The Renazzo-like carbonaceous chondrites as resources to understand the origin, evolution, and exploration of the Solar System

1N.M.Abreu,2,3J.C.Aponte,4E.A.Cloutis,5A.N.Nguyen
Geochemistry (Chemie der Erde) (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2020.125631]
1Earth Science, Pennsylvania State University – DuBois Campus, DuBois, PA, 15801, USA
2Solar System Exploration Division, Code 691, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
3Department of Chemistry, Catholic University of America, Washington, DC 20064, USA
4Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB R3B 2E9, Canada
5Jacobs Engineering Group Inc., NASA Johnson Space Center, Houston, Texas 77058, USA
Copyright Elsevier

We present here a review of the characteristics of CR carbonaceous chondrite meteorites. Over the past three decades, our knowledge and understanding of the scientific value of the CR chondrites have increased dramatically, as more samples from cold and hot deserts have become available for analysis. Based on a variety of compositional, mineralogical, isotopic, and spectroscopic studies, we have come to understand that CR chondrites are excellent samples of asteroidal meteorites to look for virtually unaltered solar nebula material and to observe asteroidal processes in progress. This paper summarizes these investigations, their similarities, and differences with other chondritic groups, their relationships to asteroids, and the questions yet to be addressed.

OXYGEN-ISOTOPE HETEROGENEITY IN THE NORTHWEST AFRICA 3358 (H3.1) REFRACTORY INCLUSIONS − FLUID-ASSISTED ISOTOPIC EXCHANGE ON THE H-CHONDRITE PARENT BODY

1,2Samuel Ebert,1Kazuhide Nagashima,1Alexander N.Krot,2Addi Bischoff
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.012]
1School of Ocean, Earth Science and Technology, Hawai’i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, USA
2Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Germany
Copyright Elsevier

The nature of oxygen-isotope heterogeneity in refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)] from weakly metamorphosed chondrites is one of the outstanding problems in cosmochemistry. To obtain insights into possible processes resulting in O-isotope heterogeneity of refractory inclusions, we investigated the mineralogy, petrology, and oxygen isotopic compositions of six CAIs and two AOAs and aqueously formed fayalite grains within the matrix of the H3.1 chondrite Northwest Africa (NWA) 3358. Most of the refractory inclusions studied appear to be unmolten solar nebula condensates; some may have experienced partial melting and/or high-temperature annealing. The NWA 3358 refractory inclusions nearly completely avoided metasomatic alteration on the H-chondrite parent body: nepheline grains replacing anorthite and/or melilite are either very minor or absent. Five out of eight refractory inclusions studied have heterogeneous O-isotope composition: Δ17O ranges from ∼ −25‰ to ∼ 3.5±2‰ (2σ). This O-isotope heterogeneity appears to be mineralogically controlled with melilite and anorthite being systematically 16O-depleted compared to hibonite, spinel, Al,Ti-diopside, and forsterite all having similar solar-like Δ17O of ∼ −24±2‰. In contrast to NWA 3358 refractory inclusions, the previously studied AOAs and a fine-grained CAI from the LL3.00 chondrite Semarkona have uniform Δ17O of ∼ −25‰ (McKeegan et al., 1998; Itoh et al., 2007). Because the mineralogically-controlled O-isotope heterogeneity in refractory inclusions from ordinary chondrites appears to correlate with petrologic type of a host meteorite experienced by aqueous alteration, we suggest O-isotope exchange in NWA 3358 CAIs and AOAs resulted from aqueous fluid-rock interaction on the H-chondrite parent asteroids. This is supported by the presence of 16O-depleted anorthite (Δ17O ∼ 3.5±2‰) and aqueously formed fayalite similar depleted in 16O (Δ17O ∼ 4±2‰). The Δ17O of NWA 3358 fayalite is comparable to that of magnetite and fayalite in Semarkona and other weakly metamorphosed L3 and LL3 chondrites (Choi et al., 1998; Doyle et al., 2015) suggesting similar Δ17O of aqueous fluids on the H, L, and LL chondrite parent asteroids.

