Element abundances, solar

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 ... 

Extraterrestrial dust particles can be proven to be nonterrestrial by a variety of methods, depending on the particle si2e. Unmelted particles have high helium. He, contents resulting from solar wind implantation. In 10-)J.m particles the concentration approaches l/(cm g) at STP and the He He ratio is close to the solar value. Unmelted particles also often contain preserved tracks of solar cosmic rays that are seen in the electron microscope as randomly oriented linear dislocations in crystals. Eor larger particles other cosmic ray irradiation products such as Mn, Al, and Be can be detected. Most IDPs can be confidently distinguished from terrestrial materials by composition. Typical particles have elemental compositions that match solar abundances for most elements. TypicaUy these have chondritic compositions, and in descending order of abundance are composed of O, Mg, Si, Ee, C, S, Al, Ca, Ni, Na, Cr, Mn, and Ti. 

Table 2-1 Solar abundances of the elements (atoms/lO atoms of Si)...

Fig. 3.42. Depletion below solar abundances of elements in the H I gas towards f Ophiuchi plotted against atomic mass number in (a) and condensation temperature in (b), based in part on the curve of growth shown in Fig. 3.11. Vertical boxes indicate error bars. The dotted curve in the left panel represents an A-1/2 dependence expected for non-equilibrium accretion of gas on to grains in the ISM. The condensation temperature gives a somewhat better, though not perfect, fit, suggesting condensation under near-equilibrium conditions at a variety of temperatures either in stellar ejecta or in some nebular environment. Note the extreme depletion of Ca ( Calcium in the plane stays mainly in the grain ). After Spitzer and Jenkins (1975). Copyright by Annual Reviews, Inc.

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.)... 

Table 14.3 Abundances of major elements and molecules in the atmospheres of the giant planets, relative to solar abundances (Lunine, )...

Finally, we mention the central star of NGC 246. This star is known to display CIV as the strongest absorptions in the blue spectrum and broad shallow lines of Hell hydrogen can not be detected (Heap, 1975 Husfeld, 1986). Analysis by Husfeld (1986) revealed that this CSPN is extremely hydrogen-deficient solar abundance of this element cannot be excluded. 

Using the line strengths and parameters given in Table 1, and the solar abundances for So, Fe, Sr, and Ba listed by Cameron (1982), we have oomputed the relative abundances of these elements with respect to their solar values as a function of exoitation temperature from eqn, (4), and the results are shown in Figure 1. The corresponding color temperature for the supernova at this time has... 

It should be emphasized that solar abundance ratios are used here only as a convenient referenoe point. The LMC is known to have a total heavy element abundance that is approximately two to three times less than solar (van Genderen, van Driel, and Greidanus 1986 Dufour 1984). The abundances of Sc, Sr, and Ba in the LMC are not known because of the difficulty in detecting lines of these elements in objects. They are probably not solar however, unless the history of nucleosynthesis in the Large Cloud is completely different from that in our Galaxy, the relative abundances of the s-process elements with respect to each other and to Fe should not differ greatly from those of the sun. 

Figure 3 Spectrum as observed by Menzies et al. (1987) at Apr. 2. in comparison with the reddened synthetic spectrum (thin line) as calculated by model VI (see table 1) assuming half solar abundances for all elements but Sc, Ti, V,Cr,Sr and Ba (2.5 solar). The flux as calculated by the same model but enlarged Ba abundance (50 solar) is shown between 5600 and 6200 A (dotted line). In addition identifications of some strong features are given.

Chondrites. Over 909, of the meteorites lhat are observed lo fall out of the sky are classified as chondrites, samples lhat are distinguished from terrestrial rocks in many ways. One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Chondrites also have basically undifferentiated elemental compositions lor most nonvolatile elements and match solar abundances except for moderately volatile elements. The imtsl cunipositionally primitive chondrites are members for the type I carbonaceous f Cl I class. 

The solar abundances of all of the chemical elements are shown in Figure 12.2. These abundances are derived primarily from knowledge of the elemental abundances in Cl carbonaceous chrondritic meteorites and stellar spectra. Note that 99% of the mass is in the form of hydrogen and helium. Notice that there is a general logarithmic decline in the elemental abundance with atomic number with... 

Typical cross sections for these spallation reactions are 1 -100 mb for //, > 0.1 GeV. The time scale of the irradiation is 1010 y. The product nuclei are not subject to high temperatures after synthesis and can survive. A further testimonial to this mechanism is the relative abundances of the elements in the GCR relative to the solar abundances (Fig. 12.21), which shows the enhanced yields of Li, Be, and B in the GCR. This pattern is similar to the yield distributions of the fragments from the reactions of high-energy projectiles. 

Table 3.2. Elemental (atomicj abundance ratios of prominent noble gases in the solar system...

Figure 7.15 Noble gas elemental abundance (log10 >) relative to the solar abundance in planetesimals of various sizes are plotted against a mass number. The amount of noble gases is normalized to Si. In the case of atmospheric values (thick line), M, stands for the noble gases in the atmosphere and Si, in the whole Earth.

To set an abundance scale for listing the abundances of the elements, astronomers usually set H = 1012 atoms. Other elemental abundances in stars are then given by their numbers per thousand billion H atoms. In the Sun the ratio H/He = 10. For 3He more reliable information about relative He isotopic abundances comes from the primitive classes of meteorites, which are dominated by silicon. Thus the scale frequently used for geochemistry and for stellar nucleosynthesis takes a sample containing one million Si atoms, so that abundances of the elements are then their numbers per million Si atoms. Since helium in the Sun is observed by astronomers to be 2720 times more abundant than silicon, the He total solar abundance is therefore... 

This is still a high abundance, exceeding those of the elements Na and Al, for example. These solar abundances yield an isotopic ratio 12C/13C = 89. Interestingly different values are found in other samples of matter in the universe. (See 12C, Anomalous isotopic abundance). 

K constitutes 93.26% of natural potassium. The elemental K abundance in the solar photosphere and in meteorites are in good agreement 3770 atoms of K per million Si atoms. This isotopic abundance is then... 

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