Early asteroids in the Main Asteroid Belt may have formed far from the Sun
Many objects found today in the asteroid belt between the orbits of Mars and Jupiter may have formed in the far reaches of the solar system, according to an international team of astronomers led by scientists from the Southwest Research Institute ( SwRI).
The team used numerical simulations to show that some comet-like objects residing in a disk outside the planets’ original orbit were dispersed across the solar system and into the outer asteroid belt for a violent phase of planetary evolution.
Usually, the solar system is thought of as a place of relative permanence, with changes occurring gradually over hundreds of millions to billions of years. New models of planet formation, however, indicate that at specific times the architecture of the solar system has undergone dramatic upheavals.
In particular, it now seems likely that around 3.9 billion years ago the giant planets of our solar system – Jupiter, Saturn, Uranus and Neptune – rearranged in a tumultuous spasm. “This last major event in planet formation appears to have affected nearly every nook and cranny of the solar system,” says lead author Dr. Hal Levison of SwRI.
Key evidence for this event was first identified in samples returned from the Moon by Apollo astronauts. They tell us about an ancient cataclysmic bombardment where large asteroids and comets rained down on the Moon.
Scientists now recognize that this event was not limited to just the Moon; it also affected the Earth and many other bodies in the solar system. “The existence of life on Earth, as well as the conditions that made our world habitable for us, are strongly linked to what happened in that distant time,” says Dr David Nesvorny of SwRI.
The same dynamic conditions that devastated the planets also led to the capture of some potential impactors in the asteroid belt. “In the classic ‘Casablanca’ movie, everyone comes to Rick’s house. Apparently, throughout the solar system, the cool meeting place for small objects is the asteroid belt,” says SwRI’s Dr William Bottke.
Once in the asteroid belt, the comet-like objects began to fight each other and the asteroids. “Our model shows that comets are relatively easy to shatter when hit by something, at least compared to typical asteroids. It’s inevitable that some of the debris landed on asteroids, the Moon, and Earth. In fact , some of the remains may still be arriving today,” says Dr. Alessandro Morbidelli of the Côte d’Azur Observatory in Nice, France.
The team thinks the startling similarities between some micrometeorites landing on Earth and comet samples returned by NASA’s Stardust mission are no accident. “There has been a lot of debate about the nature of micrometeorites reaching Earth,” says Dr Matthieu Gounelle of the Muséum National d’Histoire Naturelle in Paris. “Some think they’re asteroids, while others claim they’re comets. Our work suggests that, in a sense, both sides may be right.”
“Some of the meteorites that once resided in the asteroid belt show signs that they were hit 3.5 to 3.9 billion years ago. Our model allows us to prove that they were hit by comets captured or perhaps their fragments”, adds Dr Kleomenis Tsiganis. from the Aristotle University of Thessaloniki, Greece. “If so, they tell us the same intriguing story as the lunar samples, which is that the solar system apparently went berserk and reconfigured itself around 4 billion years ago.”
Overall, the main asteroid belt contains a surprising diversity of objects ranging from primitive ice/rock mixtures to igneous rocks. The standard model used to explain this assumes that most asteroids formed in place from a primordial disk that underwent drastic chemical changes in that area. This model shows, however, that the observed diversity of the asteroid belt is not a direct reflection of the variation in the intrinsic composition of the proto-planetary disk. These results fundamentally change our view of the asteroid belt.
Further testing of this model will come from studies of meteorites, the asteroid belt, planet formation, and the Moon. “The Moon and the asteroid belt are perhaps the best and most accessible places in the solar system to understand this critical part of the solar system’s history,” says Levison. “We believe key evidence from these airless cold bodies can help us unlock the greatest ‘cold case’ ever.”
Funding for this research was provided by NASA’s Outer Planets Research and Origins of Solar Systems programs. Additional support was provided by NASA’s Lunar Science Institute.
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