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Research projects:
The final structure and composition of a planet can be significantly different from its original building blocks. Important modifications happen when planets grow, since new physical conditions re-distribute elements between the deep planetary layers. Despite these modifications, some features remain similar in accreting meteorites and final planets: abundances of stable isotopes that are not affected by planetary differentiation. One family of meteorites (enstatite chondrites) has isotopic compositions that are remarkably similar to Earth and could represent the best candidates as the Earth's building blocks. However, some chemical features remain drastically different than Earth. For example, Earth appears to be low in silicon, potassium and sodium, and enriched in magnesium, calcium and aluminum, compared with enstatite chondrites. Using lab experiments and simulating the effect of a crust formation in early growing planetary embryos, we found that stripping these early crusts through impacts could explain these differences. As these crusts would be extremely enriched in silica, this process would resolve one of the most important puzzle for the origin of Earth's formation: Earth preserved isotopic signatures of enstatite chondrites, and remaining chemical divergence derived from early impact stripping of differentiated crusts on early planetary embryos (Boujibar et al. 2015). Once Earth reached a larger size, stronger impacts formed deep layers of magma (magma ocean), where fractional crystallization lead to extensive mantle heterogeneities, progressively attenuated over geologic times through mantle stirring (Boujibar et al. 2016 ).
More than for 4000 exoplanets have been discovered beyond our Solar System. Most commonly observed exoplanets are super-Earths, which are larger than Earth, but still containing a silicate mantle and an iron core. To understand their physical and chemical properties, astronomical data on their masses and radii are combined with numerical models and equations of state of planetary materials. Using available experimental data at ultra-high pressure, I updated the internal structure of observed super-Earths, to better constrain their core-mantle ratios. Their thermal state was also constrained with latest experimental data on iron melting temperature. Our results show that large super-Earths are likely to have a solid and a liquid core (Boujibar et al. 2020). Since this configuration enhances convection in the liquid core, this property increases the likelihood to develop a geodynamo, which is an important factor for habitability.
Volatiles like H, S, C and N represent an infinitesimal fraction of the mass of terrestrial planets, though their presence is vital for the development of life. Their ubiquity in terrestrial planets is unclear, and even for Earth, it remains uncertain whether these volatiles were present all along its formation or were added later through comets or asteroids. While their abundances in planetary crusts can be estimated through space missions, meteorites and terrestrial rocks, their concentrations in planetary interiors are difficult to estimate. In the laboratory, we can replicate planetary formation to investigate reactions happening when planets grow, using apparatuses creating high pressures and temperatures. By studying these reactions, we can redraw the processes leading to surface composition. If it is impossible to sample material from the core, we can still replicate its formation in the lab and estimate its composition relative to the mantle and crust. This type of work enabled me determining that the low sulfur budget in Earth’s rocks are a result of both core segregation and sulfur volatilization (Boujibar et al. 2014 ). Alkali elements (Na, K, Cs, Rb) are considered as moderately volatile elements in the context of planetary formation, and prefer concentrating in planetary crusts. High pressure experiments suggest they could still be slightly segregated into planetary cores if iron sulfides were present during core formation (Boujibar et al. 2020).
Mercury is notorious for being an "end-member planet". It is the closest to the Sun, the smallest and has the largest iron core compared to other planets in our Solar System. One specific feature giving Mercury its special makeup is its very low oxygen fugacity (amount of oxygen and effect on chemical elements). NASA spacecraft MESSENGER provided invaluable data on the planet and showed high sulfur abundance on the surface. If Mercury is extensively enriched in sulfur, it could contain sulfides in its interior possibly forming a sulfide layer between the Fe core and silicate mantle. By conducting experiments in the lab at high pressures to simulate Mercury's differentiation, I found that significant fractions of radioactive elements (uranium, thorium and potassium) would be trapped in this possible sulfide layer (Boujibar et al. 2019). Heat generated by the radioactive decay of these elements would have an important role on Mercury's geologic history. One other interesting result is that the segregation of uranium (U), thorium (Th) and potassium (K) is not uniform. U and Th incorporate sulfides at a higher rate than K, which can modify U/K and Th/K ratios of the surface compared to the core, and possible sulfide layer. This fractionation can explain how measurements of these ratios by MESSENGER on the surface were different than original expectations. If a sulfide layer actually exists between core and mantle, ratios on the surface would not be representative of the bulk planet and would be considered as apparent ratios. Corrections for sulfides would lead lead to different implication for Mercury's volatile content compared other planets (Boujibar et al. 2019).
Planetary differentiation is accompanied with various chemical exchanges and physical processes that can drastically modify the structure and physico-chemical state of a planet, including its oxidation state. For instance, during planetary growth, the increase of internal pressures and temperatures of the Earth's core segregation increases the mantle FeO concentration. This effect leads to a higher oxidation state of the bulk planet. Evidence for this redox reaction comes from experimental partitioning of Fe between metal and silicate and its thermodynamic modeling. I also found the same result using an empirical model based on experimental data, which shows that the final FeO/Fe ratio of terrestrial planets increases throughout core segregation (Boujibar et al. 2015). Another process that can increase the oxidation state of a terrestrial planet is the crystallization of the magma ocean. For planets that are large enough to start crystallizing bridgmanite (Mg-perovskite) in its mantle can increase Fe3+ content of the mantle through the coupled substitution of Al and Fe3+ in bridgmanite. Using high pressure experiments, I found that ~20 % of Fe would be present in as Fe3+ during the early Earth's magma ocean crystallization (Boujibar et al. 2016).
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