Reflection spectra of shocked ordinary chondrites and their relationship to asteroids

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, Stöffler, 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.