^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.