It is widely accepted that the sun and planets formed from a common, relatively homogeneous, interstellar gasdust cloud. Comparison of the isotopic compositions of elements in the solar wind with those in planetary materials is a general test of this assumption. A survey of present knowledge of solar system isotopic compositions is given in Document A. It is wellestablished that solar system matter represents a mixture of materials having different histories of stellar nucleosynthesis and that these materials were isotopically very heterogeneous. Thus, if the sun and planets formed from an inhomogeneous cloud, differences in isotopic composition should result between solar and terrestrial/meteoritic matter. The uniformity of most isotopic compositions among terrestrial, lunar, and meteoritic materials at the 1% or better level in bulk (>0.1g) samples does not preclude systematic isotopic variations between solar and planetary matter. Variations at the sub-1% level are found among different planetary materials. It is of fundamental importance to test for solar-planetary differences. Variations of 1-10% could realistically be expected, although what is known about solar system isotope ratios (Document A) probably rules out large (factor of 2) variations. The only well known solar wind isotopic ratio (22Ne/20Ne) differs from the terrestrial atmosphere by 30% It is certain that measurable differences will be found for N, O, and Ti, as well as the noble gases (Document A).
More specifically, astrophysical studies of star formation make it likely that the Sun formed from a parent molecular cloud along with many other stars. It is likely that the initial stellar mass distribution of this cluster would contain massive stars which would evolve through their whole life cycle in a time comparable to that required for the Sun to evolve to the main sequence, producing local supernovae, which in principle could add a component of mass to planetary, but not to solar, material. The existence of isotopic anomalies in meteorites, particularly those attributable to the relatively shortlived radioactive nuclei, 26Al and 107Pd, (see e.g. Kelley and Wasserburg, 1978), indicates that such injections probably did occur, but these data do not require any significant amount of mass injected, nor are there any constraints on the timing of the injection(s) relative to the time of isolation of solar and planetary material. Comparison of potentially sensitive isotopic ratios, e.g., 26Mg/24Mg, between the solar wind and terrestrial matter provide constraints on these questions.
Because most of the mass in the solar system is in the Sun, measurements of the solar isotopic composition of these elements provide the natural reference points in understanding which samples are "anomalous" compared to the average solar system value. For example, it would be of considerable interest to know which, if any, meteoritic materials or which planetary atmospheres have solar isotope composition for N,O, and noble gases.
In general any solar wind isotopic ratio measured to approximately 1% would be important in clarifying the initial heterogeneity of the solar nebula. In some cases, where large variations might be expected, e.g. 124Xe/130Xe, measurement even to 10% would be important.
Theories of stellar nucleosynthesis predict widely different isotopic/chemical products of nuclear burning in individual stars depending on the specific history of stellar evolution (mass, initial composition, details of mass return to the interstellar medium, etc.). Nevertheless, because most of the injection of heavy elements may come from massive stars in associations, the resulting interstellar clouds could in fact be relatively homogeneous on a scale of parsecs, although not on a scale of millimeters. However, it is likely that our parent molecular cloud was formed by the coalescence of fragments from several pre-existing "grandparent" molecular clouds, and there could still be age differences in various clouds which would show up in differences in the isotopic composition of elements which have radioactive isotopes (K, U, Rb, etc.) if solar matter contained a different distribution of grandparent cloud fragment ages than planetary matter. The half lives of 40K and 235U are most appropriate for this test, but solar wind measurements of these nuclei would be very difficult. However, measurement of 85Rb/87Rb is probably feasible and should be carried out for these reasons.
Kelly W.R. and Waserburg G.J. (1978) Evidence for the existence of 107Pd in the early solar system. Geophys. Res. Lett. 5, 1079-1082.
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