^1,2Klaus Mezger,¹Alessandro Maltese,^1,2Hauke Vollstaedt
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114497]
¹Institute for Geology, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland
²Center for Space and Habitability, University of Bern, Switzerland
Copyright Elsevier

The abundances of the chemical elements and radiogenic isotopes in the silicate Earth provide key information on the composition of planetary building blocks, the accretion process, including its timing, and the early planetary-wide chemical differentiation. The abundances of the lithophile and highly siderophile elements in the bulk silicate Earth can be modeled as a mixture of three distinct components. Component A (proto-Earth) consisted of volatile-element depleted and strongly reduced material to which a highly oxidized component B (impactor, Theia) was added with chondritic element abundances for the refractory elements to slightly depleted in the volatile elements. Finally, a late veneer (component C) added more material with a composition similar to carbonaceous chondrites. These components make up ~85%, ~15% and ~ 0.4% of the mass of the silicate Earth, respectively. The sequence of their accretion led to a first core formation that produced a metallic core and depleted the silicate portion in siderophile elements including most of the Fe. Addition of the oxidized and volatile richer component B was followed by a second core formation event with removal of a sulfide melt and depletion of the mantle in chalcophile and siderophile elements. The final addition of a late chondritic veneer established a near CI-chondritic abundance among the highly siderophile elements, but also among S, Se and Te. The significant chemical differences between the two first and major components imply that they formed in different regions of the solar system and from isotopically distinct material. The homogeneity of the isotopes of refractory elements in the Earth-Moon system then requires a giant impact that was energetic enough to homogenize the material from the two bodies. The combination of the two major components that formed the Earth is contemporaneous with the formation of the Moon. The initial Sr-isotope composition of the Moon indicates that this impact occurred at 4.507 (15) Ga. The most-likely major source for the highly volatile elements, including water on Earth, is the Moon-forming impactor. Thus, the habitability of Earth and its ability to develop plate tectonic processes is the result of the chance collision of proto-Earth with a planetary body that had formed dominantly from material originating beyond the orbit where Earth formed and therefore had accreted a higher amount of volatile elements.

¹G.David et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114481]
¹Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, CNES, Toulouse, France
Copyright Elsevier

The Curiosity rover has been characterizing mineralogical and chemical compositions of Gale crater soils on Mars since 2012. Given its sub-millimeter scale of analysis, the ChemCam instrument is well suited to study the composition of soil constituents. However, the interpretation of LIBS data on soils in the martian environment is complicated by the large diversity of particle sizes (from dust to sand), combined with the unknown physical arrangement of their mineral constituents (i.e., the type of grain mixtures). For example, martian soils contain a significant amount of X-ray amorphous materials whose physical form remains unclear. In this study, we reproduced martian soil analyses in the laboratory to understand how the LIBS technique can provide specific insights into the physical and chemical properties of granular soils. For this purpose, different types of samples were studied with various ranges of grain sizes, mimicking two possible mixtures that may occur in martian soils: mechanical mixtures of two populations of grains made of distinct chemical compositions; and material forming a compositionally distinct coating at the surface of grains. Our results, also supported by in situ ChemCam data, demonstrate that both the sizes and the type of mixture of soil particles have a strong influence on the LIBS measurement. For mechanical mixtures of two populations of grains larger than 125–250 μm, the scatter of the data provides information about the chemical composition of the end-members. On the other hand, the chemistry recorded by LIBS for grains with surface coatings is fully dominated by the outer material for grains smaller than 500 μm in diameter. This is due to the small penetration depth of the laser (~0.3–1.5 μm per shot), combined with the ejection of small grains at each shot, which leads to a constant replenishment of fresh material. This experimental work will thus improve our understanding of martian soils analyzed by ChemCam, and more broadly, will benefit LIBS studies of granular materials.

¹C.Pilorget,²J.Fernando
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114498]
¹Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Orsay 91405, France
²Independent scholar, Orsay 91400, France
Copyright Elsevier

Over the last few decades, visible and near-infrared spectroscopy has proven to be an efficient technique to characterize planetary surface mineralogy, in particular thanks to the presence of diagnostic features appropriate for the identification of most minerals of interest. A more quantitative analysis of the VIS-NIR reflectance spectra constitutes the next major step in understanding the planetary bodies’ history as the retrieval of the mineral assemblages and their relative abundances enables to constrain the chemical and physical conditions of their formation and, thus, the past and present geologic and climatic processes.

