THE CASE OF THE WELL-STUDIED, INCOMPLETELY-UNDERSTOOD ASTEROID 433 EROS

A milestone occurred on 14 Feb 2000, when the NEAR spacecraft entered orbit around asteroid 433 Eros for a year of detailed observation of the geology, mineralogy and chemistry, before landing on 12 Feb 2001. Eros is irregularly shaped (35x10x10 km) with a surface that includes a global regolith, asymmetrically distributed boulders, heavily cratered surface, and grooves and ridges recording deformation. The spectrally-derived mineral compositions and olivine:pyroxene ratio, absence of a global magnetic field, and the Mg/Si, Al/Si and Ca/Si ratios and K concentrations point to a primitive, chondritic body, while a depletion in sulfur and differences in Fe/Si and Fe/O ratios from the X-ray and gamma-ray experiments are likely due to metal and sulfide migration due to partial melting throughout the entire asteroid or, more likely, processing of the regolith layer by micrometeorite bombardment, sputtering and/or size/density segregations. Synthesis of the spectral and chemical data suggest the most likely meteoritic analog is either an ordinary chondrite composition that experienced alteration at the surface or less likely a primitive achondrite formed from an ordinary chondritic precursor. Recent work has renewed debate about the geological history of Eros. Newly-derived determinations of chondritic levels of Cr, Mn and Ni from X-ray spectra strongly argue against a partial melting origin for the S depletion, while a spectrally-derived FeO-rich pyroxene composition might favor partial melting and/or a chondritic composition enriched in FeO relative to ordinary chondrites. A similar debate is taking place about the geologic history of asteroid 25143 Itokawa, the target asteroid for the Hayabusa mission. Given inherent uncertainties in orbital data and our incomplete knowledge of the chemical and physical processes occurring in asteroidal regolith, these debates will likely remain unresolved pending asteroid sample return.