Other
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusm.p41a..09h&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #P41A-09
Other
6205 Asteroids, 6008 Composition
Scientific paper
The reflectance spectra of V-type asteroids from SMASS1, SMASS2 and other sources were processed by the optimal smoothing method. Faint absorption features near 600-650 nm have been found in 28 V-asteroid reflectance spectra with a low noise level. The examined spectra have been divided into five groups in accordance with characteristics of the faint absorption bands. The first group consists of two V-asteroids spectra with a narrow absorption near 610 nm in the red part. The similar feature is observed in some spectra of the Fe-low orthopyroxenes as enstatites, bronzites, and hypersthenes. This absorption appears to be attributed to Fe3+ or forbidden transition in Fe2+-cations located in these minerals. The second group consists of four V-asteroids spectra which contain a weak broad feature centered near 620 ¨C 640 nm. Such a band was not found in the reflectance spectra of the HED meteorites and terrestrial orthopyroxenes. Simulation test by pyroxene-olivine mixture spectra was carrying out to interpret the second type of vestoids spectra which confirms that a little amount of olivine on the asteroids would be enough for an appearance of this band in their spectra. For this test we used a model of spectral albedo by Yurij Shkuratov. The third group is composed of nine V-asteroid spectra which contain a broad isolated feature centered less than 600 nm. We have paid an attention to a similar absorption band in spectra of some diogenites and terrestrial orthopyroxenes which can be attributed to Cr3+. To verify our supposition we calculated spectra of mixture of HED meteorites with terrestrial Cr-containing orthopyroxenes. A little amount of chromite is enough for an appearance the wide feature near 600nm in theoretical spectra. The fourth group contains spectra of ten asteroids with pair of broad bands (center positions are shortward of 600 nm and near 650 nm). We did not find these bands in spectra of eucrites and howardites but spectra of the terrestrial Cr-containing orthopyroxenes have the spin-allowed chromium bands near 600 nm and a composite band around 690 nm. Just Cr3+ cations produce a spin-allowed band near 600 nm and less intensive spin-forbidden bands near 640, 660, and 690 nm. Simulation test by spectra of the eucrite-terrestrial chromite mixture was carried out to interpret the fourth type of the vestoid spectra.At last, the vestoid spectra of the fifth type contain a very broad feature (overlapping region ~100 nm) centered near 600 nm. Interpretation of this feature is still unclear. This may be the Cr3+-feature, which is observed in the spectra of some Cr-containing enstatites, or the charge transfer band Fe2+ - Fe3+ in pyroxene spectra, or, alternatively, spectral contribution of a stranger material accumulated on asteroid surfaces. Each spectral type of the vestoids refers to definite surface compositions.
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