Vestoid surface composition from analysis of faint absorption bands in visible reflectance spectra
Introduction
Low-calcium pyroxenes are well known to be the most abundant phase on the Vesta-like asteroid (also called vestoid) surfaces (Burbine et al., 2001, Duffard et al., 2004). Discovery of the spin-forbidden bands of Fe2+ in 460–560 nm range of the vestoid spectra (Vilas et al., 2000, Cochran et al., 2004, Golubeva et al., 2005, Shestopalov et al., 2007) is in complete agreement with this assertion obtained by means of the near-infrared spectral observations and subsequent comparative analysis of the spectral properties of the vestoids and howardite, eucrite, and diogenite (HEDs) meteorites. The broad and week features near 600 and 650 nm, also found in the spectra of some vestoids (Shestopalov et al., 2007), are hardly interpreted because Fe2+, Fe3+, and Cr3+ cations produce overlapping absorptions in the visible spectra of common meteorite minerals (pyroxenes, olivines, chromites).
In this paper, we continue the study of the faint absorption bands in the visible reflectance spectra of Vesta-like asteroids, focusing mainly on the features around 600 and 650 nm. The probable causes of these absorptions in the vestoid spectra are also discussed.
Section snippets
Absorption bands in the V-asteroid visible reflectance spectra
Previously we investigated spectra of vestoids collected in the catalog SMASS1 (Shestopalov et al., 2007). Here we study vestoid spectra from the catalogs SMASS2 (Bus and Binzel, 2002) and S3OS2 (Lazzaro et al., 2004) as well as spectra obtained by Duffard et al. (2004) and Alvarez-Candal et al. (2006). The characteristic property of the SMASS2 vestoid spectra is a strong “noisy” variation of reflectance coefficients in the range of 435–500 nm and gaps in the region between 500–550 nm resulting
Analyzing the 600- and 650-nm features
Let us try to understand the nature of the absorption bands centered around 600 and 650 nm. To this end, we use the reflectance spectra of the minerals and HED meteorites, which are available at RELAB (http://www.planetary.brown.edu/relab) and USGS SpecLab (http://speclab.cr.usgs.gov). The main details of laboratory facilities were described in Pieters (1983) and Clark (1999).
Among samples of HEDs we selected the unweathered ones, spectra of which contain absorptions around 600 and 650 nm. We
Simulating vestoid spectra
To verify the assumptions about vestoid surface compositions obtained with the faint absorption bands, the simulation of the vestoid spectra has been carried out. For that we used an optical model developed by Shkuratov et al. (1999). Validity of the scattering theory by Shkuratov et al. (1999) for solid planetary surfaces have been repeatedly examined by us (Shestopalov and Golubeva, 2000), other investigators (Poulet et al., 2002), and author himself (Shkuratov and Grynko, 2005). In this
Discussion
Small Vesta-like asteroids are usually considered as fragments of a large differentiated body(ies) (Burbine et al., 2003, and references therein). To clarify a question if the studied vestoids in Table 3 are fragments of a single body or several bodies, we indicated their membership of the Vesta family in accordance with Mothé-Diniz et al. (2005) definitions of asteroid dynamic families. Only half of objects in Table 3 belong to the Vesta family and, consequently, they appear to be Vesta's
Conclusions
In our paper, we have concentrated attention on the properties of the faint absorption bands centered around 600 and 650 nm in the vestoid spectra. It is shown, in particular, there can be olivine or chromite as minor phases along with low-Ca pyroxene on the vestoid surfaces. Simulating the vestoid spectra helps to estimate content variations of olivine () and chromite (). Apparently, mineralogy of the Type-I vestoids is similar to howardites and eucrites, whereas
Acknowledgments
Reflectance spectra of the minerals and meteorites discussed here were measured at RELAB and USGS Digital Spectral Library. We thank all Principal Investigators for spectral measurements used in our work, and D. Lazzaro for kindly providing us with some vestoid spectra before their publication. We are grateful to T.H. Burbine and other reviewer for their helpful comments which contributed strength to an improvement of our work.
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