MINERALOGICAL COMPOSITION OF (25143) ITOKAWA 1998 SF36 FROM VISIBLE AND NEAR-INFRARED REFLECTANCE SPECTROSCOPY: EVIDENCE FOR PARTIAL MELTING

Abstract

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/near‐infrared (VNIR) spectra (∼∼0.48 to 0.9 μm) obtained on March 24, 26, and 27 UT, and near‐infrared (NIR) spectra (∼∼0.75 to 2.5 μm) obtained on March 10, 11, 12, 23, and 24 UT collectively show absorption features centered near 1.0 and 2.0 μm, 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 ∼∼Wo14 ± 5Fs43 ± 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 Fs14–26 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 °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. Itokawa's 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.