MIXING FRACTION OF INNER SOLAR SYSTEM MATERIAL IN COMET 81P/WILD2

The presence of crystalline silicates in the comae of comets, inferred through infrared observations, has been a long-standing puzzle. Crystalline silicates are unexpected if comets are composed of pristine interstellar material, since interstellar silicates are almost entirely amorphous. Heating to > 1100 K can anneal silicates to crystallinity, but no protoplanetary heating sources have been identified that were sufficiently strong to heat materials in the outer nebula to such high temperatures. This conundrum led to the suggestion that large-scalemixing was important in the protoplanetary disk. Reports of refractory calcium-aluminum-rich inclusion-like objects and large concentrations of noble gases in Stardust samples underscore the need for such mixing. However, the evidence from the Stardust samples until now has been largely anecdotal, and it has not been possible to place quantitative constraints on the mixing fraction. Here we report synchrotron-based X-ray microprobe measurements of the relative concentrations of the chemical state of iron in material from a known comet, the Jupiter-family comet 81P/Wild2. We find that the comet is rich in iron sulfides. The elemental S/Fe ratio based on the sulfide concentration, S/Fe > 0.31(2 sigma), is higher than in most chondritic meteorites. We also found that Fe-bearing silicates are at least 50% crystalline. Based on these measurements, we estimate the fraction psi of inner nebular material in 81P/Wild2. With the lower bound on the crystalline Fe-bearing silicate fraction, we find that psi > 0.5. If the observed S depletion in the inner solar system predated or was contemporaneous with large-scale mixing, our lower bound on the S/Fe ratio gives an upper bound on psi of similar to 0.65. This measurement may be used to test mixing models of the early solar system.