Spectrophotometry of (32) Pomona, (145) Adeona, (704) Interamnia, (779) Nina, (330825) 2008 XE3, and 2012 QG42 and laboratory study of possible analog samples

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2015-12, Vol.262, p.44-57
• Main Authors: Busarev, Vladimir V., Barabanov, Sergey I., Rusakov, Vyacheslav S., Puzin, Vasiliy B., Kravtsov, Valery V.
• Format: Article
• Language: English
• Subjects: Absorption spectra; Asteroids; Iron; Mineralogy; Reflectance; Reflectivity; Serpentine; Spectra; Spectrophotometry; Surface chemistry
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2015.08.001

•We study obtained reflectance spectra of six asteroids including two NEAs.•Spectral comparison of asteroids and their proper analog samples is made.•Intensity of an absorption band at 0.44-μm and Fe3+-content are strongly correlated.•Spectral signs of sublimation activity are found on Adeona, Interamnia, and Nina.•We discuss why weak absorption bands absent in reflectance spectra of NEAs. 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 0.35–0.90μm with the aim to study features of their reflectance spectra. A high-altitude position of our Terskol Observatory (3150m above sea level) favorable for the near-UV and visible-range observations of celestial objects allowed us to probably detect some new spectral features of the asteroids. Two subtle absorption bands centered at 0.53 and 0.74μm 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 0.67–0.71μm 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 0.44μm 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 o