Asteroids spectral reflectances

McCord T. B., Adams J. B., and Johnson T. V. (1970) Asteroid Vesta spectral reflectivity and compositional implications. Science 168, 1445-1447. 

The major tool that allows us to relate classes of meteorites to classes of asteroids is spectral reflectance (Burbine et al., 2002). Unfortunately, iron meteorites, with their paucity of silicate phases, have relatively featureless spectra with red spectral slopes and moderate albedos (e.g.. Clouds et al., 1990) (Figure 10). There appears to be no simple correlation between nickel abundance and spectra redness and, thus, distinguishing different chemical groups (e.g., low-nickel IIAB from... 

Figure 8 Histograms of the spectral reflectivity gradient of KBOs, Centaurs (bodies with perheilia between Jupiter and Neptune), active comet nuclei, dead comets, and Trojan asteroids. The similarity of dead comets and Trojan...

Asteroids are a highly diverse group of bodies despite the fact that they formed in a very small region of the solar nebula. Spectral reflectance studies of asteroids show a variety of different... 

Prior to discussing specific meteorite-asteroid (minor planet) links, we note that while >10 asteroids have been discovered, the spectral reflectances of only 1000-2000 are known. As white (Solar) light impinges on an asteroid s surface, its constituent minerals differ in reflectivity. Thus, the wavelength-reflectance dependence indicates the asteroid s major surficial constituent minerals and allows the asteroid to be classified by spectral type. The distribution of asteroid spectral type roughly varies with distance from the Sun [cf (/, 5, 6)]. The same spectrometer can be used to determine the spectral reflectance of a meteorite on Earth and asteroid/meteorite spectra can be compared (7/). 

Besides long gas retention ages, three lines of evidence link all undifferentiated and most differentiated meteorites to asteroids mineralogy spectral reflectance and the orbits of nine meteorite falls. 

Spectral reflectances of some meteorites match those of some asteroid types irons or E chondrites with M type enstatite achondrites with E type the unique carbonaceous chondrite Tagish Lake with D type HED achondrites with V type stony-irons with most S type thermally metamorphosed carbonaceous chondrites (see below) with C, G, B and F types. (Relative... 

Among the fractionated meteorites, such as the eucrites, the lanthanides show some variation. These meteorites, the basaltic achondrites, are basalts, originating as lavas on small asteroidal bodies (Basaltic Volcanism Study Project 1981, p. 214). The eucrites form one important class. Since the asteroid 4 Vesta, 555km in diameter, has a basaltic-like surface from spectral reflectance data, it is a prime candidate for the source of such meteorites, although considerable dynamical problems remain (Gehrels 1979). 

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. 

Many asteroid spectra tend to be "redder" than the spectra of the corresponding meteorites. Reddened spectra have lower reflectivity, weaker absorptions, and spectral slopes that are flatter in the red end. This phenomenon has been attributed to "space weathering," a catchall term referring to any process that modifies the optical properties of surfaces of airless bodies exposed to the space environment. 

Anhydrous planetesimals, and especially the meteorites derived from them, provide crucial cosmochemical data. Spectroscopic studies of asteroids do not provide chemical analyses, but the spectral similarities of several asteroid classes to known meteorite types provide indirect evidence of their compositions. The few chemical analyses of asteroids by spacecraft are consistent with ordinary chondrite or primitive achondrite compositions. Laboratory analyses of anhydrous meteorites - chondrites, achondrites, irons, and stony irons - allow us to study important chemical fractionations in early solar system bodies. Fractionations among chondrites occur mostly in elements with higher volatility, reflecting the accretion of various components whose compositions were determined by high- and low-temperature processes such as condensation and evaporation. Fractionations among achondrites and irons are more complex and involve partitioning of elements between melts and crystals during differentiation. 

Figure 10 Normalized reflectance versus wavelength ( jLm) for M-type 16 Psyche versus iron and enstatite chondrite meteorites (lines) (Gaffey, 1976 Bell et al., 1988 Bus, 1999). All spectra are normalized to unity at 0.55 xm. Spectra for all three are relatively featureless with red spectral slopes and moderate albedos. While some M-class asteroids may be metallic core material, existing spectral and density data are inconsistent with this explanation for all M asteroids.

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