Mixing Fraction of Inner Solar-system Material in Comet 81p/wild2

Introduction: The presence of crystalline silicates in the co-mae of comets, inferred through infrared astronomical observations , has been a long-standing puzzle (e.g., [1]). Crystalline silicates are unexpected if comets are composed of pristine interstellar material, since interstellar silicates are almost entirely amorphous[2]. Heating to > 1100K 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-scale mixing was important in the protoplanetary disk[3]. Reports of refractory CAI-like objects[4] and large concentrations of noble gases in Stardust samples[5] underscore the need to assume 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. Based on measurements of the fraction of iron in sulfides and in crystalline materials, we estimate the fraction ψ of inner nebular material in 81P/Wild2. This measurement constrains models of large-scale mixing in the early solar system. Methods: We removed eleven cometary tracks from the Star-dust cometary aerogel tiles in keystones[6]. We used a mi-crofocused tunable hard x-ray beam on beamline 10.3.2 at the Advanced Light Source at Lawrence Berkeley National Laboratory to perform micro x-ray absorption measurements. We first performed micro-XRF Fe elemental mapping of the tracks in the keystone, using an incident energy of 10keV. We then analyzed Fe particles of interest using X-ray absorption near edge structure (XANES) spectroscopy. Here the absorption spectrum is measured in the region of the Fe K-edge at ∼ 7110 eV, with high photon statistics and energy resolution ∼ 1 eV. We then used an extensive library of standards (54 standards including metals, sulfides, and Fe 2+-and Fe 3+-bearing minerals and glasses) to fit the absorption spectra and to determine the relative fraction of metal, sulfide, and oxidized Fe in each particle. Although there is degeneracy among standards within a group (e.g., sulfides), identification of the mineral group is reliable , principally because of the systematic shift in position of the K-edge as a function of oxidation state[7]. Glass standards are also highly degenerate with each other and their spectra are distinct from crystalline materials[8]. We …