¹Andrew S. Rivkin,²Ellen S. Howell,³Joshua P. Emery
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005833]
¹Johns Hopkins University Applied Physics Laboratory
²University of Arizona
³University of Tennessee
Published by arrangement with John Wiley & Sons

Low‐albedo, hydrated objects dominate the list of the largest asteroids. These objects have varied spectral shapes in the 3‐μm region, where diagnostic absorptions due to volatile species are found. Dawn’s visit to Ceres has extended the view shaped by ground‐based observing, and shown that world to be a complex one, potentially still experiencing geological activity.

We present 33 observations from 2.2‐4.0 μm of eight large (D > 200 km) asteroids from the C spectral complex, with spectra inconsistent with the hydrated minerals we see in meteorites. We characterize their absorption band characteristics via polynomial and Gaussian fits to test their spectral similarity to Ceres, the asteroid 24 Themis (thought to be covered in ice frost), and the asteroid 51 Nemausa (spectrally similar to the CM meteorites). We confirm most of the observations are inconsistent with what is seen in meteorites and require additional absorbers. We find clusters in band centers that correspond to Ceres‐ and Themis‐like spectra, but no hiatus in the distribution suitable for use to simply distinguish between them. We also find a range of band centers in the spectra that approaches what is seen on Comet 67P. Finally, variation is seen between observations for some objects, with the variation on 324 Bamberga consistent with hemispheric‐level difference in composition. Given the ubiquity of objects with 3‐μm spectra unlike what we see in meteorites, and the similarity of those spectra to the published spectra of Ceres and Themis, these objects appear much more to be archetypes than outliers.

^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.