AS SPACE rocks go, asteroid 2002 AA29 looks fairly ordinary. In snapshots taken with powerful telescopes it shows up as a tiny black speck, a pipsqueak compared with the enormous asteroid that wiped out the dinosaurs 65 million years ago. Heck, it isn’t even on a life-threatening collision course with Earth. Yet to Richard Gott and Edward Belbruno, 2002 AA29 is the most important rock in the solar system. They believe it shares a history with the Mars-sized planet that crashed into the Earth 4.5 billion years ago to make the moon.
Where this world came from has always been a mystery. All the signs point to an unthinkable place, but now Gott and Belbruno think their space rock might help to solve the puzzle. It could even prove the heavens are full of cosy corners where life might flourish.
Back in the early days of the solar system, the Earth wasn’t really the Earth. It was a protoearth, a smaller world growing rapidly as it gobbled up rubble that had condensed from the dust cloud swirling round the young sun. But protoearth had a rival, another planet about a tenth of its mass, spinning around the sun in a dangerously similar orbit.
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One day the two collided. The impact melted Earth’s crust and obliterated the smaller planet, which some astronomers call Theia.
Theia’s iron core sank into the Earth, while its molten rock layers splashed out into space. Once in orbit, the splattered pieces were pulled together by gravity to form a protomoon. Originally whirling only 22,000 kilometres from Earth, the moon gradually picked up more rubble and was pushed out to its present position. And the rest, as they say, is history.
In astronomy circles, this “big splash” scenario is now taken as read. First proposed in 1975 by William Hartmann at the Planetary Science Institute in Tucson, Arizona, and independently by Al Cameron at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, the idea has been tremendously successful. It explains why the moon is made of lightweight rock, and why it contains far less iron than the Earth: iron makes up as little as 8 per cent of the moon compared to 30 per cent of Earth.
Since then, planetary scientists have bolstered the case with scores of computer models showing planets of different sizes and characteristics careering into the Earth and congealing into a variety of Earth-moon systems. As a result, we know a surprising amount about the events that led to the moon’s formation. The most detailed simulations yet were published earlier this year by Robin Canup of the Southwest Research Institute in Boulder, Colorado (Icarus vol 68, p 433).
Canup’s model shows that the big splash must have happened when the Earth had grown to almost 95 per cent of its present mass. After ploughing into protoearth, a portion of Theia sheared off and flew into space. Badly stretched and distorted, it swung round Earth and struck the planet again. This time, the iron core sank into the Earth, while lighter rocks from Theia’s mantle spiralled out into orbit (see Graphic). Eventually these clumps stuck together to form the moon. Canup’s simulation shows that as much as 80 per cent of the moon came from the smaller planet, rather than Earth.
That’s all well and good. But it leaves a problem. Where did Theia come from in the first place? “The clues seem to point to an apparently impossible location,” says Gott who is an astrophysicist at Princeton University.
One important clue lies in samples of lunar rock brought back to Earth in the 1960s and 1970s by the Apollo astronauts. Planetary scientists believe that the elements and isotopes present in the swirling cloud of dust and rock that formed the planets changed dramatically with distance from the newborn sun. Meteorites that have landed on Earth from Mars have wildly different ratios of oxygen isotopes from terrestrial rocks. So too do chunks of rock chipped off the large asteroid Vesta, which orbits between Mars and Jupiter.
Yet the levels of oxygen isotopes in the Apollo samples are an almost exact match for rocks found on Earth. Together with Canup’s finding, this implies that Theia originated much closer to home than anyone suspected. In fact the isotope match is so good that planetary scientists are forced to conclude that the smaller planet formed at the same distance from the sun as the Earth.
This notion is backed up by computer simulations which show that Theia was travelling relatively slowly when it smacked into Earth. Knowing its speed, planetary scientists can work out its orbit before it struck. To move so slowly, the planet must have originated in the inner solar system, rather than its chilly outer reaches.
