Spectrophotometry of (32) Pomona, (145) Adeona, (704) Interamnia, (779) Nina, (330825) 2008 ХЕ3, and 2012 QG42 and laboratory study of possible analog samplesстатья

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1. Полный текст IcarusBusarev_etal15_Printed.pdf 2,4 МБ 2 декабря 2016 [BusarevVV]

[1] Spectrophotometry of (32) pomona, (145) adeona, (704) interamnia, (779) nina, (330825) 2008 ХЕ3, and 2012 qg42 and laboratory study of possible analog samples / V. V. Busarev, S. I. Barabanov, V. S. Rusakov и др. // Icarus. — 2015. — Т. 262. — С. 44–57. Six asteroids including two NEAs, one of which is PHA, accessible for observation in September 2012 were investigated using a low-resolution (R ≈ 100) spectrophotometry in the range 0.35-0.90 μm with the aim to study features of their reflectance spectra. A high-altitude position of our Terskol observatory (3150 m above sea level) favorable for the near-UV and visible-range observations of celestial objects let us probably to detect some new spectral features of the asteroids. Two subtle absorption bands centered at 0.53 and 0.74 μm were found in the reflectance spectra of S-type (32) Pomona and interpreted as signs of presence of pyroxenes in the asteroid surface matter and its different oxidation. Very similar absorption bands centered at 0.38, 0.44 and 0.67-0.71 μm have been registered in the reflectance spectra of (145) Adeona, (704) Interamnia, and (779) Nina of primitive types. We performed laboratory investigations of ground samples of known carbonaceous chondrites, Orguel (CI), Mighei (CM2), Murchison (CM2), Boriskino (CM2), and seven samples of low-iron Mg serpentines as possible analogs of the primitive asteroids. In the course of this work, we discovered an intense absorption band (up to ∼25%) centered at 0.44 μm in reflectance spectra of the low-Fe serpentine samples. As it turned out, the equivalent width of the band has a high correlation with content of Fe3+ (octahedral and tetrahedral) in the samples. It may be considered as a confirmation of the previously proposed mechanism of the absorption due to electronic transitions in exchange-coupled pairs (ECP) of Fe3+neighboring cations. It means that the absorption feature can be used as an indicator of ferric iron in oxidized and hydrated low-Fe compounds on the surface of asteroids and other atmosphereless celestial bodies. Moreover, our measurements showed that the mechanism of light absorption is partially or completely blocked in the case of intermediate to high iron contents. Therefore, the method cannot probably be used for quantitative estimation of Fe3+ content on the bodies. Based on laboratory study of the analog samples, we conclude that spectral characteristics of Adeona, Interamnia, and Nina correspond to a mixture of CI-CM-chondrites and hydrated silicates, oxides and/or hydroxides. The obtained reflectance spectra of two NEAs, (330825) 2008 XE3 and 2012 QG42, are predominantly featureless and could be attributed to S-C and S-B-type bodies, respectively. We discuss reasons why spectral features seen in reflectance spectra of the main-belt asteroids are not observed in those of NEAs.

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