• Asteroid 3628 Božněmcová: Covered with angrite-like basalts?

      Cloutis, E. A.; Binzel, R. P.; Burbine, T. H.; Gaffey, M. J.; McCoy, T. J. (The Meteoritical Society, 2006-01-01)
      A detailed analysis of the reflectance spectrum of asteroid 3628 Božněmcová, previously identified as a possible ordinary chondrite parent body, indicates that its surface consists of an assemblage dominated by clinopyroxene and plagioclase feldspar. The clinopyroxene is Fe2+-bearing (likely in the range Fs~10-20), with >90% of the Fe2+ being present in the M1 crystallographic site (spectral type A). The clinopyroxene:plagioclase feldspar ratio is between ~2 and 3 (~55-75% clinopyroxene, ~20-33% plagioclase feldspar). If olivine is present, the clinopyroxene:olivine ratio is >~3 (<20% olivine). The derived mineralogy of Božněmcov is most similar, but not identical, to the known angrite meteorites. The data suggest that Božněmcov formed by melting and differentiation of an oxidized chondritic precursor and probably represents an unsampled angrite-like body.
    • Composition of 298 Baptistina: Implications for the K/T impactor link

      Reddy, V.; Emery, J. P.; Gaffey, M. J.; Bottke, W. F.; Cramer, A.; Kelley, M. S. (The Meteoritical Society, 2009-01-01)
      Bottke et al. (2007) suggested that the breakup of the Baptistina asteroid family (BAF) 160+30 /-20 Myr ago produced an asteroid shower that increased by a factor of 2-3 the impact flux of kilometer-sized and larger asteroids striking the Earth over the last ~120 Myr. This result led them to propose that the impactor that produced the Cretaceous/Tertiary (K/T) mass extinction event 65 Myr ago also may have come from the BAF. This putative link was based both on collisional/dynamical modeling work and on physical evidence. For the latter, the available broadband color and spectroscopic data on BAF members indicate many are likely to be dark, low albedo asteroids. This is consistent with the carbonaceous chondrite-like nature of a 65 Myr old fossil meteorite (Kyte 1998)and with chromium from K/T boundary sediments with an isotopic signature similar to that from CM2 carbonaceous chondrites. To test elements of this scenario, we obtained near-IR and thermal IR spectroscopic data of asteroid 298 Baptistina using the NASA IRTF in order to determine surface mineralogy and estimate its albedo. We found that the asteroid has moderately strong absorption features due to the presence of olivine and pyroxene, and a moderately high albedo (~20%). These combined properties strongly suggest that the asteroid is more like an S-type rather than Xc-type (Moth-Diniz et al. 2005). This weakens the case for 298 Baptistina being a CM2 carbonaceous chondrite and its link to the K/T impactor. We also observed several bright (V Mag. 16.8) BAF members to determine their composition.
    • High-albedo asteroid 434 Hungaria: Spectrum, composition and genetic connections

      Kelley, M. S.; Gaffey, M. J. (The Meteoritical Society, 2002-01-01)
      New data in the wavelength region of approximately 0.4-2.5 micrometers have been obtained for asteroid 434 Hungaria. This is the most complete visible to near-infrared spectrum to date for this object. The near-infrared portion of the spectrum (about 0.8-2.5 micrometers) is smooth, featureless, and agrees well in the overlap region with new visible region data. However, visible region (about 0.45-0.9 micrometers) data appear to exhibit weak, broad spectral absorption features near 0.5, 0.6-0.7, and 1 micrometers. If real, the presence of such features would strongly constrain the compositional determination of Hungaria since it has a relatively high albedo of 46%. Most minerals that exhibit similar absorption features, and are commonly found in meteorites, have a much lower albedo. Asteroid 434 Hungaria has been observed more than six times in these overlapping spectral regions, and it is now possible to assess its mineral composition with some confidence. The dominant phase on this asteroid is an iron-free mineral, probably enstatite. Hungaria may contain secondary phases causing subtle, visible-region absorption features. Alternatively, the surface layer(s) of the asteroid may be contaminated by an absorbing species from an external source.
    • Mineralogical composition of (25143) Itokawa 1998 SF36 from visible and near-infrared reflectance spectroscopy: Evidence for partial melting

