Icarus 148, 494–507 (2000) doi:10.1006/icar.2000.6488, available online at http://www.idealibrary.com on Rotationally Resolved Spectra of Some S-type Asteroids 1 Thais Moth´ e-Diniz, Daniela Lazzaro, and Jorge M´ arcio Carvano Depto. Astrof´ ısica, Observat ´ orio Nacional, 20921 Rio de Janeiro, Brazil and Marcos Florczak Depto. F´ ısica, CEFET-PR, 80000 Curitiba, Brazil Received February 8, 2000; revised July 11, 2000 We report rotationally resolved spectra of six S-type asteroids obtained at ESO in the wavelength range 4900–9200 ˚ A. The sample was chosen to span most of the sub-classes defined by M. Gaffey et al. (1993, Icarus 106, 573–602). Three other asteroids of different taxonomic class were also analyzed in orderto compare the varia- tions observed among the S-type asteroids. Only two of the S-types belonging to sub-classes mineralogicaly distinct presented signifi- cant variations, as well as the P-type asteroid present in oursample. All the other asteroids have variations of the order of the noise of the spectrum and were not considered as significant. c  2000 Academic Press Key Words: asteroids; asteroids, composition; mineralogy; spec- troscopy;surfaces, asteroids. 1. INTRODUCTION Asteroids are possibly the major source of meteorites that fall on Earth. Studies on these bodies reveal that their composition is largely influenced by heating processes during the first few million years of Solar System history (Scott et al. 1989). The thermal history and mineralogical changes are well documented by the most abundant type of meteorites that fall on Earth: the ordinary chondrites. These are primitive bodies, in the sense that they have not undergone amounts of partial melting or fractional crystallization like the differentiated or igneous meteorites. The association between asteroids and meteorites is possible by the analysis of their spectral signatures. The nature and location of the meteorites’ parent bodies are, therefore, important for a correct understanding of the composition and nature of the planetesimals, of how they accreted in these zones, and of the geologic processing on planets, satellites, and comets. It has long been realized that asteroids of different miner- alogical compositions occur at different distances from the Sun 1 Based on observations made with the 1.52-m telescope at the European Southern Observatory (La Silla, Chile) under agreement with the CNPq/ Observat´ orio Nacional (Brazil). (see Gradie et al. 1989, for a complete review). Those spectrally analogous to the igneous meteorites dominate the inner part of the main belt while those analogous to the primitive meteorites are preferentially located in the outer part. The relative percent- ages of asteroid and meteorite types, however, indicate that the meteorite flux is a biased and incomplete sample of asteroids. One of the most intriguing aspects is the apparent lack of ordi- nary chondrite-like assemblages in the asteroid belt. This is the so-called “ordinary chondrites paradox” (Chapman 1996 and references therein): on one hand the S-type asteroids, the sec- ond most numerous taxonomic class, dominate the innermost part of the main belt having, therefore, the best conditions to deliver meteorites to Earth; on the other hand, the primitive or- dinary chondrites are the most common meteorites to fall on Earth. However, the spectra of S-type asteroids have a redder continuum and weaker absorption bands than those of the ordi- nary chondrites measured in laboratories. Moreover, according to some investigators, the meteorites most closely resembling the S-type asteroids are the stony iron (Gaffey 1984, and refer- ences therein), which have undergone different degrees of ig- neous melting and separation of mineral phases. The ordinary chondrites would then derive from the very uncommon Q-type asteroids (McFadden et al. 1984, Gaffey et al. 1993, Hiroi et al. 1993). Another hypothesis argues that the difference in spec- tral properties between the S-types and the ordinary chondrites can be attributed to some optical alteration process that reddens and decreases the absorption band depth in asteroidal regoliths, or what is called the “space weathering” phenomenon (Pieters et al. 1993, Chapman 1996). The spectral diversity observed among the S-type asteroids has led to works subdividing the S-class (Chapman and Gaffey 1979, Dermott et al. 1985, Barucci et al. 1987, Gaffey et al. 1993 (hereafter G93), Howell et al. 1994, Bus 1999) into several sub- classes ranging from three to seven. Among these contributions it is noteworthy that only in the works of Gaffey et al. and Howell et al. are the sub-classes interpreted in terms of their mineralogical differences. 494 0019-1035/00 $35.00 Copyright c  2000 by Academic Press All rights of reproduction in any form reserved.