¹Adrian P.Broz,²Joanna Clark,³Brad Sutter,⁴Doug W.Ming,³ValerieTu,⁵Briony Horgan,⁶Lucas C.R.Silva
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114965]
¹Department of Earth Sciences, University of Oregon, Eugene, OR 97405, United States of America
²Geocontrols Systems – Jacobs JETS Contract, NASA Johnson Space Center, Houston, TX 77058, United States of America
³Jacobs JETS Contract, NASA Johnson Space Center, Houston, TX 77058, United States of America
⁴NASA Johnson Space Center, Houston, TX 77058, United States of America
⁵Department of Earth, Atmospheric and Planetary Science, Purdue University, IN, 47907, United States of America
⁶Environmental Studies Program, Department of Geography, University of Oregon, Eugene, OR 97405, United States of America
Copyright Elsevier

Ancient (4.1–3.7-billion-year-old) layered sedimentary rocks on Mars are rich in clay minerals which formed from aqueous alteration of the Martian surface. Many of these sedimentary rocks appear to be composed of vertical sequences of Fe/Mg clay minerals overlain by Al clay minerals that resemble paleosols (ancient, buried soils) from Earth. The types and properties of minerals in paleosols can be used to constrain the environmental conditions during formation to better understand weathering and diagenesis on Mars. This work examines the mineralogy and diagenetic alteration of volcaniclastic paleosols from the Eocene-Oligocene (43–28 Ma) Clarno and John Day Formations in eastern Oregon as a Mars-analog site. Here, paleosols rich in Al phyllosilicates and amorphous colloids overlie paleosols with Fe/Mg smectites that altogether span a sequence of ~ 500 individual profiles across hundreds of meters of vertical stratigraphy. Samples collected from three of these paleosol profiles were analyzed with visible/near-infrared (VNIR) spectroscopy, X-ray diffraction (XRD), and evolved gas analysis (EGA) configured to operate like the SAM-EGA instrument onboard Curiosity Mars Rover. Strongly crystalline Al/Fe dioctahedral phyllosilicates (montmorillonite and nontronite) were the major phases identified in all samples with all methods. Minor phases included the zeolite mineral clinoptilolite, as well as andesine, cristobalite, opal-CT and gypsum. Evolved H₂O was detected in all samples and was consistent with adsorbed water and the dehydroxylation of a dioctahedral phyllosilicate, and differences in H₂O evolutions between montmorillonite and nontronite were readily observable. Detections of hematite and zeolites suggested paleosols were affected by burial reddening and zeolitization, but absence of illite and chlorite suggest that potash metasomatism and other, more severe diagenetic alterations had not occurred. The high clay mineral content of the observed paleosols (up to 95 wt%) may have minimized diagenetic alteration over geological time scales. Martian paleosols rich in Al and Fe smectites may have also resisted severe diagenetic alteration, which is favorable for future in-situ examination. Results from this work can help differentiate paleosols and weathering profiles from other types of sedimentary rocks in the geological record of Mars.

¹Keisuke Sugiura,²Makiko K.Haba,¹Hidenori Genda
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114949]
¹Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan
²Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8550, Japan
Copyright Elsevier

Mesosiderites are a type of stony-iron meteorites composed of a mixture of silicates and Fe-Ni metals. The mesosiderite silicates and metals are considered to have originated from the crust and metal core, respectively, of a differentiated asteroid. In contrast, mesosiderites rarely contain the olivine that is mainly included in a mantle. Although a giant impact onto a differentiated asteroid is considered to be a probable mechanism to mix crust and metal materials to form mesosiderites, it is not obvious how such a giant impact can form mesosiderite-like materials without including mantle materials. We conducted three-dimensional numerical simulations of giant impacts onto differentiated asteroids, using the smoothed particle hydrodynamics method, to investigate the detailed distribution of mixed materials on the resultant bodies. For the internal structure model of a target body, we used a thin-crust model derived from the magma ocean crystallization model of the asteroid Vesta. We also considered, as another possible internal structure for the target body, a thick crust and a large metal core suggested from the proximity observation of Vesta by the Dawn probe. In the simulations with the former model, excavation of the metal core requires nearly catastrophic impacts and mantle is exposed over large surface areas. Thus, stony-iron materials produced on its surface are likely to include mantle materials, and it is difficult to produce mesosiderite-like materials with this internal structure. Conversely, in the simulations with the latter model, mantle materials are exposed only at impact sites, even when the impacts excavate the metal core, and we confirmed that the formation of a surface with little mantle material and the formation of mesosiderite-like materials are possible from such a surface. Therefore, our simulations suggest that an internal structure with a thick crust and a large core is more likely as a mesosiderite parent body rather than the thin-crust internal structure inferred from the conventional magma ocean model.