What puzzled Gott was how a planet that shared Earth’s orbit could grow as big as Mars. According to the accepted theory of planet formation, protoearth’s bigger gravitational pull means it should have guzzled up all the rocks in its vicinity. With rubble in such short supply, the smaller planet’s growth would have stalled, and it too should have been gobbled up by the Earth long before it reached the mass of Mars.
Gott and his mathematician colleague Belbruno set out to solve the mystery. One thing they did not want to do was throw out the big splash scenario, because it is so successful at explaining the moon’s peculiar make-up. “We therefore asked: is there some special location where a body could grow to the mass of Mars?” says Gott. It didn’t take long for them to realise that there is.
Safe havens
In 1772, the French mathematician Joseph Louis Lagrange calculated that there are five regions in space where the gravitational pulls of the Earth and the sun balance. Naively, you might expect the two forces to balance at just one point, somewhere along a line joining the Earth and the sun. But Lagrange realised that other forces come into play because both bodies are moving. This complex web of forces produces several other regions in space, called Lagrange points, where gravity balances. Gott and Belbruno were most intrigued by the two regions known as L4 and L5, lying some 150 million kilometres away on Earth’s orbit. Millions of kilometres across, one trails 60 degrees behind our planet, while the other presses ahead of Earth by the same distance.
L4 and L5 are the only stable Lagrange points in the Earth-sun system: any rock that happens to drift into them becomes hopelessly trapped forever unless gravity or an impact kicks it out. Evidence of this kind of trapping is visible all over the solar system. The most famous example is a group of asteroids called the Trojans, which tag along with Jupiter as it orbits the sun. Mars also plays follow-my-leader with an asteroid called 5261 Eureka.
Gott and Belbruno believe that 4.5 billion years ago enough rubble could have amassed at L4 or L5 to clump together and form a planet. In the safety of the Lagrange point, Theia could easily have grown as big as Mars without threatening protoearth.
But protoearth was not the only body in the solar system tugging on this growing world. Planetary scientists are almost certain that all the planets in the solar system formed at the same time. As they puffed up, their gravitational pulls would have strengthened too. Over millions of years, Theia would have been yanked this way and that by the other growing planets.
For what happened next, Gott and Belbruno turned to a computer simulation. They found that the constant gravitational tugging tried to jerk the planet from its haven. Yet the complex web of forces around the Lagrange point meant it did not release its grip easily. Nudged from its Lagrange point, at first the planet picked up speed. But then the Coriolis force – the same force that drives the rotation of ocean currents and weather systems on Earth – pushed it back towards the equilibrium point.
Gott and Belbruno’s model shows that all this pushing and pulling sent the planet on a bizarre path known as a horseshoe orbit. After rushing towards Earth, it skidded to a halt and bounced back again. With each yo-yo, it picked up speed and crept a bit closer to Earth. Yet in spite of all this wild swinging, the planet also continued to orbit the sun in step with the Earth.
For millions of years, the planet swung back and forth in its orbit, until it eventually picked up enough speed to break free from its gravitational prison. All this time, the planet would have been a spectacular sight from protoearth. Far outshining Venus, it would have appeared as a bright morning star before sunrise or as an evening star after sunset.
Once the planet broke free from the Lagrange point, events took a final dramatic turn. According to Gott and Belbruno’s simulation, Theia would have ended up on a chaotic orbit. Within a few centuries, it found itself on an inevitable collision course with the Earth.
Everything appears to fit, Belbruno says. The model shows that the smaller planet struck the Earth a glancing blow at a speed of 40,000 kilometres per hour, exactly as suggested in earlier simulations carried out by Canup and her colleague Erik Asphaug at the University of California at Santa Cruz. And Gott and Belbruno’s own computer model of the big splash shows that 1 in 4 encounters produces a body just like the moon.