Here, we evaluate the capability to retrieve quantitative properties (abundance, grain size) of intimately mixed materials (the most common mineral mixture among planetary surfaces) from typical space VIS-NIR reflectance spectroscopic data. Such results are key to correctly assess the accuracy and relevance of the retrieved mineral information. For that purpose, we developed an inversion model based on a Monte-Carlo Markov Chains (MCMC) scheme with a Bayesian approach to invert VIS-NIR spectra. This approach allows to properly propagate the uncertainties from the data to the retrieved properties, and finally assess what such uncertainties imply for the interpretation. Different binary and ternary mixtures with minerals of interest in planetary sciences and displaying a large variety of albedos and spectral features were tested. Typical uncertainties, both for the abundance and the grain size, were derived and sensitivities on specific parameters/trends were identified. In particular, the role of absorption features in the spectra is quantified. Tests were performed using either the Hapke or the Shkuratov radiative transfer model. The case of unidentified endmembers in the mixture is also discussed. In particular, results show that if the unidentified phase does not display any significant spectral feature, the lack of knowledge about its optical properties does not significantly impact the inversion. These different results will be key in the quantitative analyses of VIS-NIR spectra from planetary bodies.

Finally, we analyze more specifically the case of phyllosilicates and carbonates, two families of minerals of high importance in understanding the Mars geologic and climate history. Typical uncertainties on their relative abundances and grain sizes are derived in various cases, providing a critical supporting dataset for the characterization of the martian mineralogy and the associated geological processes.

¹David T.Blewett,¹Brett W.Denevi,¹Joshua T.S.Cahill,¹Rachel L.Klima
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114472]
¹Planetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, MS200-W230, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA
Copyright Elsevier

We performed an analysis of spacecraft multispectral images for lunar swirls in order to gain an improved understanding of optical space weathering on the Moon and its causes. LROC WAC data provide information on the slope of the spectrum in the near-UV (NUV), as measured by the 321-nm/415-nm or 321-nm/360-nm reflectance ratios. Kaguya MI data were used to assess the near-infrared (NIR) continuum slope (1548-nm/749-nm reflectance ratio). Context for interpreting the spectral variations found in the remotely observed regions of the lunar surface is provided by laboratory reflectance spectra of lunar rocks and soils, as well as spectra for transparent silica gel analogs (Noble et al., 2007) containing different sizes and abundances of nanometer-sized iron (nsFe) particles. We gain additional insights into the spectral effects of sample maturity by considering the ferromagnetic resonance parameter (Is) values for mare and highland soils, as well as the number density of nsFe particles in the silica gels.

We examined a set of three mare swirls (Reiner Gamma, Ingenii, and Mare Marginis) and three highland swirls (Airy, Descartes, and Gerasimovich). The NIR continuum slopes of both mare and highland swirls are shallower than those of the nearby normal mature regolith. Bright swirl surfaces have higher NIR slopes than normal fresh material of the same albedo. The NUV ratios within mare swirls are lower than in the mature background, but for highland swirls, the NUV ratios are approximately the same as the mature background. We do not see definitive evidence for “over-maturation” (excessive darkening and reddening beyond that found in the normal background surfaces) in dark lanes at the swirls we examined, although saturation of weathering effects at a high-iron location like Reiner Gamma could prevent over-maturation from appearing – even if enhanced solar-wind bombardment related to deflection by local magnetic fields is taking place.

Evaluation of the NIR character of swirls and comparison with lab spectra of lunar soils and nsFe-bearing silica gel analogs leads to the conclusion that swirl materials contain abundances of nsFe that are lower than that of normal non-swirl background surfaces; the nsFe content of swirls corresponds to immature (though not pristine) or submature soils. However, the size distribution of nsFe in swirls is anomalous compared with normal lunar surfaces, with a deficiency in the smaller size range (< ~15 nm), as inferred from the NUV character of swirls. Because the flux of solar-wind ions reaching the surface in swirls is attenuated by shielding by crustal magnetic fields, we conclude that solar-wind exposure is the primary agent for production of small nsFe in normal lunar space weathering. Micrometeoroid bombardment, which is unimpeded by the presence of magnetic fields, is mainly responsible for production of larger nsFe in space weathering.

