THE leading theory of how the Moon formed is in trouble. A physicist announced this week that if it was born after a planet-sized body collided with the Earth in its youth, as many scientists assumed, the Earth and Moon should have far more angular momentum than they do today.
A decade ago, Al Cameron of Harvard University in Cambridge, Massachusetts, showed that if an object the size of Mars hit the young Earth about 4.5 billion years ago, it would have splattered a lunar mass of material into orbit. Astronomers thought that most of the orbiting material would have clumped together, neatly explaining how the Moon formed.
However, Robin Canup of the University of Colorado at Boulder told a meeting of the American Astronomical Society鈥檚 Division of Planetary Sciences in Boston that the object that struck the Earth would have to be much more massive than Mars for the Moon to form.
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Her simulations, carried out in collaboration with Cameron and others, have shown that after a collision with a Mars-sized planet the debris would form a disc around the Earth, stretching to many Earth radii. First, small 鈥渕oonlets鈥� would form within the disc, says Canup. Over thousands to tens of thousands of years, these could have coalesced into a single Moon, probably at about 3.5 Earth radii. However, it turns out that because the disc was so spread out, only 20 to 50 per cent of the initial disc material would form the Moon-the rest would fall back to the Earth.
鈥淵ou need to produce a much larger disc initially because you lose most of the material back onto the Earth,鈥� Canup says, concluding that the object that hit the Earth would have to contain 2.5 to 3 Martian masses to create a big enough disc. Such a 鈥減rotoplanet鈥� could have been orbiting near enough to the Earth to have collided with it. But the model shows that the impact would have left the Earth-Moon system with over twice its present angular momentum. Explaining how all that angular momentum dissipated is very difficult.
Canup isn鈥檛 about to abandon the giant impact theory because all the other theories 鈥渉ave even more severe problems鈥�, she says. But, she concedes, 鈥渢his does raise important questions鈥�.
