Solar system abundance

Percentage of meteorites seen to fall. Chondrites. Over 90% of meteorites that are observed to fall out of the sky are classified as chondrites, samples that are distinguished from terrestrial rocks in many ways (3). One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Elemental composition is also a property that distinguishes chondrites from all other terrestrial and extraterrestrial samples. Chondrites basically have undifferentiated elemental compositions for most nonvolatile elements and match solar abundances except for moderately volatile elements. The most compositionaHy primitive chondrites are members of the type 1 carbonaceous (Cl) class. The analyses of the small number of existing samples of this rare class most closely match estimates of solar compositions (5) and in fact are primary source solar or cosmic abundances data for the elements that cannot be accurately determined by analysis of lines in the solar spectmm (Table 2). Table 2. Solar System Abundances of the Elements ... 

Fig. 2. The deuterium abundance (by number relative to hydrogen), yi> = 105(D/H), derived from high redshift, low metallicity QSOALS [3] (filled circles). The metallicity is on a log scale relative to solar depending on the line-of-sight, X may be oxygen or silicon. Also shown is the solar system abundance (filled triangle) and that from observations of the local ISM (filled square).

Fig. 3. The 3He abundances (by number relative to hydrogen), j/3 = 10B(3He/H), derived from Galactic H n regions [4], as a function of galactocentric distance (filled circles). Also shown for comparison is the solar system abundance (solar symbol). The open circles are the oxygen abundances for the same H n regions (and for the Sun).

Fig. 5.11. Amounts, in units of relative Solar-System abundances, of nuclear species resulting from hydrostatic evolution of an average pre-supernova. Filled circles represent an initial mass function with slope -2.3 and plus signs one with slope -1.5. The dashed box encloses 28 species co-produced within a factor of 2 of solar values, assuming a vlC.(u. y)16O rate 1.7 x that given by Caughlan and Fowler (1988). Reprinted from Weaver and Woosley (1993). Reproduced with kind permission of Elsevier Science. Courtesy Tom Weaver.

Fig. 5.25. Nucleosynthetic outcome (after radioactive decay) of model W7 for Type la supernovae (Nomoto, Thielemann Yokoi 1984, and Thielemann, Nomoto Yokoi 1986), compared to Solar-System abundances. Dominant isotopes of multi-isotope elements are circled. Adapted from Tsujimoto (1993).

Fig. 6.5. Development of the convective region, neutron density from 13C and 22Ne sources and maximum temperature as functions of time during a thermal pulse in a low-mass star with Z Z0/3, which seems to give the best fit to Solar-System abundances from the main s-process. However, more recent models imply that 13C is all used up in the radiative phases. After Kappeler et al. (1990). Courtesy Maurizio Busso and Claudia Raiteri.

Fig. 6.10. Results of a dynamical calculation of the r-process in the hot neutrino bubble inside a 20 Mq supernova (continuous curve) compared to the observed Solar-System abundance distribution (filled circles). After Woosley etal. (1994). Courtesy Brad Meyer.

Some sample results of calculations of stellar yields, i.e. the mass of a nuclear species freshly synthesized and ejected from a star of given initial mass and chemical composition, are given in Tables 7.2, 7.3 and 7.4. The last column of Table 7.2 gives an IMF-integrated yield which can be compared with the Solar-System abundances. 

Fig. 9.1. Abundances in primary cosmic rays reaching the top of the Earth s atmosphere, compared to Solar-System abundances. (Both normalized to C = 100.) After Rolfs and Rodney (1988). Copyright by the University of Chicago. Courtesy Claus Rolfs.

Hubert Reeves once summed up the similarity of cosmic-ray source and Solar-System abundances in the form of a graffito seen at times in Paris CRS = SS ... 

Harper CL (1996) Evidence for Nb in the early solar system and evaluation of a new p-process cosmochronometer from Nb/ Mo. Astrophys J 466 437-456 Harper CL, Wiesmann H, Nyquist LE, Hartmann D, Meyer B, Howard WM (1991) Interpretation of the Ti- Zr anomaly correlation in CAI NNSE Zr production limits and S/ R/ P decomposition of the bulk solar system abundances. Lunar Planet Sci XXII 517-518... 