Thermophysical properties of the surface of asteroid 162173 Ryugu: Infrared observations and thermal inertia mapping

1Yuri Shimaki et al. (>10)
Icarus (in Press) Link to article [https://doi.org/10.1016/j.icarus.2020.113835]
1Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
Copyright Elsevier

TIR, the thermal infrared imager on Hayabusa2, acquired high-resolution thermal images of the asteroid 162173 Ryugu for one asteroid rotation period on August 1, 2018 to investigate the thermophysical properties of the asteroid. The surface temperatures of Ryugu suggest that the surface has a low thermal inertia, indicating the presence of porous materials. Thermophysical models that neglect or oversimplify surface roughness cannot reproduce the flat diurnal temperature profiles observed during daytime. We performed numerical simulations of a thermophysical model, including the effects of roughness on the diurnal brightness temperature, the predictions of which successfully reproduced the observed diurnal variation of temperature. The global thermal inertia was obtained with a standard deviation of 225 ± 45 J m−2 s−0.5 K−1, which is relatively low but still within the range of the value estimated in our previous study (Okada et al., Nature 579, 518–522, 2020), confirming that the boulders on Ryugu are more porous in nature than typical carbonaceous chondrites. The global surface roughness (the ratio of the variance of the height relative to a local horizontal surface length) was determined as 0.41 ± 0.08, corresponding to a RMS surface slope of 47 ± 5°. We identified a slightly lower roughness distributed along the equatorial ridge, implying a mass movement of boulders from the equatorial ridge to the mid-latitudes.

Evidence of extensive lunar crust formation in impact melt sheets 4,330 Myr ago

1,2L. F. White,1,2,3A. Černok,4J. R. Darling,5M. J. Whitehouse,6K. H. Joy,7C. Cayron,4J. Dunlop,1,2 K. T. Tait,3,8M. Anand
Nature Astronomy (in Print) Link to Article [DOIhttps://doi.org/10.1038/s41550-020-1092-5]
1Centre of Applied Planetary Mineralogy, Department of Natural History, Royal Ontario Museum, Toronto, Ontario, Canada
2Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada
3School of Physical Sciences, The Open University, Milton Keynes, UK
4School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth, UK
5Swedish Museum of Natural History, Stockholm, Sweden
6Department of Earth and Environmental Science, University of Manchester, Manchester, UK
7Laboratory of ThermoMechanical Metallurgy (LMTM), PX Group Chair, École Polytechnique Fédérale de Lausanne (EPFL), Neuchâtel, Switzerland
8Department of Earth Sciences, The Natural History Museum, London, UK

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Onset of magma ocean solidification on Mars inferred from Mn-Cr chronometry

1Thomas S.Kruijer,1Lars E.Borg,1Josh Wimpenny,1Corliss K.Sio
Earth and Planetary Science Letters 542, 116315 Link to Article [https://doi.org/10.1016/j.epsl.2020.116315]
1Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue (L-231), Livermore, CA 94550, USA
Copyright Elsevier

The mantle of Mars probably differentiated through the crystallization of a magma ocean during the first tens of million years (Ma) of Solar System evolution. However, the exact timescale of large-scale silicate differentiation of the martian mantle is debated, and in particular, it remains unclear when differentiation commenced. Here we applied the short-lived 53Mn-53Cr system to martian meteorites in order to date the onset of large-scale mantle differentiation on Mars. The new Cr isotope data demonstrate that martian meteorites exhibit no resolvable radiogenic 53Cr variations, and instead have a uniform +20.2±1.2 (95% conf.) parts-per-million excess in 53Cr/52Cr relative to the terrestrial mantle. The investigated groups of martian meteorites are lithologically varied and derive from diverse mantle sources that probably had variable Mn/Cr. Hence, the lack of 53Cr variability among martian meteorites demonstrates that silicate differentiation on Mars occurred after the extinction of 53Mn. Provided that the sources of the martian meteorites have Mn/Cr variations that are typical of the terrestrial planets, this result implies that the onset of large-scale silicate differentiation must have occurred later than 20±5 Ma after Solar System formation. The onset of silicate differentiation on Mars inferred here is significantly later than time estimates for segregation of the martian core which conservatively occurred within <10 Ma after Solar System formation. Thus, the new Mn-Cr data imply that there was a small, but resolvable, time gap of at least 5 Ma between core formation and magma ocean solidification on Mars. If the age of core segregation is taken at face value, our results imply that the martian magma ocean remained mostly molten over several Ma. This inferred longevity of the magma ocean is inconsistent with thermal models predicting rapid (<1 Ma) solidification of the martian magma ocean. Although there is currently no unique solution to this conundrum, our results can potentially be explained by a protracted history of impact bombardment that delayed differentiation in a shallow magma ocean on Mars, or perhaps more readily, by the presence of an early and dense atmosphere that acted as an insulator and prevented the magma ocean from cooling quickly.