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What are they?
The Noblesville and Sikhote-Alin meteorites are examples of the two major types of meteorites. Noblesville is a stony meteorite, the most common type, while Sikhote-Alin is a much rarer iron meteorite. |
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16. Stony meteorite
Stony meteorites are commonly made of familiar minerals like plagioclase, pyroxene, and olivine. Scientists believe that they were formed in the outer parts of asteroids. Stony meteorites look a lot like Earth rocks, and are often not recognized as meteorites. Their outer surfaces are usually melted as they pass through our atmosphere, giving them dark "fusion crusts." That is why the outside of the Noblesville meteorite looked so dark. (Photo by Cecilia Satterwhite, NASA JSC photo S94-44343)
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17. Iron meteorite
Iron meteorites are made almost entirely of iron metal with some nickel. As with stony meteorites, iron meteorites also have fusion crusts and show distinct molten metal shapes and flow markings, like those on the fragment of Sikhote-Alin.
Inside, many iron meteorites are made of criss-crossing intergrown crystals of two different iron-nickel minerals. The sample of the iron meteorite in the Meteorite Sample Disk has an etched surface that shows these patterns. (Photo by Carl Allen, NASA JSC photo S94-43470)
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18. Stony-iron meteorite
Some meteorites are mixtures of iron and fragments of rock; they are called stony-iron meteorites. This sample, like the one in the Meteorite Sample Disk, formed at the boundary between the metal core and the rocky mantle of an asteroid. (Photo by James Holder, NASA JSC photo S94-44546)
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19. Solar nebula
The story of meteorites begins 4.6 billion years ago. The solar system began as a spinning cloud of gas and dust, called the solar nebula, which collapsed under its own weight to form a new star, our Sun. As the solar nebula spun and churned, dust grains stuck together to form dustballs, and huge bolts of lightning melted them into small spheres. These solidified into rocky balls called chondrules. (Painting by Don Dixon, NASA JSC photo S76-25001)
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20. Differentiation
Some very primitive meteorites are just masses of chondrules stuck together. In most meteorites, though, the chondrules have been partially or totally destroyed. To destroy chondrules takes a lot of pressure and cooking time, so much that these meteorites could not have formed as small rocks floating in the solar nebula. They must have come from inside larger objects, the asteroids, where the weight of overlying rock created enough pressure to obliterate the chondrules.
But more happened inside asteroids than merely squeezing chondrules. In the largest asteroids, heat and pressure got so great that rock could melt and crystals move under the force of gravity. Heavy iron crystals sank toward the center and lighter silicates floated toward the surface. The results were differentiated asteroids with metal cores and rocky mantles and crusts. (Artist's conception, NASA JSC photo S94-43469)
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