The mystery of 506.5 nm feature of reflectance spectra of Vesta and Vestoids: Evidence for space weathering?

Although asteroid 4 Vesta and Vestoids have been believed to be the source of a group of basaltic meteorites called HEDs, there have been detailed spe ctral analyses on the spectral redness and the 506.5 nm absorption band, su ggesting controversy on their space weathering processes and origins. In or der to evaluate a possibility that such an apparent inconsistency may be ex plained by the space weathering, the 506.5 nm spectral feature and reddenin g trend are examined for Vesta and Vestoids, HED meteorites, lunar soils, a nd laser irradiated pyroxene samples in this paper. Our results indicate th at all fresh HED meteorites have the 506.5 nm band at different wavelengths according to their classes, lunar soils seem to lose the 506.5 nm band as they mature, and pulse laser irradiation on the pyroxene sample seems to re duce the 506.5 nm band. Therefore, absence of the 506.5 nm band on some Ves toids can be due to space weathering although the relationship between the visible redness and presence/absence of the 506.5 nm band of Vesta and Vest oids is inconsistent with the assumed HED-lunar space weathering trend base d on the above laboratory results. Other possible explanations are that som e Vestoids experienced shock heavy enough to erase the 506.5 nm band and th at pyroxenes on some Vestoids are not similar to those in HED meteorites. E ven if the latter case is true and some Vestoids are not made of HED materi als, HED meteorites could still come from Vesta unless we assume all Vestoi ds have to be fragments of Vesta.