2019ApJ...872...66W

The relative fractions of Fe in metal, sulfide, or oxide-the Fe oxidation state-in the interstellar medium (ISM) can provide constraints on the processes that operated on material in the protosolar disk. We used synchrotron-based X-ray absorption spectra of three mineral standards and two kinds of primitive extraterrestrial materials to constrain the oxidation state and mineralogy of the host phase of ISM Fe as measured by X-ray observations of Fe-L ISM absorption from the Chandra X-ray Observatory. Oxidation of cometary material appears to have taken place in the nebula, before incorporation into cometary nuclei, although the mechanism is unknown. We also test the hypothesis of Ishii et al. that ISM solids consist largely of Glass with Embedded Metals and Sulfides (GEMS), enigmatic components of primitive interplanetary dust particles. The hypothesis is not consistent with observations. Using Fe L-edge absorption spectra of comet 81P/Wild 2 samples returned by the Stardust mission, we also find that Chandra observations are inconsistent with the hypothesis the cometary fine-grained material consists of unaltered interstellar dust.