¹Elizaveta Kovaleva,²Monika A.Kusiak,³Gavin G.Kenny,³Martin J.Whitehouse,⁴Gerlinde Habler,⁵Anja Schreiber,²Richard Wirth
Earth and Planetary Science Letters 565, 116948 Link to Article [https://doi.org/10.1016/j.epsl.2021.116948]
¹Department of Earth Sciences, University of the Western Cape, Robert Sobukwe Road, Bellville 7535, South Africa
²Institute of Geophysics, Polish Academy of Sciences, Księcia Janusza 64, PL-01452 Warsaw, Poland
³Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
⁴Department of Lithospheric Research, University of Vienna, 1090 Vienna, Austria
⁵Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, 3.5 Surface Geochemistry, D-14473 Potsdam, Germany
Copyright Elsevier

Granular neoblastic zircon (ZrSiO4) with systematically oriented granules has been proposed as evidence for extreme shock pressures (>30 GPa) and subsequent high temperatures (>1200 °C). It is widely agreed to reflect the solid-state phase transition from zircon to its high-pressure polymorph reidite and subsequent reversion to zircon. This model is based on crystallographic relationships between granules of a single type of granular zircon and does not explain the formation of other types of granular zircon textures, for example, grains with randomly oriented granules or with large, often euhedral granules. Here we report the first nano-scale observations of granular neoblastic zircon and the surrounding environment. We conducted combined microstructural analyses of zircon in the lithic clast from an impact melt dike of the Vredefort impact structure. Zircon granules have either random or systematic orientation with three mutually orthogonal directions of their c-axes coincident with [110] axes. Each 1-2 μm zircon granule is a mosaic crystal composed of nanocrystalline subunits. Granules contain round inclusions of baddeleyite (monoclinic ZrO2) and amorphous silica melt. Tetragonal and cubic ZrO2 also occur as sub-μm-sized inclusions (<50 nm). Filament-like aggregates of nanocrystalline zircon are present as “floating” in the surrounding silicate matrix. They are aligned with each other, apparently serving as the building blocks for the mosaic zircon crystals (granules). Our results indicate shock-related complete melting of zircon with the formation of immiscible silicate and oxide melts. The melts reacted and crystallized rapidly as zircon granules, some of which experienced growth alignment/twinning and parallel growth, causing the characteristic systematic orientation of the granules observed for some of the aggregates. In contrast to the existing model, in which this type of granular zircon is considered to be a product of reversion from the high-pressure polymorph reidite, our nano-scale observations suggest a formation mechanism that does not require phase transition via reidite but is indicative of instant incongruent decomposition, melting and rapid crystallization from the melt.

¹d’Ischia, M.,¹Manini, P.,²Martins, Z.,³Remusat, L.,⁴O’D. Alexander, C.M.,⁵Puzzarini, C.,⁶Barone, V.,⁷Saladino, R.
Physics of Life Reviews 37, 65-93 Link to Article [DOI: 10.1016/j.plrev.2021.03.002]
¹Department of Chemical Sciences, University of Naples Federico II, Via Cintia 4, Naples, 80126, Italy
²Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, Lisboa, 1049-001, Portugal
³Institut de minéralogie, de physique des matériaux et de cosmochimie, UMR CNRS 7590, Sorbonne Université, Muséum National d’Histoire Naturelle, 61 rue Buffon, Paris, 75005, France
⁴Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, NW Washington, DC 20015-1305, United States
⁵Department of Chemistry “Giacomo Ciamician”, University of Bologna, Via F. Selmi 2, Bologna, I-40126, Italy
⁶Scuola Normale Superiore, Piazza dei Cavalieri 7, Pisa, I-56126, Italy
⁷Biological and Ecological Sciences Department (DEB), University of Tuscia, Via S. Camillo de Lellis, Viterbo, 01100, Italy

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¹Yang Xiu et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114471]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
Copyright Elsevier

