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J. M. Sunshine (SAIC), S. J. Bus (IfA, U. Hawaii), T. H. Burbine, T. McCoy (National Museum of Natural History/ Smithsonian Inst.), R. P. Binzel (MIT)
A-asteroids are spectrally defined by the presence of broad 1-micron absorption features superimposed onto moderately reddened spectral slopes. These broad 1-micron features are characteristic of olivine. The surfaces of A-asteroids are therefore inferred to be rich in olivine.
Three groups of meteorites are also known to be olivine-dominated (excluding martian meteorites): pallasites, brachinites, and R-chondrites. The groups have many distinctions implying unique petrologic histories, including differences in olivine composition (Mg/Fe ratios).
Numerous laboratory spectral analyses have shown that the broad 1-micron olivine absorption feature is composed of three individual bands. The position, width, and relative strength of these absorptions are also known to vary with both composition (e.g. Mg/Fe ratio) and temperature. Given sufficiently high quality (spectral resolution and S/N) data, it should therefore be possible to compare A-asteroids and the three meteorites groups based on olivine composition.
Such high quality spectra of A-asteroids have recently been measured using SpeX, a low- to medium-resolution infrared spectrograph, newly commissioned at the Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. In its low-resolution mode (R ~ 100), SpeX can produce spectra of faint asteroids from 0.8 to 2.5 microns with S/N comparable to data typically collected with visible wavelength CCDs. The SpeX data have been combined with visible CCD data measured during the SMASSII survey to produce high S/N spectra from 0.44 to 2.5 microns for several A-asteroids.
For the first time, these new SpeX data include subtle spectral features which reveal differences among the A-asteroid spectra. We use the Modified Gaussian Model (MGM) to model individual absorptions in the asteroid spectra. Preliminary results indicate differences in the amount and composition of minor pyroxene components and in the composition of olivine. Efforts are also underway to account for temperature effects. In a parallel analysis, representative spectra for the three meteorite groups are also being analyzed with the MGM for comparison to A-asteroid spectra.
Research funding for J.M.S. from NASA's Planetary Geology and Geophysics Program (NASW-012) is gratefully acknowledged.
The author(s) of this abstract have provided an email address for comments about the abstract: sunshinej@saic.com