Highly oxidizing aqueous environments on early Mars inferred from scavenging pattern of trace metals on manganese oxides

1,2Natsumi Noda,1,2Shoko Imamura,1Yasuhito Sekine, 2Minako Kurisu,3Keisuke Fukushi,4Naoki Terada,2Soichiro Uesugi,5Chiya Numako,2Yoshio Takahashi,6Jens Hartmann
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005892]
1Earth‐Life Science Institute, Tokyo Institute of Technology
2Department of Earth and Planetary Science, University of Tokyo
3Institute of Nature and Environmental Technology, Kanazawa University
4Department of Geophysics, Tohoku University
5Graduate School of Science, Chiba University
6Institute for Geology, Center for Earth System Research and Sustainability, Universität Hamburg
Published by arrangement with John Wiley & Sons

The Curiosity and Opportunity rovers have found depositions of manganese (Mn) (hydr)oxides within the veins of the sedimentary rocks at Gale and Endeavour craters. Since Mn is a redox sensitive element, revealing the chemical form of the Mn (hydr)oxide provides unique information on the redox state of the near‐surface/groundwater at the time of deposition. Here, we report results of laboratory experiments that investigated scavenging patterns of trace metals (zinc, nickel, and chromium) on different Mn (hydr)oxides in order to constrain the chemical form of the Mn precipitates found on Mars. Our results show manganese dioxide (MnO2) scavenges zinc and nickel effectively but not for chromium. The agreement of this scavenging pattern with the observations strongly suggests that the Mn (hydr)oxides found on Mars are highly likely to be MnO2. To form MnO2, oxidizing aqueous environments are required (e.g., Eh > 0.5 V at pH ~ 8). The candidates of the oxidant include molecular oxygen, ozone, nitrates, and perchlorate acids; all of which are considered to be produced by photochemical processes. The presence of MnO2 veins in sediments suggests that such atmospheric high‐Eh oxidants may have been supplied to the subsurface, possibly through hydrological cycles activated by transient warming.

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