* Astronomy

Chronicle of a Chondrule's Travels

Cometary particles returned in 2006 by NASA's Stardust spacecraft provide crucial information not only about the mission's target, comet 81P/Wild 2, but also about the dynamics in the solar nebula that brought the comet's materials together. Ryan Ogliore (University of Hawaii) and colleagues from the universities of Hawaii, California, and Washington, and the Lawrence Berkeley National Laboratory have focused attention on a tiny chondrule fragment from one of the Wild 2 particles. Using petrology, oxygen isotopes, and aluminium-magnesium isotopic measurements, they determined this chondrule formed relatively late (as chondrules go) in the inner solar nebula and moved out to the comet-forming region before Jupiter could have blocked its way. The timing implies Jupiter formed more than three million years after the formation of the first solids in our Solar System.
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Title: Experimental investigation of the nebular formation of chondrule rims and the formation of chondrite parent bodies
Authors: Eike Beitz, Jürgen Blum, Romain Mathieu, Andreas Pack, Dominik C. Hezel

We developed an experimental setup to test the hypothesis that accretionary dust rims around chondrules formed in the solar nebula at elevated temperatures. Our experimental method allows us to form dust rims around chondrule-analogues while being levitated in an inert-gas flow. We used micrometer-sized powdered San Carlos olivine to accrete individual dust particles onto the chondrule-analogue at a temperature of 1100°C. The resulting dust-rims were analysed by means of two different techniques: one sample was investigated with non-destructive micro computer tomography, the other with a scanning electron microscope. Both methods give very similar results for the dust-rim structure and a mean dust-rim porosity of 60 percent, demonstrating that both methods are equally well suited for sample analysis. The chondrule-analogue's bulk composition has no measurable effect on the accretion efficiency of the dust. We measured the chemical composition of chondrule-analogue and dust-rim to check whether elemental exchange between the tow components occurred. Such a reaction zone was not found, thus we can experimentally confirm the sharp border between chondrules and dust-rims described in the literature. The experimental dust-rim porosities and reported porosities in carbonaceous and ordinary chondrites depend on the meteorite type, indicating different degrees of compaction subsequent to parent

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Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids. Prominent among the components present in chondrites are the enigmatic chondrules, millimetre-sized objects that originated as freely floating, molten or partially molten droplets in space; most chondrules are rich in the silicate minerals olivine and pyroxene. Chondrites also contain refractory inclusions (including Ca-Al Inclusions), which are among the oldest objects to form in the solar system, particles rich in metallic Fe-Ni and sulphides, and isolated grains of silicate minerals. The remainder of chondrites consists of fine-grained (micrometer-sized or smaller) dust, which may either be present as the matrix of the rock or may form rims or mantles around individual chondrules and refractory inclusions. Embedded in this dust are presolar grains, which predate the formation of our solar system and originated elsewhere in the galaxy.

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