Their idea also backs up another feature of Canup and Asphaug’s simulations. For the big splash to produce such a light moon, the impact must have happened towards the end of the planet-forming process, when the Earth was almost fully grown. By this time, dense molten iron would have sunk to the centre of both Earth and the smaller planet, where it would have formed a solid core. “We’re perfectly OK with this,” says Gott. “In our scenario, a late impact is essential.”
Gott and Belbruno speculate that the impact was in fact so late that very few other big bodies were left orbiting near Earth. This would explain why nothing else seems to have smashed into the moon to contaminate it with iron.
Gott and Belbruno say they can even estimate how many big bodies were still around. “The Earth is about 160 times better at sweeping up debris than the moon,” says Gott. “So, as long as there were fewer than 160 bodies, the moon would likely have suffered no hits from these big bodies whatsoever.”
Other researchers are impressed by the theory. “It is a clever idea which would solve some obvious problems,” says Carl Murray of Queen Mary University in London. However he doubts that L4 and L5 were always the stable havens that they are now. Other growing planets in the solar system might have altered the pattern of forces around protoearth, preventing rubble from clumping together to form a planet.
“While I am quite prepared to believe that a large object can remain at L4 or L5 before eventually impacting Earth, I would want to see good evidence that it was possible for such objects to accrete there,” Murray says. Asphaug is also critical: “The impact models aren’t good enough to confirm such a detailed scenario.”
If Gott and Belbruno are right, then Lagrange points are a natural refuge for growing planets that can later be wrenched free to smash into their planetary neighbours. And if moon-making collisions really are a normal part of a planet’s adolescence, why stop with Earth? Another planet that might show the scars of a catastrophic collision is Uranus. At some point in its history, this gas giant was knocked over to leave it spinning on its side. Outside our own solar system giant moons might be more common than anyone suspects. “There may even be planetary systems where collisions have resulted in two or more terrestrial planets having big moons,” says Gott.
That boosts the prospects for finding life on planets outside the solar system. Most scientists believe that the moon was essential for the evolution of life on Earth. Its gravity has prevented our planet’s axis of spin from wobbling chaotically, ensuring that Earth has remained a relatively warm and cosy place for life to flourish. Yet the big splash scenario has always had a chance-in-a-billion feel about it. So many planetary scientists think the prospects for finding life on planets around other stars are slim. If Gott and Belbruno’s theory is right, ET might have many more homes than we think.
But is there any way to prove their theory? Since the moon was formed in such a violent manner, it is unlikely that any material from that time could have survived. “But perhaps not impossible,” says Gott. And that’s where asteroid 2002 AA29 comes in. Gott and Belbruno are prepared to stick their necks out and make an extraordinary claim about this rather ordinary-looking space rock.
Discovered in 2002 by astronomers poring over images from the LINEAR sky survey, 2002 AA29 measures less than 100 metres across. Later that year, a team led by Martin Connors of Athabasca University in Alberta, Canada, discovered that it follows a horseshoe orbit that brings it within 5.8 million kilometres of the Earth. “You have to ask yourself, how did it get in that orbit?” says Belbruno. “An intriguing possibility is that it might have been associated with L4 or L5 in the distant past.” If his hunch is right, then 2002 AA29 is made from the same pristine rock that clumped together to form Earth and Theia.
To confirm whether the space rock really is a relic from the infant solar system, researchers would need to get their hands on it. That may not be such a distant dream: 2002 AA29 has already been picked out as an asteroid that would be relatively easy to visit and quarry.
Gott and Belbruno say that if rock samples proved to have the same oxygen isotope ratios as the Earth and moon, plus a sprinkling of iron, it would support their idea. And if the asteroid contained no iron, then it might be a bit of the molten rock that splashed out from the giant collision itself. “Either way, we think 2002 AA29 has the potential to tell us about the origin of the Earth and moon,” they say.
- “Where did the moon come from?” Edward Belbruno and J. Richard Gott