ABSTRACT
Apart from the elements in Mendeleev's periodic table (H, Li, Be, the chemical elements and their isotopes are nearly all the result of stellar nudeosynthesis (Clayton , 1 983 ; Fowler, 1 9 84) . By 1 970, it was traditionally admitted that one of the best supported hypotheses stated that the solar system was isotopically homogenous since the protosolar gas was thoroughly mixed (Suess , 1 965) . The planets and the other components of the solar system had condensed in a homogenous medium, as witnessed, for instance, by the fact that on a large scale, the l 3C/1 2Cratioof theplanets and of the sun is experimentally known to better than 1 0% (Marten et aI., 1 994) . No formed prior to the protosolar could survived extremely
Table 3.1 : Characteristics of the interstellar carbon-bearing phases found in meteorites . Ne E(L)andNe-E(H)aretwotypesofneonessentiallypurei n 22Ne extractedrespectivelyatlow (L) and high (H) temperature. The isotopic compositions of Ne-A are said to be planetary (close to terrestrial atmosphere) . The composition of Xe(HL) is characterised by an excess of both light and heavy isotopes (cf. the top half of Figure 3 . 1 ) . The i sotopic composi tion ofXe (S) (bottom half of Figure 3 . 1 ) corresponds to the theoretical predictions of stellar nudeosynthesis bycaptureofs lowneutrons .
