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Origin of Elements in the Solar System 589 Origin of Elements in the Solar System O. Manuel Chemistry Department, University of Missouri, Rolla, MO 65401 USA om@umr.edu If our inconceivably ancient Universe even had any beginning, the conditions determining that beginning must even now be engraved in the atomic weights. ” Theodore W. Richards (1919) Abstract: The solar system is chemically and isotopically heterogeneous. The earth contains only 0.0003% of the mass of the solar system, but the abundance pattern of non-radiogenic isotopes for each terrestrial element has been de- fined as “normal”. The outer planets consist mostly of light elements like H, He and C. The in- ner planets are rich in heavy elements like Fe and S. Isotopic irregularities are closely linked with these chemical differences in planets, as well as in the primary minerals of chondritic meteorites. Chondrites are heterogeneous, agglomerate rocks from the asteroid belt that separates the two types of planets. They contain troilite (FeS) inclusions with isotopically “normal” Xe, like that found in the inner planets. Chon- drites also contain diamond inclusions (C) with abundant He and “strange” Xe enriched in isotopes from the r- and p-processes. The Galileo probe found similar r-products in the Xe isotopes of Jupiter, a planet rich in He and C. The sun is a mixture of the chemically and isotopically distinct compo- nents found in its planetary system. Inter-linked chemical and isotopic ir- regularities, short-lived radioactivities and other post-1957 observations are used here to evaluate the two most conflicting opinions on the origin of ele- ments in the solar system. The first of these is that the elements in the solar system originated via remote element synthesis (RES). The RES model is the modern version of the classic nebular model postulated by Kant and Laplace over 200 years Proceedings of the 1999 ACS Symposium (organized by Glenn Seaborg and O. Manuel): The Origin of Elements in the Solar System: Implications of Post-1957 Observations (O. Manuel, editor, Kluwer/Plenum Pub., 2000) pp. 589-643.
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O. Manuel 590 ago for the origin of the solar system. It is a natural extension of the cos- mological view of element synthesis. According to the nebular RES view, products of nuclear reactions collected from multiple stellar sources over vast regions of space and produced a well-mixed protosolar nebula having approximately the elemental composition of the sun’s photosphere and iso- topic ratios of carbonaceous chondritic meteorites. The sun formed as a fully convective, homogeneous protostar. Elements in the planetary system were subsequently redistributed to produce the solar system’s current chemi- cal gradients. This nebular RES model was modified in the late 1970s and early 1980s to try to explain the occurrence of isotopic anomalies and decay products of short-lived nuclides by the addition of very small amounts of
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