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Asteroids scarred by solar system's violent youth

Pushed around Telltale evidence of the solar system's traumatic childhood can be found in the main asteroid belt, which contains a far more integrated assortment of bodies than previously believed, a new study shows.

Previous observations of the 2000 or so biggest asteroids in the belt — those with diameters of roughly 100 kilometers or larger — showed a neat structure, with asteroids closer to the Sun having surface temperatures warmer than those located farther away.

The observations neatly match theories about the formation of the solar system, which posits that bodies formed in warm environments would be found closer to the Sun and those formed in cold environments are farther away.

"We said, 'Oh look, this has been preserving the conditions from the original formation. Case closed. It all makes sense,'" says astronomer Francesca DeMeo, with the Harvard-Smithsonian Center for Astrophysics.

But a new analysis, this time based on 100,000 asteroids of varying sizes, tells a far different story.

"Everything is mixed. Pieces are everywhere, like they've been just kind of thrown all over the asteroid belt," says DeMeo, lead author of a study that appears in this week's journal Nature.

"It's certainly overturned a lot of traditional thinking," adds University of Arizona planetary scientist Dante Lauretta, lead researcher for an upcoming NASA asteroid sample return mission.

"There is still an underlying structure and composition, but there is evidence of mixing and that just makes so much sense to me," he says.

Gravitational elbowing

Scientists don't yet know why smaller asteroids buck the trend of their larger siblings, but that it is related to the gravitational elbowing by jostling planets early in the solar system's history.

"What we're leaning toward now is that asteroids, rather than forming in the asteroid belt, formed throughout the entire solar system ... as close (to the sun) as Mercury and as far away as Neptune, and then, through the planetary migration, you scatter them all over the place. What's left is what you see in the asteroid belt today," DeMeo said.

A more detailed understanding of what happened to the asteroids is one of the primary goals of NASA's OSIRIS-REx sample return mission, which is slated for launch in September 2016. Samples from a cold asteroid named Bennu are due back on Earth in 2023.

Already in hand are samples from a warm asteroid collected by Japan's Hayabusa spacecraft in 2005 and returned in 2010.

Together, the analyses will help scientists match information about asteroids collected by telescopes to the actual chemical and physical processes that shaped them.

"Asteroids' compositions tell us about where they formed. Where they are today tells us the whole evolution of where they've gone since," DeMeo says.