Eros Sulfur Deficiency: a Closer Look at Meteorite Comparisons

Introduction: The NEAR Shoemaker spacecraft that orbited and eventually landed on asteroid 433 Eros carried two experiments to determine the chemistry of the surface layer: an x-ray spectrometer (XRS) and a gamma ray spectrometer (GRS). Arguably the most surprising result obtained by these instruments was the discovery that, in spite of an otherwise chondritic chemistry, the sulfur content of Eros regolith is very low. Here we address several possible explanations, consider their plausibility, and propose steps toward resolving the conundrum. NEAR results: The sulfur x-ray signal was barely detectable by XRS. Nittler et al. [1] suggest an upper limit of S/Si=0.05 (weight ratio); the model calculations of McCoy et al. [2] use an average of 0.014. Normalized to CI chondrites, the upper limit corresponds to (S/Si)*=0.1, the McCoy estimate (S/Si)*=0.027 [where (S/Si)*=(S/Si)sample/(S/Si)CI]. For comparison with meteorites two other results are important: the K content determined by GRS (0.070±0.028%) and the Fe/Si weight ratio determined by XRS (1.65±0.27). For comparison with meteorites the absolute value of the K determination has to be converted to a K/Si ratio. We have adopted the nominal value of 17.65wt% Si of McCoy for this purpose; any error in the Si estimate is probably trivial compared to the uncertainty in K. Possible causes: There are three classes of explanations for the low S content found by NEAR: (1) the depletion is primary, (2) sulfur has been lost into space, or (3) sulfur has migrated from the surface layer analyzed by XRS to a deeper level in the regolith. The issue of primary depletion can be addressed to some extent by comparison with meteorites. The other two possible explanations for sulfur depletion can be best studied by laboratory simulations of asteroidal processes. Primary depletion: Comparisons are complicated by the fact that there are at least two major fractionation processes affecting chondrites: volatility and mineral separation. The mineral fractionation has primarily been conceived as metal/silicate fractionation. Sulfur, however, is not hosted in either fraction, but in a separate mineral, FeS. The behavior of FeS during metal/silicate fractionation is poorly understood. Figures 1 and 2 show (S/Si)* of major chondrite groups plotted against (K/Si)* and (Fe/Si)*, respectively. The former is a measure of fractionation related to volatility, the latter mostly a measure of metal/silicate fractionation. Since many chondrite groups are apparently affected by both processes, neither diagram shows a very tight correlation. The width of the boxes representing Eros are defined by the stated errors for the determination of K [3] and Fe/Si [1]; the height of boxes spans the range