Rotational spectral variations of asteroid (8) Flora: Implications for the nature of the S-type asteroids and for the parent bodies of the ordinary chondrites

Abstract

The new interpretive calibrations (olivine-pyroxene abundance, spectral signature of chondritic metal grains) have been applied to visible and near-infrared spectral reflectance data for the typical S-type asteroid, (8) Flora. Chondritic metal grains do not exhibit the steeply reddened spectral signature previously associated with a nickel-iron (NiFe) metal phase. The conditions required to produce a S-type spectral curve from a chondritic material are much more restrictive and improbable than previously supposed. Five mineralogic and patrologic parameters have been derived for the surface material of Flora from the analysis of the spectral reflectance and rotational spectral variations of this object. These include bulk mineralogy (NiFe metal, olivine, pyroxene), mafic mineral abundance (ol/py ≈ 2.8), metal nature [abundant (≈50%) and coarse-grained in the substrate], pyroxene composition (orthopyroxene > Fs₅₀ and/or abundant clinopyroxene), and spatial mineralogic variations (olivine/pyroxene ratio and pyroxene composition do not covary in a chondritic manner). All of the diagnostic mineralogic properties of the Flora surface material indicate that this is a differentiated body. None of the derived surface properties unambiguously support a chondritic-type material and several contradict any type of undifferentiated assemblage. Flora is most probably the residual core of an intensely heated, thermally evolved, and magmatically differentiated planetesimal which was subsequently disrupted. The present surface samples layers formed at and near the core-mantle boundary in the parent body. Several lines of evidence suggest that the S-type asteroids as a class are predominantly the core fragments of disrupted, differentiated planetesimals. A main belt source of the ordinary chondrites remains elusive.