The composition of 433 Eros: A mineralogical—chemical synthesis

Abstract

Abstract-The near-Earth asteroid rendezvous (NEAR) mission carried x-ray/gamma-ray spectrometers and multi-spectral imagerhear-infrared spectrometer instrument packages which gave complementary information on the chemistry and mineralogy, respectively, of the target asteroid 433 Eros. Synthesis of these two data sets provides information not available from either alone, including the abundance of non-mafic silicates, metal and sulfide minerals. We have utilized four techniques to synthesize these data sets. Venn diagrams, which examine overlapping features in two data sets, suggest that the best match for 433 Eros is an ordinary chondrite, altered at the surface of the asteroid, or perhaps a primitive achondrite derived from material mineralogically similar to these chondrites. Normalized element distributions preclude FeO-rich pyroxenes and suggest that the x-ray and gammaray data can be reconciled with a common silicate mineralogy by inclusion of varying amounts of metal. Normative mineralogy cannot be applied to these data sets owing to uncertainties in oxygen abundance and lack of any constraints on the abundance of sodium. Matrix inversion for simultaneous solution of mineral abundances yields reasonable results for the x-ray-derived bulk composition, but seems to confirm the inconsistency between mineral compositions and orthopyroxene/clinopyroxene ratios. A unique solution does not seem possible in synthesizing these multiple data sets. Future missions including a lander to fully characterize regolith distribution and sample return would resolve the types of problems faced in synthesizing the NEAR data. INTRODUCTIONThe near-Earth asteroid rendezvous (NEAR)-Shoemaker mission spent almost a year in orbit around the asteroid 433 Eros, studying its mineralogy, surface chemistry, surface geology, surface topography, and gravitational and magnetic fields. This is the first time that such a diverse suite of data has been collected simultaneously from an asteroid by a spacecraft. 433 Eros is a particularly worthy target for this study, since ground-based spectral studies (Murchie and Pieters, 1996) indicate that it is a member of the S( IV) spectral class of Gaffey et al. (1993). Debate has raged for nearly 20 years about the relationship between S asteroids and ordinary chondrites (Wetherill and Chapman, 1988), the most abundant types of asteroids in the inner belt and meteorites observed to fall to Earth, respectively, and NEAR may allow solution of this conundrum, if only in this one specific case. While each data set can be interpreted individually, as evidenced by the series ofpapers Prelude prcprint MS#4616

Towards understanding the dynamical evolution of asteroid 25143 Itokawa: constraints from sample analysis

“…The parent asteroid spectral type for ordinary chondrites was hypothesized to be the S-type asteroids (Binzel et al 2001;McCoy et al 2001;Binzel et al 2010;Nesvorny et al 2010;Nakamura et al 2011). The Hayabusa samples provide a key data point for referencing an S-type asteroid to ordinary chondrites, supporting the above hypothesis.…”

443 Eros: Problems with the meteorite magnetism record in attempting an asteroid match

“…433 Eros may be a match with the L/LL chondrites based on the analysis of spectral and x-ray and gamma-ray data (McCoy et al, 2001). The NEAR-Shoemaker spacecraft was inserted initially into a nearly circular 200 km orbit around Eros and subsequent orbit adjustments brought the spacecraft to near-circular orbits of 100,50 and 35 km radius with respect to Eros' center of mass.…”

The NEAR‐Shoemaker x‐ray/gamma‐ray spectrometer experiment: Overview and lessons learned

“…In a series of companion papers, we describe the methods for analyzing these data and comparing them to meteoritic compositions, the results and interpretations of the x-ray and gamma-ray spectrometers individually, and the synthesis of these data with results from the multi-spectral imagerhear-infrared spectrometer experiments (Nittler et al, 2001; Evans et al, 2001b;McCoy et al, 2001). In this paper we focus on the instrumentation itself to give the reader a brief overview of the design, capabilities and limitations of these instruments, an overview of results from the NEAR XGRS experiment, and some thoughts on the design of instrumentation for hture missions.…”