modeling the Moon's formation for eight years. On previous studies, she has worked with William Ward and Alastair Cameron, who represent one of two separate research groups that developed the original impact theory back in the mid-70s. (William K. Hartmann and Donald R. Davis were the other team.)
As Canup knows, all ideas about how the Moon formed must contend with one important fact: The Moon contains very little iron. Earth, on the other hand, is loaded with iron, the bulk of it tied up in the planet's core.
So the Moon is thought to have been pieced together by the bits that got blown off the upper layers of Earth, as well as the outer portions of the object that hit Earth.
Canup's latest effort, produced with the help of Erik Asphaug of the University of California, Santa Cruz, is like a small scene in a blockbuster disaster movie -- the first 24 hours of time in the epic calamity that made the Moon. It is detailed in the Aug. 16 issue of the journal Nature.
The model treats the debris created by the collision as more than 20,000 computational lumps, or particles, all of which are given their own gravity to play with as the cataclysm unfolds.
In a telephone interview, Canup described the day the Moon was made:
A dark, lifeless object less than half as massive as Earth careens around a newborn Sun. It is one of many planet-sized bodies hoping for a long career. But its orbit is shaky. It's future grim. It is a character actor on the grand stage of the solar system, a player of great ultimate consequence but one destined to never see its name in lights.
This doomed "protoplanet" travels a path that crosses the orbits of similar objects and, ultimately, cannot last. Eventually, the nameless protoplanet meets up with a fledgling Earth.
It is not a head-on collision, but rather a glancing blow. The impact imparts what astronomers call angular momentum into the system. It sets Earth to spinning on its axis and creates a Moon that would go round and round the host planet for billions of years.
The shock of the impact strips material from the outer layers of Earth and the impacting object. The mostly iron cores of both bodies meld into Earth's core. It is like a compact car merging onto the highway and colliding with an S.U.V. -- glass, trim and hubcaps fly, but the two chassis remain hopelessly tangled.
All told, about 2 percent of the combined mass of the objects -- mostly rocky stuff that's largely bereft of iron -- begins to orbit the Earth. About half of this eventually becomes the Moon.
Some of the stripped material is heated so fantastically that it vaporizes and expands into the surrounding vacuum of space.
"The material that was vaporized expands into a cloud that envelops the whole planet," Canup explained.
Meanwhile, a long arm of solid matter is winging its way around Earth. Some of it develops into a clump that slams back into the planet. The rest is flung into orbit, all pretty much along a plane that mimics the path of the incoming object. This plane slices through what is now Earth's equator, and it is roughly the same plane along which the Moon orbits.
"The object came in and hit, and that's what set the Earth's rotation and what its equator would be," Canup said.
The model assumes Earth was not spinning before the impact, though it might have been. If it were already spinning, Canup said the model could be tweaked to account for that fact and would still work.
"For the first time, we demonstrated with simulations that a single impact can give you an iron-depleted Moon of the right mass, and the current mass of the Earth, and the current angular momentum of the Earth-Moon system," Canup said.
Though the model covers only a day's time, Canup said shortly thereafter the material in outer regions began to cool. Gradually, small clumps would have formed, collided with one another, and grown. Based on other models, she said it would have taken between 1 and 100 years to make a Moon after the impact.
http://www.space.com/scienceastronomy/solarsystem/moon_making_010815-1.html
