When worlds collide

In astronomical circles, it is pretty much official. The Moon was created when a body about the size of Mars slammed into the newborn Earth. In the cataclysm, the molten iron core of the impacting body sank to the Earth's core while its molten mantle splashed out into space to form a ring of debris. This congealed into the Moon. The Moon, originally about 20 times closer to the Earth, gradually moved out to its current location. This "Big Splash" picture, proposed by William Hartmann, Al Cameron and their colleagues in 1975, is very well-received. For instance, it explains why the Moon contains essentially no iron.

In astronomical circles, it is pretty much official. The Moon was created when a body about the size of Mars slammed into the newborn Earth. In the cataclysm, the molten iron core of the impacting body sank to the Earth's core while its molten mantle splashed out into space to form a ring of debris. This congealed into the Moon. The Moon, originally about 20 times closer to the Earth, gradually moved out to its current location. This "Big Splash" picture, proposed by William Hartmann, Al Cameron and their colleagues in 1975, is very well-received. For instance, it explains why the Moon contains essentially no iron.

Unfortunately, it has a big problem. It concerns the body that collided with the Earth. "Where did it come from?" says Richard Gott of Princeton University in New Jersey. "The clues suggest a seemingly impossible location." One such clue comes from comparing the composition of the Earth and Moon. Cosmologists are pretty sure that the disc of swirling debris from which the planets congealed had a different composition at different distances from the newborn Sun. The Mars-mass body would, therefore, not have had the same make-up as the Earth. In the impact, the Earth and Moon would have been contaminated by different amounts of this material, which means, when we examine terrestrial and lunar rocks, we should see marked differences in composition. "The bizarre thing is, we don't," says Gott.

Take oxygen. It comes in three types - oxygen-16 and two heavier and rarer types, oxygen-17 and oxygen-18. The relative proportions of these are like a chemical "fingerprint". The prediction of the Big Splash scenario is that the Earth's oxygen fingerprint will be quite different from the Moon's. But it isn't. It's pretty much identical.

The oxygen evidence forces the conclusion that the body that hit the Earth and created the Moon formed at exactly the same distance from the Sun as the Earth. This is also indicated by computer simulations of the birth of the Moon, which show that the impactor came in at relatively low speed, characteristic of bodies in the Earth's vicinity. "But if the impactor formed at the same distance from the Sun as the Earth, there is a big problem understanding how it ever managed to grow as big as Mars," says Gott.

The accepted theory of the birth of the planets is that they gradually "accreted" from debris pulled in by their gravity. The bigger they got, the stronger was their gravity and the more matter they pulled in. Since it is a process in which the rich get richer and the poor poorer, the impactor should have been gobbled up by the proto-Earth long before it reached the mass of Mars. So, why wasn't it?

Gott set out to solve the puzzle with Princeton colleague Edward Belbruno. They began by asking: is there some special location at the Earth's distance from the Sun where a body could grow to the mass of Mars? Immediately, they realised there is. In fact, there are two places. These are the "Lagrange-4" and "Lagrange-5" points, whose existence was first suggested by the French mathematician Joseph Louis Lagrange in 1772. One lags 60 degrees behind the Earth as it orbits the Sun and the other precedes the Earth in its orbit by the same amount. At the Lagrange points, all the forces in the Sun-Earth system miraculously balance each other. What's more, any slow-moving debris that happens to find its way there becomes hopelessly trapped in a kind of interplanetary Sargasso Sea.

Gott and Belbruno say the Lagrange points are places where matter would naturally have accumulated and where a body could have grown in peace without being affected by the fast-growing Earth. Eventually, when it had reached the mass of Mars, the gravity of other embryonic planets in the Solar System, such as Jupiter, would have tugged it repeatedly, perhaps over millions of years, until it was ejected from the Lagrange point.

In computer simulations, Gott and Belbruno have followed the subsequent course of events. They find nothing can prevent the inevitable - a titanic collision with the Earth. Everything appears to fit. The impactor comes in on a low-velocity orbit, delivering a glancing blow on the Earth. Gott and Belbruno's simulations show that, in a quarter of encounters, the end result is a body exactly like the Moon.

If Gott and Belbruno are right, the Earth had once had a planetary which shared its orbit round the Sun. "It's a clever idea which would solve some obvious problems," says Carl Murray of Queen Mary University in London. But he thinks work still needs to be done to prove it. The most interesting consequence of Gott and Belbruno's scenario is its implications for our prospects of finding extraterrestrial life. The Earth has the biggest moon compared to its size of any planet in the Solar System (Pluto also has a big moon but is rarely considered a full-blown planet nowadays). And a giant moon has been important for the evolution of life.

The Earth, for instance, spins around its axis like a top. And, in common with all tops, it has a tendency to wobble wildly. Such wobbles would cause severe changes in the Earth's climate, with grave consequences for life. But every time the Earth tips too far over on its axis, the Moon's gravity rights it. The Moon has, therefore, ensured a relatively stable climate for the evolution of life over billions of years.

And this is not the only way that the Moon has been important in the evolution of life. The tides created by the Moon, which are three times bigger than those created by the Sun, leave large areas of the ocean margins high and dry twice a day. Hundreds of millions of years ago, this enabled marine creatures to gradually adapt to arid conditions - the first step in the conquest of the land. But the Moon's key importance in the evolution of life has a depressing consequence for our prospects of finding ET life. The reason is that the kind of collision needed to create a big moon has always seemed an extremely unlikely event.

Gott and Belbruno don't see it like that. They say that the formation of a large Mars-mass body at one of the Lagrange points of other planetary systems may not be that uncommon at all. And, since their simulations show a big moon created in a quarter of cases, the formation of a big moon may be more likely than anyone expected. They even speculate that there may exist planetary systems in the Galaxy, where two or more terrestrial planets have big moons.

Is there any way of proving Gott and Belbruno's scenario? At first sight, it would appear to be difficult. After all, the Moon was formed in a tremendously violent manner and the impactor was utterly destroyed. It would be highly unlikely that any unprocessed material from that time could have survived to the present day. "But perhaps not impossible," says Gott.

Gott and Belbruno point to an asteroid, or chunk of interplanetary rubble, discovered in 2002. "2002 AA29" is barely the size of a football pitch and is currently in a orbit which periodically brings it within a mere 5.8 million kilometres of the Earth. The peculiar orbit is very similar to the one the impactor that created the Moon would have been in 4.55 billion years ago. "You have to ask yourself, how did 2002 AA29 get in that orbit?" says Belbruno.

An intriguing possibility is that it might have been associated with Lagrange-4 or Lagrange-5 in the distant past and at some point was kicked out. If so, 2002 AA29 may carry the imprint of the material from which the impactor and the Earth were formed. Bizarrely, 2002 AA29 has been picked out by planetary physicists as an asteroid that would be relatively easy for a space probe to visit. Gott and Belbruno suggest that a mission to return a sample would be most interesting. If it found iron and material with the same oxygen fingerprint as the Earth and Moon, it would support the Lagrange point scenario. If it contained no iron, it could be a bit of the splashed out material from the impact that formed the Moon. "Either way, we think 2002 AA29 could tell us about the origin of the Earth and Moon," says Gott. "It may be the most valuable chunk of rock in the Solar System."

Marcus Chown is the author of 'The Universe Next Door: Twelve Mind-Blowing Ideas from the Cutting Edge of Science', published by Headline, £7.99