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L. F. Lim (Cornell University and NASA/GSFC), L. R. Nittler (Carnegie Institution of Washington), T. J. McCoy (National Museum of Natural History), R. D. Starr (Catholic University of America)
The NEAR-Shoemaker X-ray Spectrometer (XRS) measured the relative abundances of six elements on the surface of 433 Eros via their solar-induced X-ray fluorescence: Mg, Al, Si, S, Ca, and Fe [1]. The next most abundant elements in ordinary chondritic meteorites are Mn, Cr, and Ni. Of these, Mn and Cr should be measurable in the XRS data during major solar flares. Although their abundances are low in comparison with those of the major elements, their fluorescence lines will be enhanced because of their location just below the energy of the iron lines in the solar flare spectra. Preliminary spectral modeling of Eros spectra using ordinary chondrite compositions both including and excluding Mn and Cr suggest that these elements are essential to producing a correct fit. Thus, we should be able to derive their abundances.
Mn and Cr are particularly interesting because of their geochemical behavior. The XRS-derived composition for 433 Eros was chondritic for most major element ratios, but notably depleted in S/Si. This may have resulted from either impact dissociation of FeS and volatilization of S or removal of S in the early Fe,Ni-FeS cotect melt. While Mn is carried principally by mafic silicates, Cr occurs mainly in chromite, which is a component of the Fe,Ni-FeS melt. Partial melting can therefore produce strong fractionation in Mn/Cr ratios. For this reason, primitive achondritic meteorites exhibit a much wider range in Mn/Si and Cr/Si than do ordinary chondrites [2]. In contrast, neither element is particularly volatile or incorporated into phases that are easily dissociated by impact, and a chondritic Mn/Cr ratio would favor impact volatilization as the cause of the S depletion.
[1] Nittler, L.R., et al. (2001) Meteorit. Planet. Sci. 36, 1673-1695. [2] Nittler, L.R., et al. (2000) LPSC 31, 1711+.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.