Meteorites parent bodies

Fig. 2. The plot of total reduced iron, Fe, and oxidized iron, Fe, normalized to Si abundance shows how the chondrite classes fall into groups distinguished by oxidation state and total Fe Si ratio. The soHd diagonal lines delineate compositions having constant total Fe Si ratios of 0.6 and 0.8. The fractionation of total Fe Si is likely the result of the relative efficiencies of accumulation of metal and siUcate materials into the meteorite parent bodies. The variation in oxidation state is the result of conditions in the solar nebula when the soHds last reacted with gas. Terms are defined in Table 1 (3).

Chondrite classes are also distinguished by their abundances of both volatile and refractory elements (3). For volatile elements the variation among groups results from incomplete condensation of these elements into soHd grains that accrete to form meteorite parent bodies. Volatile elements such as C,... 

The fractionation of these refractory elements is beheved to be the result of relative efficiencies of incorporation of condensed sohds rich in early high temperature phases into the meteorite parent bodies at different times and locations in the solar nebula. The data are taken from Reference 3. 

Within each chondrite class there are petrographic grades that relate to alteration processes that occurred within the meteorite parent body. The... 

Another isotopic anomaly, discovered in Allende inclusions, concerns magnesium, for which an intrinsically low abundance in these samples makes its isotope ratios sensitive to small effects. Certain of the inclusions show a correlation between 26Mg and 27 Al, indicating an origin of excess 26Mg from radioactive decay of 26 A1 (mean life 1 Myr), the existence of which had previously been postulated as a heat source for meteorite parent bodies (Fig. 3.32). Other short-lived activites that seem to have been alive in the early Solar System are 10Be (mean life 2.2 Myr) from a correlation of 10B with 9Be, and 41Ca (mean life 0.15 Myr) from a correlation of... 

Possible heat sources for melting in meteorite parent bodies are discussed in Chapter 11. Differentiated meteorites generally have old radiometric ages (Chapter 9), which indicate igneous activity began and ended soon after their parent bodies accreted. 

Endress, M., Zinner, E. and Bischoff, A. (1996) Early aqueous activity on primitive meteorite parent bodies. Nature, 379, 701-703. 

Isotopic chronometers can provide information about the collisional history of the meteorite parent bodies. As discussed in Chapter 8, isotopic chronometers can give the time of... 

As already noted, spectral similarities between the various asteroid classes and specific types of meteorites provide a way to identify possible meteorite parent bodies. The Tholen and Barucci (1989) asteroid taxonomy has been interpreted as representing the types of meteorites shown in Table 11.1. Using the Bus et al. (2002) taxonomy, the C-complex asteroids are probably hydrated carbonaceous chondrites (e.g. Cl or CM). These carbonaceous chondrite asteroids probably accreted with ices and will be considered in Chapter 12. Some S-complex asteroids are ordinary chondrite parent bodies, but this superclass is very diverse and includes many other meteorite types as well. The X-complex includes objects with spectra that resemble enstatite chondrites and aubrites, and some irons and stony irons, although other X-complex asteroids are unlike known meteorite types. A few asteroid spectra are unique and provide more definitive connections, such as between 4 Vesta and... 

Cooling rates have important implications for the sizes of meteorite parent bodies. Because of its high thermal conductivity, a metallic core should have a uniform temperature, but its rate of cooling is controlled by outer silicate layers that act like insulation. Larger asteroids cool more slowly, and Haack et al. (1990) developed an approximate relationship between the radius of an asteroid R and the cooling rate CR of its metallic core ... 

Using this eguation, the calculated radii of most iron meteorite parent bodies are found to have been -10-100 km. 

C60 has not yet been detected in primitive meteorites, a finding that could demonstrate its existence in the early solar nebular or as a component of presolar dust. However, other allotropes of carbon, diamond and graphite, have been isolated from numerous chondritic samples. Studies of the isotopic composition and trace element content and these forms of carbon suggest that they condensed in circumstellar environments. Diamond may also have been produced in the early solar nebula and meteorite parent bodies by both low-temperature-low-pressure processes and shock events. Evidence for the occurrence of another carbon allotrope, with sp hybridized bonding, commonly known as carbyne, is presented. 

McFadden, L. A. (1989) Remote sensing and the Shergottite-Nakhlite-Chassignite meteorite parent body. Bull. Amer. Astronom. Soc., 21, 967. 

An interesting consequence of this model is that bodies from the terrestrial-planet region may be scattered outwards while the planets are forming and implanted in the Asteroid Belt. With dynamical and collisional modeling of this process, Bottke et al. (2006) find that this may be the origin of most iron meteorites. This would explain the diversity of iron-meteorite types, why there is little observational or meteoritical evidence of mantle material from differentiated bodies in the Asteroid Belt, and the fact that most iron-meteorite parent bodies appear to have formed >1 Myr before the parent bodies of the chondritic meteorites (Kleine et al. 2005). 

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