Elsevier

Icarus

Volume 195, Issue 2, June 2008, Pages 649-662
Icarus

Vestoid surface composition from analysis of faint absorption bands in visible reflectance spectra

https://doi.org/10.1016/j.icarus.2008.01.011Get rights and content

Abstract

Faint absorption bands in the visible range of the smoothed vestoid spectra have been found. The bands centered near 505, 530, and 550 nm are attributed to ferrous iron in low-calcium pyroxene and are typical for pyroxene-bearing vestoid surfaces. In accordance with characteristics of the faint absorption bands around 600 and 650 nm the studied vestoid spectra can be sorted into five types. Since the same absorptions are also seen in the laboratory spectra of the minerals and meteorites, which appear to be similar to vestoid material, spectral types of the vestoids can be related to their surface compositions. Regolith of the Type-I vestoids consists of pure low-calcium pyroxenes. Minor amount of olivine along with pyroxene appear to be on the Type-II vestoids whereas the mixtures of low-calcium pyroxene with minor chromite define the Type-III and -IV. The causes for the fifth spectral type in terms of minor mineral phases are unclear now. Simulation of the spectra of vestoids was employed to estimate content of olivine (612 vol%) and chromite (1230 vol%) on their surfaces.

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 (612 vol%) and chromite (1230 vol%). 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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