Spectral study of the Eunomia asteroid family: I. Eunomia

Abstract

We present color ratio curves of the S-Asteroid 15 Eunomia, which have been extracted from high-precision photometric lightcurves obtained in three different VNIR wavelength bands at the Bochum Telescope, La Silla. The measured color ratio curves and near infrared spectra were used to derive a detailed surface composition model whose shape has been computed by V-lightcurve inversions. According to this analysis, the asteroid shows on one hemisphere a higher concentration of pyroxene, which causes an increased 440/700 nm and a reduced 940/700 nm reflectance ratio as well as a pronounced 2-μm absorption band. The remaining surface shows a higher concentration of olivine, leading to a reduced 440/700 nm and slightly increased 940/700 nm color ratio. In addition, we found that the maximum of the 440/700 nm color ratio curve coincide with the minimum of the 940/700 nm color ratio curve and vice versa. We demonstrate on the basis of USGS laboratory spectra that this anti-cyclical behavior can be explained by choosing Fe-rich olivine and a pyroxene with moderate Fe content as varying mineral phases. Furthermore, our observations confirm that 15 Eunomia is an irregular elongated and at least partially differentiated body. Previous spectral investigations of several smaller fragments of the Eunomia asteroid family revealed that the amount of fragments showing an increased pyroxene content exceeds the amount of pyroxene-poor fragments (Nathues, 2000, DLR Forschungsbericht, ISSN 1434-8454). This finding together with the observation that the major fraction of Eunomia's surface is enriched in olivine let us claim that a large fraction of the original pyroxene-enriched crust layer has been lost due to a major collision that created the Eunomia asteroid family. Significant spectral evidences, consistent with high concentrations of metals have been found neither in the rotational resolved spectra of 15 Eunomia nor in its fragments. This led to the conclusion that either no core consisting mainly of metals exists or that an eventual one has not been unearthed by the impact.