USING NEW SPECTRAL CALIBRATIONS TO DERIVE MINERALOGIES OF NEAR EARTH ASTEROIDS

Approximately 8,000 Near-Earth Asteroids (NEAs), bodies with orbits that cross or come close to Earth’s orbit, have been discovered to date. Several studies have suggested that many NEAs have ordinary chondrite-like spectra. This can be tested by measuring the spectral parameters of NEAs (band area ratio and band center) in the visible and near-infrared (VISNIR) wavelengths (out to ~2.5 µm). Mineralogies can be estimated due to the characteristic olivine and pyroxene absorption bands in the VISNIR wavelengths. However, one problem with using spectral parameters is that formulas used previously for deriving compositions do not appear to work very well for chondrite assemblages. Using formulas derived by Dunn and others, we characterized the mineralogies (mafic mineral abundances and compositions) of 19 NEAs with ordinary chondrite-like infrared spectra. The band area ratio, or BAR, (area of Band II/ area of Band I) of each NEA was calculated using the trapezoidal rule, and the Band I center (BIC) was calculated by dividing out the continuum and then fitting a second-degree polynomial over the bottom third of the band. We corrected BARs and BICs for asteroid surface temperatures by estimated the temperature of each NEA and then using data from Moroz and others to predict the shift in wavelength with decreasing temperature. The calibrations of Dunn and others were then used to derive ol/(ol+px) ratios, Fa content (mol%) in olivine, and Fs content (mol%) in pyroxene from the measured spectral parameters. Derived mineralogies are ordinary chondrite-like, with ol/(ol+px) ratios from 0.53 to 0.71, Fa from 18.4 to 30.3 mol%, and Fs from 12.7 to 24.9 mol%. Derived ol/(ol+px) and derived Fa values for seventeen NEAs plot within fields previously established for ordinary chondrites by Dunn and others. The same is true for ol/(ol+px) versus Fs content. All ordinary chondrite-like NEAs except one have compositions that are best fitted by L or LL chondrites; asteroid 1998 QS42 is similar in composition to the H-chondrites. Our results are consistent with previous studies, which suggest that most NEAs are LL chondrite-like. This result again highlights the dichotomy between recovered ordinary chondrites and the asteroid population from which they are derived, as the meteorite collection is dominated by H chondrites.