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Asteroid belt "contaminated" by distant comets
Posted: JULY 16, 2009

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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.

Did a shake-up of the giant planets' orbital architecture send comets into the Asteroid Belt? Image: NASA/JPL-Caltech.

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.

Click here for animation simulating the first 1.2 billion years of Solar System history. The orbits of the four giant planets are shown as colour ellipses. The green dots show small comet-like objects. A small fraction of these objects become trapped in the Asteroid Belt when the orbits of the planets become unstable. Credit: SwRI.

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."

Micrometeorites frequently fall to the Earth, and may be derived from either asteroids or comets originating in the Asteroid Belt. Image: CSNSM-Orsay-CNRS / IPEV.

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.

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!


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