      Abell, P. A.; Vilas, F.; Jarvis, K. S.; Gaffey, M. J.; Kelley, M. S. (The Meteoritical Society, 2007-01-01)
      In March 2001, asteroid (25143) Itokawa, the target of the Japanese Hayabusa spacecraft mission, was in a favorable viewing geometry for ground-based telescopic study. Visible/nearinfrared (VNIR) spectra (~0.48 to 0.9 micrometers) obtained on March 24, 26, and 27 UT, and near-infrared (NIR) spectra (~0.75 to 2.5 micrometers) obtained on March 10, 11, 12, 23, and 24 UT collectively show absorption features centered near 1.0 and 2.0 m, which are indicative of olivine and pyroxene. Analyses of these absorption features indicate an abundance ratio of olivine to pyroxene of approximately 75:25 +/- 5, respectively, with no significant variation in the relative abundance of these minerals across its surface on a regional scale. The band center positions indicate that the mean pyroxene chemistry is ~Wo(14 5)Fs(43 5). There appear to be at least two pyroxene components: primarily a low-Ca orthopyroxene accompanied by a spectrally significant (~15-20%) high Fe-rich pigeonite phase. The mean pyroxene composition is significantly more Fe-rich than the Fs1426 range found in ordinary chondrites. These pyroxene compositions are suggestive of phases crystallized from partial melts. This would indicate that the parent body of (25143) Itokawa reached temperatures sufficient to initiate partial melting (~1050 to 1250 degrees C), but that it did not attain the degree of melting required for significant melt mobilization and efficient segregation of the basaltic melt component from the unmelted residual olivine portion. Itokawas spectral band parameters place it near the S(III)/S(IV) boundary, but within the S(III) taxonomic field. In meteoritic nomenclature, Itokawa would be most analogous to an olivine-rich primitive achondrite. Alternatively, if the high Fs value is not related to partial melting, then Itokawa could also represent a rare atypical LL chondrite, or a previously unsampled oxidized Fe-rich chondritic-like assemblage.
    • Spectral properties and geologic processes on Eros from combined NEAR NIS and MSI data sets

      Izenberg, N. R.; Murchie, S. L.; Bell, J. F.; McFadden, L. A.; Wellnitz, D. D.; Clark, B. E.; Gaffey, M. J. (The Meteoritical Society, 2003-01-01)
      From April 24 to May 14, 2000, the Near Earth Asteroid Rendezvous (NEAR) Shoemaker mission's near infrared spectrometer (NIS) obtained its highest resolution data of 433 Eros. High signal-to-noise ratio NIS reflectance spectra cover a wavelength range of 800-2400 nm, with footprint sizes from 213 x 427 m to 394 x 788 m. This paper describes improvement in instrument calibration by remediation of internally scattered light; derivation of a "pseudo channel" for NIS at 754 nm using Multispectral Imager (MSI) Eros approach maps at 951 and 754 nm; synthesis of a 3127-spectrum high-resolution data set with the improved calibration and expanded wavelength coverage; and investigation of global and localized spectral variation with respect to mineralogy, composition, and space weathering of Eros, comparing the findings with previous analyses. Scattered light removal reduces the "red" slope of Eros spectra, though not to the level seen by telescopic observations. The pseudo channel completes sampling of Eros' 1 micron (Band I) absorption feature, enabling direct comparison of NIS data with other asteroid and meteorite spectra without additional scaling or correction. Following scattered light removal and wavelength range extension, the spectral parameters of average Eros plot well inside the S(IV) field of Gaffey et al. (1993) and are consistent with the L6 chondrite meteorite fields of Gaffey and Gilbert (1998). Although Eros shows no evidence of mineralogical heterogeneity, modest spectral variations correlate with morphologically and geographically distinct areas of the asteroid. Eros bright-to-dark spectral ratios are largely consistent with laboratory "space weathering" experiment results and modeling of space weathering effects. Eros brightness variation unaccompanied by significant spectral variation departs from "lunar-type"--where band depths, slopes, and albedoes all correlate--and "Ida-type"--where significant spectral variation is unaccompanied by corresponding brightness variation. The brightest areas on Eros--steep crater walls--have lesser spectral slope and deeper Band I, consistent with exposure of "fresher," less space weathered materials. Bright crater slope materials have opx/(opx + olv) of 0.24-0.29 and may be more representative of the subsurface mineralogy than "average" Eros, which is probably affected by space weathering. The floors of the large craters Psyche and Himeros have lower albedo and contain the most degraded or altered looking materials. NIS spectra retain a "red" spectral slope at greater than 2 microns. The recalibrated and expanded NIS spectra show better agreements with mixing models based on space weathering of chondritic mixtures.