Excitation in the Early Solar Nebula — New Experimental Findings
Inferences about the formation of primordial matter in our solar system rest on analysis of the earliest preserved materials in meteorites, of the structure of the solar system today, and of matter in evolving stellar systems elsewhere.
The isotope distribution in meteorites suggests that molecular excitation processes similar to those observed today in circumstellar regions and dark interstellar clouds were operating in the early solar nebula. Laboratory model experiments together with these observations give evidence on the thermal state of the source medium from which refractory meteoritic dust formed. They indicate that rl o9gnce excitation of the broad isotopic bands of molecules such as 12C 16O, MgO, O2, AlO and OH by strong UV line sources such as H Lyd, Mg II, HB and Ca II may induce selective reactions resulting in the anomalous isotopic composition of oxygen and possibly other elements in refractory oxide condensates in meteorites.
The temperature of the grains condensing from this medium can be determined from the interdiffusion of elements between metal grains in contact with each other; the results of such analyses illustrate the large temperature differential between condensing dust and the surrounding source plasma. The metal diffusion couples mostly consist of platinum or platinum metal alloys in contact with nickel iron, encased in refractory oxide grains. These consist of minerals such as magnesium aluminate (spinel) and calcium aluminum silicates (melilite and pyroxene). The metal interdiffusion shows that they have formed at temperatures ≤ 1000 K; this is less than or about one half of the temperature surmised from consideration of thermodynamic rather than thermal radiation equilibrium.
KeywordsIsotope Fractionation Interdiffusion Coefficient Source Medium Nickel Iron Space Medium
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