¹Laura Schaefer,²Kaveh Pahlevan,³Linda T. Elkins-Tanton
Journal of Geophysical Research (Planets) Link to Article [https://doi.org/10.1029/2023JE008262]
¹Department of Earth and Planetary Sciences, Stanford University, Stanford, CA, USA
²Carl Sagan Center, SETI Institute, Mountain View, CA, USA
³School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
Published by arrangement with John Wiley & Sons

Magma ocean crystallization models that track fO2 evolution can reproduce the D/H ratios of both the Earth and Mars without the need for exogenous processes. Fractional crystallization leads to compositional evolution of the bulk oxide components. Recent work suggests that metal-saturated magma oceans may contain near-present-day Fe3+ concentrations. We model the fractional crystallization of Earth and Mars, including Fe2+ and Fe3+ as separate components. We calculate Fe3+ partition coefficients for lower mantle minerals and compare the results of fractional crystallization for both Earth and Mars. We calculate oxygen fugacity (fO2) at the surface as the systems evolve and compare them to constraints on the fO2 of the last magma ocean atmosphere from D/H ratios, both with and without metal saturation. For Earth, we find that Fe3+ likely behaves incompatibly in the lower mantle in order to match the D/H constraint for whole mantle models, but shallow magma ocean models also provide reasonable matches. Disproportionation in whole mantle magma oceans likely overpredicts the amount of Fe3+ and metal that form or require subsequent reduction to return to present-day values. For Mars, we cannot match the D/H constraints on last fO2 unless the magma ocean begins with <50% of the predicted Fe3+, but better match the present day mantle redox. We show that Fe3+ partitioning has a measurable effect on magma ocean redox, and that it evolves throughout the magma ocean’s lifetime. We highlight the need for additional experimental constraints on ferric iron mineral/melt partitioning and more thermodynamic data for the Fe-disproportionation reaction.

¹A. Nathues,¹M. Hoffmann,¹R. Sarkar,¹P. Singh,¹J. Hernandez,²J. H. Pasckert,²N. Schmedemann,³G. Thangjam,⁴E. Cloutis,¹K. Mengel,¹M. Coutelier
Journal og Geophysical Research (Planets) (in Press) Open Access Link to Article [https://doi.org/10.1029/2023JE008150]
¹Max Planck Institute for Solar System Research, Goettingen, Germany
²Institut für Planetologie, Universität Münster, Münster, Germany
³School of Earth and Planetary Sciences, National Institute of Science Education and Research, NISER, HBNI, Khurda, Odisha, India
⁴University of Winnipeg, Winnipeg, MB, Canada
Published by arrangement with John Wiley & Sons

Ceres is a partially differentiated dwarf planet located in the main asteroid belt. Consus crater (diameter ∼64 km) is one of the oldest impact features (∼450 Ma) on the Cerean surface that surprisingly still shows a large variety of color lithologies, including exposures of bright material, which are thought to be brine residues. Here, we present new results that help in understanding the structure and composition of the Cerean crust. These results were deduced by using newly processed Dawn Framing Camera (FC) color imagery and FC clear filter images combined with infrared spectral data of Dawn’s Visible and Infrared Spectrometer (VIR). Consus exhibits a variety of color lithologies, which we describe in detail. Interestingly, we found three spectrally different types of bright material exposed by a large old crater on Consus’ floor. One of these, the yellowish bright material (Nathues et al., 2023, https://www.hou.usra.edu/meetings/lpsc2023/pdf/1073.pdf) and its modification, shows spectral signatures consistent with ammonium-enriched smectites. We hypothesize that the ammonium in these smectites stems from contact with ascending brines, originating from a low-lying former brine ocean that has been enriched in ammonium during the differentiation and freezing process of the Cerean crust. This enrichment is mainly due to ammonium uptake by sheet silicates. If such an ammonium enrichment occurred over long-time scales on a global scale, this process may explain the vast presence of ammonium on the Cerean surface. Therefore, an outer solar system origin of Ceres is possibly not needed to explain the global presence of ammonium.