Analysis of ground-based and VIRTIS-M/ROSETTA reflectance spectra of asteroid 2687 Šteins: A comparison

The asteroid 2687 Šteins was encountered by Rosetta in 2008. Prior to the fly-by, ground-based observations of Šteins were performed [1, 2, 3, 4, 5, 6]. We present a summary of ground-based VIS and NIR reflectance spectra of Šteins and compare them with VIRTIS-M-spectra obtained during the fly-by. On the basis of these spectral data we discuss the relationship to meteorite materials, and the classification of Šteins. The ground-based spectra cover a wavelength range from 0.4-2.5 μm. All spectra show a clear absorption feature at ∼0.5 μm and a steep spectral slope between ∼0.6-0.8 μm. At wavelengths >1 μm the spectra show a neutral to slightly reddish trend. The absorption band at ∼0.5 μm is commonly linked to the feature at that wavelength in the oldhamite spectrum [7]. The oldhamite spectrum shows another weaker feature at 0.96 μm. This weaker feature at ∼0.96 μm is visible in two of the ground-based spectra. Spectral slopes of most Earth-based spectra are comparable within arrow bars. The uniform spectral characteristics indicate a homogenous surface of Šteins. The VIRTIS-M-spectra of Šteins cover the wavelength range from 0.25-1 μm (VIS) and 1-5 μm (IR). The spectra show an overall flat behavior with a steep red slope at wavelengths <1 μm. The absorption feature located at ∼0.5 μm is clearly visible. At wavelengths >3.5 μm thermal emission contributes significantly to the detected radiation. The thermal properties derived from VIRTIS-M long wavelength measurements suggest a thin regolith layer and a low porosity. The shape of the asteroid is consistent with the hypothesis that Šteins is a rubble-pile. Ground-based and fly-by spectra of Šteins are in good agreement with each other considering the overall spectral characteristics and the occurrence of the absorption feature at 0.5 μm. Prior to the Rosetta fly-by Šteins has been classified (by e.g. [1, 5]) as an E[II]-type asteroid (after [8, 9], also Xe after [10]). VIRTIS data suggest that Šteins can be classified as an igneous E-type asteroid, being a member of the E[II]-subclass. E-type asteroids are linked to aubrites, which are nearly monomineralic enstatite achondrites [11]. This interpretation is supported by comparative laboratory reflectance measurements. Although aubrites give the best agreement with Šteins spectra, several spectral features cannot be assigned unambiguously. Ti-rich minerals or space weathering implanted products were alternatively proposed to reproduce the observed spectral characteristics [1, 12]. Currently no meteorite in our present collection fits the Šteins spectra, indicating that Šteins is probably not the parent body of these meteorites. Because Šteins is a reduced anhydrous body, it can be argued that it formed in the inner planetary system and was scattered to the main belt. This opens interesting parallels between the E-type population and the formation of Mercury. [1] Barucci et al. (2005) A&A, 430, 313-317. [2] Dotto et al. (2009) A&A, 494, L29-L32. [3] Fornasier et al. (2007) A&A, 474, L29-L32. [4] Fornasier et al. (2008) Icarus, 196, 119-134. [5] Nedelcu et al. (2007) A&A, 473, L33-L36. [6] Weissman et al. (2008) Met. Planet. Sci, 43, 905-914. [7] Burbine at al. (2002) Met. Planet. Sci., 37, 1233-1244. [8] Tholen D. J. (1989) in Asteroids II, 1139-1150. [9] Gaffey and Kelley (2004) LPSC XXXV. Abstract #1812. [10] Bus and Binzel (2002) Icarus, 158, 146-177. [11] Keil (2010) Chem. Erde-Geochem., 70, 295-317. [12] Shestopalov et al. (2010) Planet. Space Sci., 58, 1400-1403