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The hot topic at the Meteoritical Society Meeting this first morning of the conference was “exactly what was the early solar system like before the asteroids and planets formed”. Some of the objects we find in meteorites are thought to have formed before their parent asteroids in the dusty gas disk, known as the solar nebula (shown opposite), that surrounded the early Sun. Particularly important are millimetre-sized spheres, known as chondrules, that formed as tiny droplets of molten rock in the nebula. How these fiery droplets formed is important to understand since it will tell us not only what the early Solar System was like but also why the materials that make up asteroids, including Near Earth Objects, are like they are.
Steve Desch, from the Carnegie Institute of Washington, thinks that chondrules were made in shock waves blasting through the gas in the solar nebula. Shock waves are rapidly moving, strong disturbances, similar to a very strong gust of wind, and chondrules may have formed by melting of dust due to heating by friction with the high speed gas behind the shock. Desch and colleages presented new, detailed calculations to show how quickly the dust would be heated and cooled in shock waves in the solar nebula. His results are similar to the estimates gleaned from the minerals and crystal shapes found in chondrules in meteorites. More than this the calculations looked at where the dust was found in the dusty gas disk and found that if it was mainly in the middle of the disk shock waves could generate the same amounts of the different kinds of chondrules that we see in meteorites.
Where the dust was in the gas disk surrounding the early Sun was described by Steve Weidenschilling, from the Planetary Science Institute in Tuscon, Arizona. Dust grains in the nebula sank to the middle of the disk, Weidenschilling says, because they all attract each other due to their gravity and it was here, at the so called mid-plane, that chondrules, and eventually asteroids and planets, were made. In his calculations, however, Weidenschilling realised that dust particles circling the early Sun are slowed down by the surrounding gas and this makes them move towards the Sun. The result of the dust moving inwards is that it stirs up the disk making it more and more difficult for dust to settle. Weidenschilling’s results, therefore, mean that chondrules, and eventually asteroids, could not have be constructed as fast as they otherwise would have been without the stirring.
Overall the results presented this morning have given us a fascinating insight into the earliest stages of the formation of our Solar System that may one day provide us with a theory to explain the how's and why's of the formation of planets and asteroids.
More info: The Meteoritical Society
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