Dramatic upheaval in the early Solar System may have supplied the Asteroid Belt with distant cometary material, providing an alternative explanation for the diversity of objects residing there.

The composition of Asteroid Belt inhabitants varies from primitive ice-rock mixtures to igneous rocks that resemble the crusts of planets, and metal-rich objects that formed from the cores of broken up proto-planets. This compositional diversity is widely attributed to variations in the primordial solar nebula as it condensed to form planets, and their subsequent evolution involving impact, melting and heating events.

Now, using computer simulations, an international team of astronomers have demonstrated that some comet-like objects residing in a disc outside the original orbit of the outer giant planets were scattered across the Solar System and into the Asteroid Belt during a violent phase of planetary evolution approximately 3.9 billion years ago, thus offering an alternative explanation for the mélange of Asteroid Belt objects.

It is well documented in the crater record preserved on the Moon that the Solar System suffered an intense bombardment at this time, thanks to the changing architecture of the outer planets' orbits. Known as the Nice model, as Jupiter migrated inwards and Uranus and Neptune outwards, the theory describes the destabilisation of the cometary disc, resulting in a cataclysmic bombardment of asteroids and comets into the inner Solar System.

"This last major event of planet formation appears to have affected nearly every nook and cranny of the Solar System," says Hal Levison of Southwest Research Institute (SwRI), and lead author of a paper published in today's issue of the journal Nature.

Levison and colleagues' computer simulations tracked the evolution of the cometary population and showed that primitive objects from outside Neptune’s orbit are thrust into the Asteroid Belt during this epoch. The team back up their findings by noting the similarities in compositions between some micrometeorites landing on Earth and comet samples returned by NASA's Stardust mission.  "There has been lots of debate about the nature of micrometeorites reaching the Earth," says Matthieu Gounelle of the Museum National d'Histoire Naturelle in Paris. "Some believe they are asteroidal, while others argue they are cometary. Our work suggests that in a sense, both camps may be right." Asteroids and comets residing in the over crowded Asteroid Belt would have continually suffered collisions, sending debris towards the Earth and Moon, as well as the other inner Solar System denizens.

"Some of the meteorites that once resided in the Asteroid Belt show signs they were hit by 3.5 to 3.9 billion years ago. Our model allows us to make the case they were hit by captured comets or perhaps their fragments," says Kleomenis Tsiganis of Aristotle University of Thessaloniki, Greece. "If so, they are telling us the same intriguing story as the lunar samples, namely that the Solar System apparently went berserk and reconfigured itself about four billion years ago."

The new results have the potential to fundamentally change our understanding of the evolution of the Asteroid Belt. The model will be put to the test by continued study of meteorites, and of airless bodies like asteroids and the Moon, which preserve key evidence from this tumultuous period in Solar System history.