Identifying Asteroidal Ordinary Chondrite Assemblages and Petrographic Types from Vnir Spectra

Introduction: Because of a poor understanding of the delivery mechanisms of meteoroids from the asteroid belt, early asteroid spectral investigations often assumed that the abundance of various meteorite types among meteorite falls was approximately proportional to the abundance of such assemblages within the asteroid population. Based on that assumption, it was concluded that a large fraction of the asteroid population was composed of ordinary chondrite-type (OC) assemblages , and especially that the S-type asteroids were predominately OC assemblages. This misconception often persists even today and is commonly used-explicitly or implicitly-as a justification for assigning an OC composition to virtually any asteroid whose spectrum is generally OC-like. Dynamical studies [1-3] have largely demolished the basis for this assumption. Instead, the meteorite flux reaching the Earth is dominated by source bodies in favorable locations adjacent to the chaotic zones associated with the mean motion (e.g., 3:1, 5:2) and secular resonances (e.g., ν 6) with Jupiter. Thus among the ~135 distinct source bodies which have contributed to the terrestrial meteorite collections [4], only three parent bodies are required for the ordinary chondrite meteorites. Dynamical and spectral studies have linked the H-chondrites (which constitute ~1/3 of the meteorite falls) and the IIE iron meteorites to asteroid 6 Hebe which is located near the intersection of the 3:1 and ν 6 resonances [5]. The parent body of the L-chondrites (which also make up ~1/3 of the meteorite falls) must be similarly favorably located. Evidence of a major shock event ~480 Myr ago on the L-chondrite parent body raises the probability that it was broken up and dispersed to form an asteroid family close to one or more of the resonances [6,7]. Thus at the present time, the " single " L-chondrite parent body may actually be represented by multiple fragments from the breakup. The LL-chondrite parent body (~5% of meteorite falls) must be located in a somewhat less favorable location with respect to the important resonances. None of this precludes the existence of asteroids composed of OC-like assemblages located away from the resonances. The short cosmic ray exposure ages of stony meteorites (as compared to iron meteorites) indicates that stones have short collisional lifetimes in space (≈≤5x10 7 years). Unless a stony parent body is located favorably with respect to an " escape hatch "