10:00 AM - 10:15 AM
[MIS26-05] EXPERIMENTAL STUDY ON MANGANESE OXIDE FORMATION: INTERPRETAION OF MANGANESE OXIDE ON GALE CRATER AND OXIDIZING ENVIRONMENT ON EARLY MARS.
Keywords:Early Mars, Planetary environment, Atmospheric evolution
In the present study, we conducted laboratory experiments in order to constrain the pO2 level at the time of deposition of the Mn oxides based on the elemental pattern of coprecipitation of Mn oxides. In the experiments, we synthesized MnO2, Mn3O4, and Mn(OH)2 from solutions of dissolved Mn, Ni, Zn, and Cr using different oxidants, such as KMnO4  or H2O2 . After the reactions, filtered solutions and solid precipitates were collected. The recovered solid precipitates were analyzed with X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD). In addition, concentrations of dissolved Mn, Cr, Ni, and Zn in the remnant solution samples were measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). We found that only formation of MnO2 can explain the enrichments of Mn together with Ni and Zn, which is consistent with the pattern of trace metal enrichments of the Mn oxides in the Gale crater. On the other hand, formations of Mn3O4 or Mn(OH)2 cannot account for the observations. Given that precipitation of MnO2 requires a high level of O2 (>0.01 bar), our results strongly suggest the presence of an O2-rich atmosphere on early Mars at the time when groundwater was active within the Gale crater. These results, in turn, imply effective formation of O2 via H2O photolysis and/or ineffective loss of O2 due to limited amounts of reductants on early Mars.
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