Solar system, formation

Table 10.3. K-ratios of actinide abundances at Solar-System formation...

For the abundance in the ISM of a stable or very long-lived element at the time of Solar-System formation, we can take... 

Busfield A, Gilmour JD, Whitby JA, Turner G (2004) Iodine-xenon analysis of ordinary chondrite halide implications for early solar system water. Geochim Cosmochim Acta 68 195-202 Busso M, Gallino R, Wasserburg GJ (1999) Nucleosynthesis in asymptotic giant branch stars relevance for galactic enrichment and solar system formation. Annu Rev Astronom Astrophys 37 239-309 Cameron AGW (1969) Physical conditions in the primitive solar nebula. In Meteorite Research. Millman PM (ed) Reidel, Dordrecht, p 7-12... 

Chondrites are the oldest and most primitive rocks in the solar system. They are hosts for interstellar grains that predate solar system formation. Most chondrites have experienced a complex history, which includes primary formation processes and secondary processes that inclnde thermal metamorphism and aqneons alteration. It is generally very difficult to distinguish between the effects of primary and secondary processes on the basis of isotope composition. Chondrites display a wide diversity of isotopic compositions including large variations in oxygen isotopes. 

The solar system abundances of the elements are the result of the Big Bang, which produced hydrogen and helium, 7.5 billion years of stellar nucleosynthesis, which produced most of the rest of the elements, and the physical processes that mixed the materials together to form the Sun s parent molecular cloud. The unique features of the solar system composition may also reflect the stochastic events that occurred in the region where the Sun formed just prior to solar system formation. 

Equation (8.47), with t = 0 and the composition of lead from meteoritic troilite used for the initial isotopic ratio of lead, was used by Clair Patterson (1955,1956) to determine the age of the Earth. In the 1950s, the largest uncertainty in determining the age of the Earth was the composition of primordial lead. In 1953, Patterson solved this problem by using state-of-the-art analytical techniques to measure the composition of lead from troilite (FeS) in iron meteorites. Troilite has an extremely low U/Pb ratio because uranium was separated from the lead in troilite at near the time of solar-system formation. Patterson (1955) then measured the composition of lead from stony meteorites. In 1956, he demonstrated that the data from stony meteorites, iron meteorites, and terrestrial oceanic sediments all fell on the same isochron (Fig. 8.20). He interpreted the isochron age (4.55+0.07 Ga) as the age of the Earth and of the meteorites. The value for the age of the Earth has remained essentially unchanged since Patterson s determination, although the age of the solar system has been pushed back by —20 Myr. 

There is now consensus that the Moon formed following a colhsion of the early Earth with a Mars-sized impactor (as summarized in the 1986 book Origin oftheMoon). The timing of the impact and subsequent assembly of the Moon are not tightly constrained, but models of radiogenic isotopes suggest an age of 40 to 50 Myr after solar system formation (Halliday,... 

Having worked our way through the parts of the solar system for which we have significant cosmochemical or geochemical data, we will now see how what we have learned can constrain models for solar system formation. 

Hester, J. J. and Desch, S. J. (2005) Understanding our origins Star formation in HII region environments. In Chondrites and the Protoplanetary Disk, ASP Conference Series, 341, eds. Krot, A. N., Scott, E. R. D. and Reipurth, A. San Francisco Astronomical Society of the Pacific, pp. 107-130. A clear and up-to-date review of astronomical observations that constrain models for solar system formation. 

Stars form in dense cores within giant molecular clouds (see Fig. 1.4, Alves et al. 2001). About 1 % of their mass is in dust grains, produced in the final phases of stellar evolution. Molecular clouds are complex entities with extreme density variations, whose nature and scales are defined by turbulence. These transient environments provide dynamic reservoirs that thoroughly mix dust grains of diverse origins and composition before the violent star-formation process passes them on to young stars and planets. Remnants of this primitive dust from the Solar System formation exist as presolar grains in primitive chondritic meteorites and IDPs. 

Armed with the results of laboratory studies of astrophysical dust processing, we are able to interpret the complex and varied history of dust in protoplanetary disks. This information is complemented by the detailed analysis of the solid material that remains from the earliest epochs of Solar System formation. 

Transient heating events were important in the formation of the Solar System and provided the energy to produce chondrules and refractory inclusions. These objects are not predicted in astrophysical models for the formation of planetary systems. They comprise 50-80% of the mass of many primitive meteorites. However, the mechanism that produced the transient heating events is still unknown. Future work must focus on putting the details of the petrographic and chemical analysis of these rocks into an astrophysical and cosmochemical framework of Solar System formation. 

Cameron (1973) speculated that grains from stellar sources survive in the interstellar medium, become incorporated into bodies of the Solar System, and may be found in meteorites, because some meteorites represent nearly unprocessed material from the time of Solar System formation. These grains may be identified by unusual isotopic abundance ratios of some elements, since material from nuclear burning zones is mixed at the end of the life of stars into the matter from which dust is formed. Indeed, these presolar dust grains3 were found in the late 1980s in meteorites (and later also in other types of primitive Solar System matter) and they contain rich information on their formation conditions and on nucleosynthetic processes in stars (see Section 2.2). By identifying such grains in primitive Solar System matter it is possible to study the nature and composition of at least some components of the interstellar dust mixture in the laboratory. 

How would Solar System formation look to an outside observer... 

Busso M., Gallino R., and Wasserburg G. J. (1999) Nucleosynthesis in asymptotic giant branch stars relevance for galactic enrichment and solar system formation. Ante Rev. Astron. Astrophys. 37, 239—309. 

Huss G. R., Hutcheon I. D., and Wasserburg G. J. (1997) Isotopic systematics of presolar silicon carbide from the Orgueil (Cl) carbonaceous chondrite implications for solar system formation and stellar nucleosynthesis. Geochim. Cosmochim. Acta 61, 5117-5148. 

The solar system was formed as the result of the collapse of a cloud of pre-existing interstellar gas and dust. We should therefore expect a close compositional relationship between the solar system and the interstellar material from which it formed. If we make the assumption that the composition of the ISM has remained unchanged since the formation of the solar system, we can use the local ISM as a measure of the original presolar composition. Differences between the solar system and current local ISM would imply that fractionation occurred during the formation of the solar system, that the local ISM composition changed after solar system formation or that the solar system formed in a different part of the galaxy and then migrated to its present location. Studies of solar system and local ISM composition are therefore fundamental to the formation of the... 

If the 0-rich composition is indeed the isotopic composition of the primordial solar nebula, the consequences for solar system formation are profound. As noted above, materials with the °0-rich composition are ubiquitous, but they are also rather rare, never amounting to more than a few percent of the host meteorite. The implication is that all the other material in the inner solar system has undergone some process that changed its 0-abundance by 4-5%. This must have been a major chemical or physical process that must leave evidence in forms other than the isotopic composition of oxygen. 

The prior presence of " Pu, the only transura-nic nuclide known to have been present in the early solar system, can be inferred from its spontaneous-fission decay branch, through production of fission tracks and, more diagnostically, by production of fission xenon and krypton. The identification of " Pu as the fissioning nuclide present in meteorites is unambiguous, since the meteoritic fission spectrum is distinct from that of but consistent with that of artificial " Pu (Alexander et al, 1971). The demonstration of the existence of " Pu in the solar system reinforced the requirement (from the presence of I) of a relatively short time between stellar nucleosynthesis and solar-system formation and made it incontrovertible, since while it might be possible to make in some models of early solar system development, the rapid capture of multiple neutrons (the r-process) needed to synthesize Pu could not plausibly be supposed to have happened in the solar system. 

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