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Planetary diversity: the experimental terapascal perspective

Periodic Reporting for period 3 - PLANETDIVE (Planetary diversity: the experimental terapascal perspective)

Reporting period: 2019-01-01 to 2020-06-30

The discovery of extra-solar planets orbiting other stars has been one of the major breakthroughs in astronomy of the past decades. Exoplanets are common objects in the universe and planetary systems seem to be more diverse than originally predicted. The use of radius-mass relationships has been generalized as a means for understanding exoplanets compositions, in combination with equations of state of main planetary components extrapolated to TeraPascal (TPa) pressures.

In the most current description, Earth-like planets are assumed to be fully differentiated and made of a metallic core surrounded by a silicate mantle, and possibly volatile elements at their surfaces in supercritical, liquid or gaseous states. This model is currently used to infer mass-radius relationship for planets up to 100 Earth masses but rests on poorly known equations of states for iron alloys and silicates, as well as even less known melting properties at TPa pressures.

This proposal thus aims at providing experimental references for equations of state and melting properties up to TPa pressure range, with the combined use of well-calibrated static experiments (laser-heated diamond-anvil cells) and laser-compression experiments capable of developing several Mbar pressures at high temperature, coupled with synchrotron or XFEL X-ray sources. I propose to establish benchmarking values for the equations of states, phase diagrams and melting curves relations at unprecedented P-T conditions. The proposed experiments will be focused on simple silicates, oxides and carbides (SiO2, MgSiO3, MgO, SiC), iron alloys (Fe-S, Fe-Si, Fe-O, Fe-C) and more complex metals (Fe,Si,O,S) and silicates (Mg,Fe)SiO3. In this proposal, we will address key questions concerning planets with 1-5 Earth masses as well as fundamental questions about the existence of heavy rocky cores in giant planets.
"A very intense effort has been made from the beginning of the program to build a target (and sample preparation LAB) for dynamic compression studies as well as to provide samples for diamond-anvil cells experiments. An important part of ERC's budget was thus devoted to the acquisition of equipment allowing the production of experimental units, which are referred to as ""targets"" for high-pressure experiments, each target being unique for a series of experiments requiring high flexibility of manufacturing equipment. The fabrication LAB is now operational and has several equipements such as a PVD chamber for metal and metal oxides deposition, a e-gun evaporation chamber, a plastic deposition unit, and several control and preparation tools such as a profilometer, ionic polishing equipment, microscope and UV gluing unit. The manufacture of our own targets started in April 2017. Each target has a unique architecture, whether it is the chemical nature of the layers that compose it, their thicknesses or the order in which the stacking is performed, as well as the nature of the supports. All of these variables mean that extensive development work is required before each production run, the iMPMC lab has then all equipment allowing a proper characterization of these samples/targets (SEM, FIB, TEM, x-ray diffraction and spectroscopy platforms). An article has been published about the fabrication of targets, inlcuding our own focus on specific targets for dynamic compression (Prencipe, I. et al. Targets for high repetition rate laser facilities: Needs, challenges and perspectives. High Power Laser Science and Engineering, 5. doi:10.1017/hpl.2017.18 2017).

High-pressure phase diagrams of Iron and Iron alloys (Fe-Si, Fe-O and Fe-C) have been studied up to 500 GPa pressures by the use of laser shock compression. For that purpose, we conducted X-ray diffraction at FEL facilities (LCLS and SACLA) as well as X-ray absorption spectroscopy at ESRF. These experiments are being exploited rigth now and several articles are in preparation. We have also performed high pressure and temperature experiments on SiC samples, combining laser heated diamond anvil cell and synchrotron X-ray diffraction. The obtained set of data provides information on the P T region between 30 – 205 GPa and 300 - 3500 K. The results show evidences of coexistence of SiC with Si or C, depending on the starting composition, without the appearance of intermediate compounds. Moreover, between 65 and 80 GPa, SiC undergoes a phase transition with the zinc blend structure (B3), typical of ambient conditions, replaced by the rock salt structure (B1). This phase transition corresponds to a change in the coordination of the atoms, and is accompanied by a 10% volume reduction. This work resulted in a publication in the Journal of Geophysical Research this year (Miozzi et al., J. Geophys. Res. Planets, 123. https://doi.org/10.1029/ 2018JE005582).

Other static compression experiments were performed at the ESRF (beamline ID27 for XRD experiments and beamline ID24 for XAS experiments). We determined melting temperature, the chemical composition of the eutectic point and the liquid density up to 150 GPa for Fe-C and Fe-O binary systems. We also determined the triple point fcc-hcp-liquid for pure Fe using X-ray Absorption Spectroscopy as a structural change diagnostic. Two articles have been published on this topic (Morard et al., Geophys. Res. Lett. 45. https:// doi.org/10.1029/2018GL079950; Morard et al., Earth and Planetary Science Letters 473. Elsevier B.V.: 94–103. doi:10.1016/j.epsl.2017.05.024).

Finally, even if it was not anticipated at the beginning of the program, we had the opportunity to work on first experimental evidence of ferrous iron in newly discovered pyrite-structured hydrogen-bearing phase FeO2Hx at conditions corresponding to deep planetary interiors (Boulard et al., 2018). Using in situ x-ray absorption spectroscopy at extreme pressure and temperature, we show direct experimental evidence of ferrous iron in FeO2Hx (Boulard et al., Geophys. Res. Lett., under review). Observation of ferrous iron in a phase stoichiometrically richer in oxygen than hematite, demonstrates that the general concept of an univocal relation between the oxygen fugacity and the oxidation state of iron at high pressure and high temperature has to be revisited. In addition, our results support a new view of the hydrogen cycle in the deep Earth as they suggest the possibility to have zero-valent hydrogen (i.e. H2) as a stable state of hydrogen at in the lower mantle. Hydrogen released in the deep Earth upon the phase transition may influence global geodynamic cycle of volatile elements and compounds such as carbon in the deep Earth."
The work on SiC and Fe-Si-C system is progressing rapidly. We had so far very few information on this system at extreme conditions. Our work is among the first of its kind addressing such questions, with a large pressure range explored, allowing us to derive models for planets up to 5 times the Earth's mass.
Our XANES spectroscopy study on FeO2Hx compounds at high pressure and temperature is also unique. I believe it will trigger a large interest in Earth siences.
We are now capable of manufacturing targets and samples for dynamic compression studies that will develop in the next years. We hope to be a group of reference with this target manufacturing for the study of planetary interiors in the future.
Several cooperation agreements are being discussed at the moment to access large laser facility such as Omega in the US and LMJ in France.
We hired two additional PhDs in charge of the study of post post-perovskite systems and preparation of targets and characterization (B. truffet), and the study of phase transitions in iron oxides using XANES spectroscopy during dynamic compression (A. Amouretti). With the post-doc of Marzena Baron, the task force is now complete to address questions about phase transitions beyond post-perovskite structures in large planets, melting of oxides systems in the 1-2 Mbar range, and the development of XANES spectroscopy coupled to dynamic compression focused on iron oxides. The one year work of G. Mogni also yielded a very efficient theroretical approach to identify structures at extreme conditions in FeSi binary system. We are working so as to include this original theoretical approach into a distributed sofware suite.
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