Fig. 5.5. Decomposition of Solar System abundances into r and s processes. Once an isotopic abundance table has been established for the Solar System, the nuclei are then very carefully separated into two groups those produced by the r process and those produced by the s process. Isotope by isotope, the nuclei are sorted into their respective categories. In order to determine the relative contributions of the two processes to solar abundances, the s component is first extracted, being the more easily identified. Indeed, the product of the neutron capture cross-section with the abundance is approximately constant for aU the elements in this class. The figure shows that europium, iridium and thorium come essentially from the r process, unlike strontium, zirconium, lanthanum and cerium, which originate mainly from the s process. Other elements have more mixed origins. (From Sneden 2001.)... 

Anders, E. and Ebihara, M., 1982. Solar-system abundances of the elements. Geochim. Cosmochim. Acta, 46 2363-2380. 

The term cosmochemistry apparently derives from the work of Victor Goldschmidt (Fig. 1.6), who is often described as the father of geochemistry. This is yet another crossover and, in truth, Goldschmidt also established cosmochemistry as a discipline. In 1937 he published a cosmic abundance table based on the proportions of elements in meteorites. He used the term cosmic because, like his contemporaries, he believed that meteorites were interstellar matter. Chemist William Harkins (1873-1951) had formulated an earlier (1917) table of elemental abundances - arguably the first cosmochemistry paper, although he did not use that term. As explained in Chapter 3, the term solar system abundance is now preferred over cosmic abundance, although the terms are often used interchangeably. 

In the final section of this chapter, we discussed the formation of galaxies and the formation and chemical evolution of the Milky Way. This sequence of events set the stage for the formation of the solar system. In Chapter 4, we will look at the resulting abundances of the elements and isotopes, both in the solar system and in the galaxy. The solar system abundances of the elements are a fundamental constraint for understanding the Sun, the planets, and the smaller bodies in the solar system. 

In this chapter, we discuss the abundances of the elements and isotopes in the solar system. First, we look at the techniques used to determine solar system abundances, including spectroscopy of the stellar photosphere, measurements of solar wind, and analyses of chondritic meteorites. The solar system abundances of the elements and isotopes are then presented. These abundances are then compared to the abundances in the solar neighborhood of the galaxy and elsewhere. Finally, we introduce how solar system abundances provide a basis for much of what we do in cosmochemistry. 

In recent decades, spectroscopy has revealed that the elemental and isotopic abundances in the galaxy vary with radial position and that the Sun has a somewhat different composition than the molecular clouds and diffuse interstellar medium in the solar neighborhood. For this reason, we can no longer think of the solar system abundances as truly cosmic abundances. 

Element Mean Cl chondrites Solar photosphere Solar system abundance ... 

As understanding of nuclear physics and stellar nucleosynthesis has improved, it has become possible to use that knowledge along with measured abundances to infer the solar system abundances of elements that cannot be directly measured, either by solar spectroscopy because their ionization potentials are too high, or in meteorites because they were not efficiently incorporated into the chondrites. In particular, the noble gases cannot be directly measured and must be inferred from theoretical considerations or other indirect means. 

Table 4.1 shows the solar system abundances of the elements as determined by the methods discussed above. For some elements, the photospheric abundances provide the best estimate, whereas for others, the meteorite data must be used. In some cases, the data are equally reliable and an average of the values determined from the solar photosphere and Cl chondrites is used. The abundances of the noble gases come from indirect measurements or theoretical considerations. The method for determining each abundance is indicated in the far right column of Table 4.1. 

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. 

We have now considered how the solar system abundances are determined and have discussed the uncertainties in the abundance numbers. Here we briefly discuss how these abundances are used in cosmochemistry. These uses will be discussed further in later chapters. 

In the cosmochemistry literature, you will often see data normalized to (that is, divided by) solar system abundances (most commonly those of Cl chondrites). An important reason for doing this is illustrated in Figure 4.6. The top panel of this figure shows a plot of the composition of a chondrule with the elements arranged in order of their volatility from most... 

In this chapter, we have introduced solar system abundances and cosmic abundances of the elements and isotopes and briefly discussed their importance in cosmochemistry. The idea of cosmic abundances has been around for over a century and is rooted in measurements of many different celestial objects. Only in the last 20-30 years has our knowledge of the compositions of the solar system, of stars and molecular clouds in our galaxy, and of other galaxies grown sufficiently that we can now detect clear compositional differences between our solar system and other objects in the galaxy and the universe. 

Solar system abundances are a cornerstone of our understanding of the origin and evolution of the solar system and the elements from which it formed. To understand... 

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