The Role of Mineralogy in Producing Unusual Polarization Effects in CAI-Rich Asteroids

Details The Role of Mineralogy in Producing Unusual Polarization Effects in CAI-Rich Asteroids The Role of Mineralogy in Producing Unusual Polarization Effects in CAI-Rich Asteroids

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.6007s&link_type=abstract

American Astronomical Society, DPS meeting #40, #60.07; Bulletin of the American Astronomical Society, Vol. 40, p.509

Astronomy and Astrophysics

Astronomy

1

Scientific paper

The Role of Mineralogy in Producing Unusual Polarization Effects in CAI-Rich Asteroids
Several recent polarization studies (Cellino et al., 2006; Gil-Hutton et al., 2008; Belskaya et al., 2008; Masiero et al., in prep.) have revealed that three mainbelt asteroids 234 Barbara, 387 Aquitania, and 980 Anacostia have unusual polarimetric properties. These asteroids have strongly negative polarization signatures, which persist over large phase angles (inversion angles 30º) and increase with wavelength. Some of these phase-polarization properties are seen with dark C-type asteroids; yet these unusual bodies are relatively bright (albedo 15-25%). These same asteroids are also spectrally anomalous in that they are dominated by 2 µm absorptions, but have weak or absent absorptions near 1 µm. This spectral signature is diagnostic of spinel ([Fe,Mg]Al2O4). Calcium-Aluminum-Rich Inclusions (CAIs) are also spectrally dominated by spinel and spectral modeling has shown that these asteroids contain CAIs with abundances of 2 to 3 times those of any known meteorites (Sunshine et al., 2008). Previous suggestions for the polarization behavior of these asteroids include increased metal components, unusual surface morphology, and/or the preferential preservation of immature light or dark components in their regoliths. However these bodies are large, with 50-100 km diameters, and their high abundance of CAI's suggests they are quite ancient. Their regoliths are therefore likely to be mature and composed of highly comminuted mixtures of all materials. We suggest that the mineralogy of these bodies may be directly responsible for the polarization effects. Spinel has among the highest indices of refraction of minerals common in meteorites. Furthermore, the index of refraction of spinel increases with increasing FeO and the spinel in some CAIs contain abundant FeO. Thus for at least these asteroids, which contain the highest known abundances of CAIs, FeO bearing spinel may explain their unique spectral and polarimetric signatures.

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