Jeffrey F. Bell

Comparison of asteroid and meteorite spectra: Classification by principal component analysis

Abstract The relationships between asteroid and meteorite spectral types are a subject of much discussion and debate. To provide an overview of spectral similarities between and among asteroids and meteorites, the spectra of 103 meteorites and 411 asteroids were statistically compared using principal component analysis. This analysis produced a “map” of statistically defined spectral similarities in which distance is a measure of relative similarity. There appears to be considerably more spectral variance in the meteorite collection than is seen in the asteroids. Also, many planet-crossing asteroids are spectrally much more similar to the meteorites than most main belt asteroids. The principal components of meteorite spectra are, in general, offset from those of the bulk of the asteroid population. The offset of the carbonaceous chondrite and iron meteorites from their most frequently suggested asteroidal analogues is particularly notable. Finally, the analysis shows no direct meteorite analogues from the outerbelt B-, D-, F-, P-, and T-type asteroids, confirming that meteorites sample only a relatively limited portion of the inner asteroid belt.

Mineralogical clues to the origins of asteroid dynamical families

Abstract The mineralogical composition of the parent bodies of the asteroid dynamical families is investigated by means of remote-sensing data. Tables of the correlation between the taxonomic types of Tholen and family membership lists of Hirayama, Brouwer, Arnold, Carusi and Massaro, Kozai, and Williams were prepared. These tables, supplemented by recent IR and radar results, suggest that while the larger, well-defined families are fragments of collisionally disrupted parent bodies, most of the smaller proposed families cannot have such an origin. The most probable explanations of this result are that most family lists were generated using excessively generous criteria, or that subtle unrecognized dynamical effects analogous to the Williams resonances selectively deplete some regions of the asteroid belt, leaving family-like asteroid groups in other regions.

Reflection spectra of shocked ordinary chondrites and their relationship to asteroids

Abstract Although ordinary chondrites are the most common meteorites falling on Earth, reflectance spectra of only a few rare asteroids resemble those of powdered chondrites measured in the laboratory. Therefore, “space weathering” processess which may have altered the surfaces of ordinary chondrite asteroids so that their spectra resemble those of the abundant S asteroids have been suggested. Recently, Britt et al. (1989, Lunar Planet. Sci. Conf. 19th , 537–545; and 1989, Lunar and Planet. Sci. XX , 111–112) and Britt and Pieters (1989, Lunar Planet. Sci. XX , 109–110) measured spectra of “shock-blackened” ordinary chondrites which possess much lower reflectance and shallower absorption bands than those of “normal” ordinary chondrites and, in some cases, resemble those of carbonaceous chondrites and C asteroids. They therefore propose that surfaces of ordinary chondrite asteroids may have been shock-blackened by impact, and that these asteroids may be hidden among the C asteroids. We measured the spectral reflectance of a number of mineralogically well-characterized, shock-blackened ordinary chondrites exhibiting four major types of black, shock-produced features: opaque melt veins (shock veins), melt pockets and irregular interconnected melt veins, melt dikes, and black chondrites, Stoffler, Keil, and Scott (1991, Geochim. Cosmochim. Acta 55, 3845–3867.) We confirm that their spectra resemble those of C asteroids. However, the occurence of these materials in impact crater basements and floors rather than on the surface, their low abundance in craters relative to brecciated and ejected material, and their low abundance among ordinary chondrite falls suggest that the surfaces of ordinary chondrite parent bodies are not likely to be covered by vast amountt of such shock-blackened materials. Thus, these materials cannot be responsible for significant large-scale spectral alterations of the parent asteroids of ordinary chondrites, and they cannot be called upon in support of the hypothesis that ordinary chondrite asteroids are hidden among C asteroids. If this hypothesis is to be upheld, then recourse may have to be taken to the suggestion of Britt and Pieters (1991, Lunar Planet. Sci. XXII , 139–142) that the surfaces of ordinary chondrite parent asteroids appear spectrally similar to those of C asteroids because they are covered by a hypothetical, thin layer of fine-grained material similar to that present in the dark portions of solar wind-bearing regolith breccias.

A spectral analysis of ordinary chondrites, S-type asteroids, and their component minerals: genetic implications

Three salient features of visible and infrared reflectance spectra of ordinary chondrites (OCs) and S-type asteroids are (1) albedo at 0.56 μm, (2) continuum slope, and (3) depth of the electronic absorption band due to octahedrally coordinated Fe2+ in olivine and pyroxene. These quantities were numerically extracted from the spectra of 23 OCs representing all metamorphic grades and 39 S-type asteroids to be plotted in a three-dimensional coordinate system. The spectral characteristics of three OCs which were comminuted, melted, recrystallized, and recomminuted are also presented in the same format. The results show that although laboratory simulation of melt alteration in an asteroidal regolith does alter OC spectra, the spectral parameters of these altered meteorites do not change enough to leave the parametric region defined by “unaltered” OC spectra. When the region containing the 39 S-asteroid spectra is compared with that of the altered and unaltered OCs, it is found that not one of the OCs falls within the S-asteroid region. The differences may be largely attributed to spectral differences between the respective metallic components. The range of S-asteroid parameters is then compared with potential pure “end-member” components most likely to result from magmatic differentiation of a chondritic protoasteroid: olivine, orthopyroxene, clinopyroxene, and Fe,Ni meteorite metal (alternatively represented by the M-asteroids). It is found that the S-asteroid array is consistent with random mixtures of the differentiated components except for a notable dominance of the spectral characteristics of the opaque (metallic) component. These results suggest that the M-asteroids may form a composition continuum with the S-asteroids. We discuss a scenario consistent with this analysis and with the Bell et al. theory of the geological structure of the asteroid belt. Ordinary chondritic protoasteroids of all sizes were probably the dominant primary condensates in the inner portion of the main asteroid belt. These were later heated by electromagnetic induction or by 26Al nuclide decay. As a result, the smaller ones were subjected to various degrees of metamorphism, while the larger ones were subjected to large-scale magmatic differentiation. Petrological domains of the S-asteroids (beneath a mixed regolith) may be large and supply achondrites, irons, and stony irons to Earth rather than well-mixed breccias of these components. The (smaller) OC protoasteroids may still be abundant in the asteroid belt but, if small enough to escape differentiation, may also be small enough to escape Earth-based identification.

Composition and size of Apollo asteroid 1984 KB

Abstract Reflection spectra and thermal-emission radiometry were obtained for the Earth-crossing asteroid 1984 KB. The spectrum exhibits the spectral signatures of olivine, pyroxene, and NiFe metal; it is typical of Class S objects and suggests a surface material similar to the rare lodranite meteorites. A standard asteroid thermal model implies a radius of ∼0.7 km and an albedo of ∼0.16 (similar to Class S objects). An analysis of the same thermal data with a bare-rock thermal model gives an albedo inconsistent with the IR spectrum, suggesting that this object has a significant regolith despite its small size.