We report the discovery of indigenous Mn-oxides in Martian regolith breccias Northwest Africa (NWA) 7034 and 7533. These Mn-oxides occur in Mn-rich regions as nanocrystals mixed with silicates, FeOOH, and possible phosphates. The Mn-rich regions contain up to 34 wt% Mn and typically display large chemical gradients on the scale of 10–20 μm. The Martian origin of Mn-oxides was established by the presence of Mn-rich glass (4.8–5.6 wt% Mn) in the fusion crust that crosscuts a Mn-oxides-bearing monzonite clast and by the absence of Mn-oxides on the environmentally exposed surfaces (exterior and fractures) of the meteorites. Manganese K-edge X-ray absorption spectrum (XAS) of the Mn-rich glass in the fusion crust indicated that this glass included high-valent Mn species. Synchrotron micro-X-ray diffraction of a Mn-rich region in a basalt clast and XAS of Mn-rich regions in three monzonite clasts indicate Mn-oxides in these regions are dominantly hollandite-structured with 67–85 mol% of the total Mn being Mn4+. The fact that Mn-rich regions are present in diverse petrological associations but are absent in the matrix of the breccias indicates that the Mn-oxides formed through surface alteration prior to the final brecciation event that assembled NWA 7034 and 7533. Thus, the age of the Mn-oxides is older than the lithification age (arguably 1.35 Ga) of NWA 7034 and 7533. Together with findings of Mn-rich phases within Noachian and Hesperian sedimentary strata in Endeavor and Gale craters, our results suggest that Mn-oxides are a common weathering product on Mars surface, suggesting aqueous environment on the Martian surface with high redox potential.

^1,2Charity M.Phillips-Lander,¹Jamie L.Miller,¹Megan Elwood Madden
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114478]
¹School of Geology and Geophysics, University of Oklahoma, Norman, OK, United States of America
²Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, United States of America
Copyright Elsevier

Plagioclase minerals have been detected over large portions of the Mars surface. Average plagioclase mineral compositions are approximately An60 for dust free areas on Mars; however, these compositions likely reflect a mixture of more and less sodic plagioclase minerals at the surface. Plagioclase minerals have also been observed in association with chloride and sulfate salts on the Mars surface and in meteorites. Understanding plagioclase dissolution in brines provides insight into post-Noachian weathering on Mars. Batch reactor dissolution experiments were conducted at 298 K to compare albite dissolution rates in water (18 MΩ cm−1ultrapure water (UPW) adjusted to pH 2 with H2SO4; activity of water (ɑH2O) = 1.0), 2.7 mol kg−1 MgSO4 (aH2O = 0.92), 1.24 mol kg−1 MgCl2 (aH2O = 0.92), 2.9 mol kg−1 MgCl2 (aH2O = 0.75), and 5.8 mol kg−1 MgCl2 (aH2O = 0.33) brines at pH 2 to determine how changing solution chemistry and activity of water influence albite dissolution. Aqueous Si-based dissolution rates indicate albite dissolution rates decreased from −8.80 ± 0.02 to −9.49 ± 0.40 log mol m−2 s−1 as the activity of water decreased from 1 to 0.33. Na-based dissolution rates followed the same trend, decreasing from −8.58 ± 0.04 in UPW to −9.45 ± 0.15 log mol m−2 s−1 in 2.9 mol kg−1 MgCl2. Anion chemistry did not appear to effect albite dissolution in high salinity brines at acid pH. Estimated 1 mm albite grain lifetimes increase from ~60–100 to ~587 of years as activity of water decreases, suggesting that post-Noachian weathering on Mars was limited in duration and/or extent.

