http://www.cosmosmagazine.com/node/1825
News Stardust blurs line between asteroids and comets Friday, 25 January 2008Cosmos Online SYDNEY: A new analysis of comet grains collected by NASA's Stardust probe, has surprised experts by revealing that they don't contain samples of the most ancient material in the Solar System. Rather than primordial dust, the samples contain material that has been more commonly in asteroids from the inner Solar System. The find reported today in the U.S. journal Science by cosmochemists led by Hope Ishii of the Lawrence Livermore National Laboratory in California, blurs the distinction between asteroids and comets. Rich source of dust When the Stardust returned to Earth in 2006 it carried material from the comet Wild 2 the first sample of material from the outer Solar System. NASA experts thought it would be a rich source of dust, dating from before the planets formed around 4.5 billion years ago. "Presolar material acts like a time capsule," said study co-author Anton Kearsley a mineralogist at the U.K.'s Natural History Museum in London "[It tells] us about the origins of our Solar System, and what was here before. We had hoped to find lots of undamaged dust from the birthplace of the Solar System on comet Wild 2, but have discovered that there are very few unambiguous presolar grains." The Stardust samples were definitely collected from an object you'd describe as a comet, added Kearsley (i.e. icy, with a gaseous head and tail, with an elliptical orbit and originating from the distant Kuiper Belt). "But it seems that even if you sample directly from a comet itself, you won't necessarily get the oldest material." 20,000 kph impact Much existing knowledge about the composition of primordial dust comes from meteorites on Earth and a type of interplanetary dust particle (IDP) scooped up by aircraft flying 15 to 20 km up in the stratosphere. IDPs contain curious particles of glass with embedded metals and sulphides, known as GEMS. Planetary scientists believe these were formed in interstellar space before being swept into the cloud of dust and gas from which our Solar System formed. Stardust collected thousands of particles in 2004, each merely micrometres across. As Wild 2 rushed past the spacecraft at over 20,000 kph, tiny dust grains were embedded in its collector. When the samples eventually returned to Earth in 2006, some of the trapped particles were thought to look like GEMS. However, laboratory analysis has now shown these GEMS-like structures were more likely created during the heat of impact on the collector. Another indicator of primitive interplanetary dust a distinctive form of the mineral enstatite is also missing from the Stardust samples, write the experts. More diverse than we knew The mission has revealed that comets are likely more diverse than we knew; some containing material forged in the swirling disk of gas and dust which then became the Solar System, while others preserve even more primitive interstellar material. The distinction between comets and asteroids has been blurred by the find, said the authors, which shows that Wild 2 may have more in common with inner some asteroids than comets. The samples from Wild 2 represented the first new extra-terrestrial material brought back to Earth since the missions to the Moon in the 1970s. Stardust flew further than any other sample-return mission 4.63 billion km, in looping orbits, out to between the orbit of Mars and the asteroid belt. ______________________________________________ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
