Spectroscopy and classification

Incident sunlight is partially absorbed by minerals on an asteroid s surface, and the fraction of light that is reflected can vary as a function of wavelength. Spectrophotometric studies of asteroids have been carried out for decades. Asteroid reflectance spectra have been compared to the spectra of meteorites measured in the laboratory (Fig. 11.3), providing a way to link some kinds of meteorites to possible parent bodies. Most measurements are made at visible wavelengths ranging from 0.4 to 1.0 pm, because the illuminating solar flux peaks 

Comparison of telescopic spectra of asteroids (shown by dots and black curves) with meteorite spectra measured in the laboratory (gray curves). Spectral similarities can be used to estimate the compositions of asteroids and infer correlations. Because absolute reflectance (albedo) depends on particle sizes and packing in surface regoliths, it is permissible to translate asteroid spectra up or down in the diagram to obtain a match. 

Asteroid taxonomy based on spectra (Tholen and Barucci, 

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 

HED meteorites. Determining the presence of minerals with specific compositions from asteroid spectra can provide more quantitative correlations with meteorites (Gaffey et al., 2002). 

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Examples illustrating the use of PCA for identification and classification are given in Chapter 9, including classification of American Indian obsidian artifacts by trace element analysis, identification of fuel spills by gas chromatography, identification of recyclable plastics by Raman spectroscopy, and classification of bees by gas chromatography of wax samples. 

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