Isotope 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. 

Table 4.2 gives current estimates for the relative abundances of the isotopes in the solar system. The isotopic compositions of most elements, especially those that exist as solids, come from measurements of terrestrial materials. Because the Earth has experienced extensive melting and differentiation, it can be considered a homogeneous isotopic reservoir. However, each of the elements can experience both equilibrium and kinetically based isotopic fractionations during igneous, evaporative, and aqueous processes. The range of compositions introduced by such processes is small for most elements and so does not obscure the overall picture. 

An assessment of the abundances of elements and isotopes in the solar system... 

It is the 36th most abundant isotope in the solar system, between iyO and 39K. 

It is but the 64th most abundant isotope in the solar system, comparable to minor calcium isotopes or to 7Li, the most abundant Big-Bang product outside ofH and He. 

A practical application of eqs. (16.6), (16.7) and (16.8) is the calculation of the age of the solar system. MS analysis of meteorites containing negligible amounts of U gives the following values for the isotope ratios of the Pb isotopes 2 Pb Pb = 9.4 and 207pb 204pb 2Q 3 jf these values are assumed to be the initial isotope ratios at the time of formation of the solar system, the age is obtained from the present isotope ratios of the Pb isotopes in the solar system and the ratios of the present abundances of U and Pb, for example by application of eq. (16.6) ... 

The principal products of helium burning are thus 0 and C. That these are the third and fourfti most abundant isotopes in the Solar System (after H and He) shows that helium burning is a significant player in the formation of the elements. Other key products of helium burning are 0 and Ne produced fi om left over from hydrogen burning ... 

Isotopic studies involving MC-ICP-MS revealed evidence for a heterogeneous distribution of Zr, Ti, Mo, and Ni isotopes in the solar system [49, 53-55, 59, 61]. Variations in relative isotopic abundances of Zr and Ti between carbonaceous chondrites and terrestrial samples provide evidence for a heterogeneous distribution of CAIs (Table 10.1) as carriers of these nucleosynthetic anomalies. This conclusion has implications for mixing processes in the solar nebula and accretion models of planets. 

As can be seen in Fig. 2-1 (abundance of elements), hydrogen and oxygen (along with carbon, magnesium, silicon, sulfur, and iron) are particularly abundant in the solar system, probably because the common isotopic forms of the latter six elements have nuclear masses that are multiples of the helium (He) nucleus. Oxygen is present in the Earth s crust in an abundance that exceeds the amount required to form oxides of silicon, sulfur, and iron in the crust the excess oxygen occurs mostly as the volatiles CO2 and H2O. The CO2 now resides primarily in carbonate rocks whereas the H2O is almost all in the oceans. 

The Zag meteorite fell in the western Sahara of Morocco in August 1998. This meteorite was unusual in that it contained small crystals of halite (table salt), which experts believe formed by the evaporation of brine (salt water). It is one of the few indications that liquid water, which is essential for the development of life, may have existed in the early solar system. The halite crystals in the meteorite had a remarkably high abundance of 128Xe, a decay product of a short-lived iodine isotope that has long been absent from the solar system. Scientists believe that the iodine existed when the halite crystals formed. The xenon formed when this iodine decayed. For this reason, the Zag meteorite is believed to be one of the oldest artifacts in the solar system. In this lab, you will use potassium-argon radiochemical dating to estimate the age of the Zag meteorite and the solar system. 

Figure 1.4 shows the local Galactic abundances of isobars, based on a combination of elemental and isotopic determinations in the Solar System with data from... 

The isotope ratios differ from the corresponding r- and p-process ratios in the Solar System, but this could be... 

Isotopic abundances in the Solar System and elsewhere, and their significance, are well described (without diagrams or equations) in... 

For purposes of comparison with stellar abundances, it is useful to have the relative contributions of s- and r-processes to the various elements (as opposed to nuclides) in the Solar System, because in most cases only element abundances without isotopic ratios are available from stellar spectroscopy. At the same time, elements formed in one process may often be expected to vary by similar factors in the course of stellar and Galactic evolution, but to be found in differing ratios to elements formed in another process. Relative contributions are listed for some key elements in Table 6.3. 

Curiously, there are some nearby interstellar diffuse clouds displaying anomalously low isotope ratios for 7Li/6Li, with a ratio apparently as small as 2 in one case (Lemoine et at. 1994 Knauth, Federman Lambert 2003), compared to the Solar-System (and more usual interstellar) ratio of 12 the anomaly here is that the low ratio in such clouds is consistent with cosmic-ray spallation whereas that in the Solar-System is not. 

For more detailed discussions of isotopic anomalies and short-lived radioactivities in the Solar System, see... 

Marcus, R. A. Mass independent isotope effect in the earliest processed solids in the solar system. A possible chemical mechanism../. Chem. Phys. 121, 8201 (2004). 

Lee DC, Halliday AN, Snyder GA, Taylor LA (1997) Age and origin of the moon. Science 278 1098-1103 Lee T (1978) A local proton irradiation model for the isotopic anomalies in the solar system. Astrophys J 224 217-226... 

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. 

Fig. 8.5. The delicious cocktail of the supernovas. Mixing 13 measures of SNll with 1 measure of SNl, we find a composition of matter that approaches observed abundances in the Solar System. Certain isotopes of chlorine, potassium and scandium, among others, are not produced in snfficient qnantities, however. (From Nomoto et at. 1997.)...

The isotopic and elemental abundance table shows that, in the Solar System, iron is more abundant than its neighbours. Analysis of stellar spectra conhrms this result, giving it a universal character. 

Neptunium, the first transuranium element, was discovered hy E. M. McMdlan and P. H. Ahelson in 1940 in Berkeley, California. It was produced in the cyclotron in a nuclear reaction by bombarding uranium-238 with neutrons. An isotope of mass 239 and atomic number 93 and ti/2 of 2.4 days was produced in this reaction. Neptunium-237, the longest-lived alpha-emitter with half-life 2.14x10 years, was discovered two years later in 1942 by Wahl and Seaborg. The new element was named after the planet Neptune, the planet next to Uranus in the solar system. 

This isotope had a half-life of about 24,000 years. It proved to be fissionable (56) and was the basis for the plutonium atomic bomb. Concentrated work on the new element was now begun by the Manhattan Project. The main work was done at Chicago. At this time it became desirable to have names for the elements which had previously been called simply 93 and 94 by the men who worked with them. The name suggested by McMillan, neptunium, was therefore adopted for 93, and by analogy 94 was named plutonium from the planet Pluto, next beyond Neptune in the solar system (53, 69). 

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