On the principle building blocks of Mars and Earth

1Christian Liebske, 2Amir Khan 
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.01.014]
1Institute of Geochemistry and Petrology, ETH Zürich, Switzerland
2Institute of Geophysics, ETH Zürich, Switzerland
Copyright Elsevier

The terrestrial planets are believed to have formed from primitive material sampling a broad region of the inner solar system. Several meteoritic mixing models attempting to reconcile isotopic characteristics of Mars and Earth have recently been proposed, but, because of the inherent non-uniqueness of these solutions, additional independent observations are required to resolve the question of the primary building blocks of the terrestrial planets. Here, we consider existing isotopic measurements of <span id="MathJax-Element-1-Frame" class="MathJax_SVG" role="presentation" data-mathml="Δ′17″>Δ′17O, ϵ48Ca, ϵ50Ti, ϵ54Cr, ϵ62Ni, and ϵ84Sr for primitive chondrites and differentiated achondrites and mix these stochastically to reproduce the isotopic signatures of Mars and Earth. For both planets we observe ∼ 105 unique mixing solutions out of 108 random meteoritic mixtures, which are categorised into distinct clusters of mixtures using principle component analysis. The large number of solutions implies that isotopic data alone are insufficient to resolve the building blocks of the terrestrial planets. To further discriminate between isotopically valid mixtures, each mixture is converted into a core and mantle component via mass balance for which geophysical properties are computed and compared to observations. For Mars, the geophysical parameters include mean density, mean moment of inertia, and tidal response, whereas for Earth upper mantle Mg/(Mg+Fe) ratio and core size are employed. The results show that Mars requires an oxidised, FeO-rich differentiated object next to chondritic material as main building blocks. In contrast, Earth’s origin remains enigmatic. From a redox perspective, it appears inescapable that enstatite chondrite-like matter constitutes a dominant proportion of the building blocks from which Earth is made. The apparent need for compositionally distinct building blocks for Mars and Earth suggests that dissimilar planetesimal reservoirs were maintained in the inner Solar System during accretion.

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