Cosmic elemental abundance

Gas density in H atoms, expressed in terms of total density p, on the basis of observed cosmic element abundances... 

Analysis of extraterrestrial materials, and in particular meteorites, is an important focus of cosmochemical research, as such samples preserve chemical and isotopic records of early solar system conditions and processes. The first studies of meteorites, which recognized that such samples have an extraterrestrial origin, date back to the late eighteenth century [3], but modem research in cosmochemistry has a much more recent origin. This is traced back by many to the founder of contemporary geochemistry, V.M. Goldschmidt, as he produced early, but well-founded, compilations of cosmic element abundances, based on data acquired for meteorites [4, 5]. Goldschmidt s work was later continued and extended by Suess in collaboration with Urey and their study on the abundances of the elements [5] is still an important milestone in cosmochemistry. 

FIGURE 17.11 The variation in cosmic nuclear abundance with atomic number. Note that elements with even atomic numbers (brown curve) are consistently more abundant than neighboring elements with odd atomic numbers (blue curve). 

From the point of view of GCE, one is interested primarily in effects averaged over long periods of time of the order of Gyr but in dwarf galaxies which may have experienced only a few star formation bursts over a Hubble time the sporadic character may have appreciable effects, especially when one bears in mind that much of the abundance data for such objects comes from H II regions which are intrinsically the result of a current burst, and there is indeed evidence for a cosmic dispersion in certain element abundance ratios such as N/O in such objects (see Chapter 11). 

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. 

Their table included isotopic abundances as well as elemental abundances. Since the Suess and Urey table was published, subsequent work has primarily refined the determinations of the cosmic abundances through improved measurements of meteorites, a better understanding of which meteorites should be considered for this work, improved measurements of the solar composition, and a better understanding of nuclear physics. 

In Chapter 1 and again above, we introduced the cosmochemical classification of elements based on their relative volatilities in a system of cosmic (solar) composition. In a cooling solar gas, elements condense in a certain order, depending on their volatility (Table 7.1). Condensation and evaporation partition elements between coexisting gas and solid (or liquid) phases, and the removal of one or the other of these phases can fractionate element abundances of the system as a whole from their original cosmic relative proportions. 

Figure 12.21 Relative elemental abundances in the solar system and cosmic rays. (From C. E. Rolfs and N. S. Rodney, Cauldrons in the Cosmos, Chicago University Press, Chicago, 1988.)...

Figure 7.1 Elemental abundance of noble gases relative to cosmic abundance (Anders Grevesse, 1989). Data for Earth (atmosphere), SW (solar wind implanted on A1 foils on the moon), Lunar (solar wind implanted on lunar soils), Q (chondrites), and Mars are from Table 3.2.

Figure 7.1 shows a noble gas elemental abundance relative to 36Ar for the Earth atmosphere, Q, SW, and lunar soils [cf. Table 3.2,3.3(a), and 3.3(b)]. We also included the supposed Martian atmospheric noble gas (e.g., Pepin, 1991). The abundances are normalized to the solar (cosmic) abundance. 

Figure 9.4 Goldschmidt s 1938 summary data of the cosmic logarithmic abundance of the elements as a function of their neutron numbers [66],...

Such search for experiments coincided with studies of the heavy element abundances in the cosmic radiation, carried out by exposure of particle track detectors - nuclear emulsions or plastic sheets - in balloon flights to high altitudes and analysis of the recorded tracks for atomic number and abundance. A survey [33] of all data obtained until 1970 showed one single... 

For a constant flux of galactic cosmic rays (for further discussion of this point, see LavieUe et al. (1999)), the time dependence vanishes. The parameters, size and depth, remain along with the elemental abundances and nuclear cross-sections in principle, the elemental abundances and nuclear cross-sections can be measured directly. 

Figure 6 Elemental abundance patterns for trapped noble gases in various planetary materials. For each gas identified on the abscissa, the ordinate shows the depletion factor in a given sample, i.e., the gas concentration in the sample divided by what the concentration would be if the gas were present in undepleted cosmic proportion (normalized for a nominal rock with 17% Si). The relative elemental abundances in the left panel illustrate the solar pattern, those in the right panel the planetary pattern. The vertical broken lines for each gas illustrate typical in situ gas concentrations (the radiogenic component for " He, spallation for the others), below which it becomes progressively more difficult to characterize or even identify trapped components (source Ozima and Podosek, 2002).

Flynn G. J. and Sutton S. R. (1992b) Element abundances in stratospheric cosmic dust indications for a new chemical type of chondritic material. In Lunar Planet. Sci. XXIII. The Lunar and Planetary Instimte, Houston, 373-374. 

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