¹D.L.Laczniak,¹M.S.Thompson,²R.Christoffersen,³C.A.Dukes,²S.J.Clemett,⁴R.V.Morris,⁴L.P.Keller
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114479]
¹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, United States of America
²Jacobs, NASA Johnson Space Center, Mail Code X13, Houston, TX 77058, United States of America
³Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, VA 22904, United States of America
⁴ARES, Mail Code X13, NASA Johnson Space Center, Houston, TX 77058, United States of America
Copyright Elsevier

We performed H+ and He+ ion irradiation experiments on slabs of the Murchison CM2 meteorite to simulate solar wind irradiation of carbonaceous asteroids. Two separate 6 mm × 6 mm regions were irradiated with 1 keV H+ and 4 keV He+, respectively, to fluences of 8.1 × 1017 ions/cm2 for H+ and 1 × 1018 ions/cm2 for He+. Unirradiated and irradiated surfaces were analyzed using X-ray photoelectron spectroscopy (XPS), visible to near infrared spectroscopy (VNIR; 0.35–2.5 μm), and microprobe two-step laser-desorption mass spectrometry (μL2MS). We also performed analytical field-emission scanning transmission electron microscopy (FE-STEM) of focused ion beam (FIB) cross-sections extracted from olivine grains and matrix material within the H+- and He+-irradiated regions. In situ XPS analyses suggest that ion irradiation results in the removal of most surface carbon and the partial reduction of surface iron to lower oxidation states. In response to He+-irradiation, we observed reddening and brightening of reflectance spectra, which is a departure from typical lunar-style space weathering. Additionally, H+- and He+-irradiation have opposing effects on organic carbon content: H+-irradiation increases the abundance of some free organic species by breaking down macromolecular material while He+-irradiation causes a decrease in overall organic content by cleaving bonds and sputtering constituent atoms. This suggests that solar wind H+-irradiation and solar wind He+-irradiation change the organic functional group chemistry of asteroidal regolith in different ways. In contrast to some previous experimental space weathering studies, we observe an increase in H2O and OH− abundances in our sample in response to both types of ion irradiation. FE-STEM and energy dispersive X-ray spectroscopy (EDX) analyses show complete amorphization of matrix phyllosilicates in ion-affected rims, partial amorphization of olivine, and changes in Si and Mg concentrations at and/or near the surface. We discuss the implications of these results for understanding the complex nature of space weathering of primitive, carbon-rich asteroids and for analyzing future returned samples from carbonaceous asteroids Bennu and Ryugu.

¹Knut Metzler et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.04.007]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
Copyright Elsevier

A coordinated study of the petrology, mineral chemistry, and bulk chemical and isotopic composition of the five ungrouped carbonaceous chondrites Coolidge, Loongana 001, Los Vientos (LoV) 051, Northwest Africa (NWA) 033, and NWA 13400 reveals that these meteorites have a similar set of properties that distinguishes them from the other carbonaceous chondrite groups and allows definition of the new Loongana (CL) group of carbonaceous chondrites. The basic characteristics of the investigated samples are: (1) Lithophile element ratios (e.g., Al/Mg, Si/Mg) are within the typical range of other carbonaceous chondrite groups. (2) Fe-Ni metal abundances are considerably higher than for CV, but similar to CR chondrites. (3) Chondrule size-frequency distributions are similar to CV, but dissimilar to CR chondrites. (4) The mean CAI abundance is ∼1.4 vol%, i.e., lower than in CV but much higher than in CR chondrites. (5) Very low amounts of matrix (17-21 vol%), the lowest among the main carbonaceous chondrite groups (CI, CM, CO, CV, CR, CK). (6) Olivine is nearly equilibrated, with mean fayalite (Fa) values between 12.5 mol% (Loongana 001) and 14.7 mol% (NWA 13400) as a metamorphic effect. (7) Lower Al2O3 and higher MgO and Cr2O3 concentrations in matrix, compared to matrix in CV, CK, and CR chondrites. (8) Volatile elements (Mn, Na, K, Rb, Cs, Zn, Se, Te, Pb, Tl) are considerably depleted compared to all other main carbonaceous chondrite groups, reflecting the low matrix abundance. (9) Bulk O isotope compositions plot along the CCAM line (Δ17O -3.96 to -5.47‰), partly overlapping with the CV and CK chondrite field but including samples that are more 16O-rich. (10) Unique positions of CL values in the є54Cr-є50Ti isotope plot, with є54Cr values similar to CV, CK, and CO, but є50Ti values similar to CR chondrites. All CL chondrites studied here are of petrologic type 3.9 to 4, indicating that they have been thermally metamorphosed on the parent body. The diagnostic features of CL chondrites detailed here provide a basis for identifying CL members of lower petrologic types. Such samples will be important for determining the pristine state of these meteorites and their components.