G. Morard

The influence of sulfur on the electrical resistivity of hcp iron: Implications for the core conductivity of Mars and Earth

Cosmochemical and geochemical studies suggest sulfur (S) as a light alloying element in the iron-rich cores of telluric planets, but there is no report of sulfurs alloying effect on the electrical and thermal transport properties of iron (Fe); a subject that is closely related to the dynamo action and thermal evolution of planetary cores. We measured the electrical resistivity of hexagonal-closed-packed (hcp) structured Fe alloy containing 3 wt.% silicon (Si) and 3 wt.% S up to 110 GPa at 300 K. Combined with the reported resistivities of hcp Fe and hcp Fe-Si alloy, we determined the impurity resistivity of S in a hcp Fe matrix at high pressures. The obtained impurity resistivity of S is found to be smaller than that of Si. Therefore, S is a weaker influence on the conductivity of Fe alloy, even if S is a major light element in the planetary cores.

Dynamic X-ray diffraction observation of shocked solid iron up to 170 GPa

Significance Iron is the main constituent of the core of rocky planets; therefore, understanding its phase diagram under extreme conditions is fundamental to model the planets’ evolution. Using dynamic compression by laser-driven shocks, pressure and temperature conditions close to what is found in these cores can be reached. However, it remains unclear whether phase boundaries determined at nanosecond timescales agree with static compression. Here we observed the presence of solid hexagonal close-packed iron at 170 GPa and 4,150 K, in a part of the iron phase diagram, where either a different solid structure or liquid iron has been proposed. This X-ray diffraction experiment confirms that laser compression is suitable for studying iron at conditions of deep planetary interiors difficult to achieve with static compression techniques. Investigation of the iron phase diagram under high pressure and temperature is crucial for the determination of the composition of the cores of rocky planets and for better understanding the generation of planetary magnetic fields. Here we present X-ray diffraction results from laser-driven shock-compressed single-crystal and polycrystalline iron, indicating the presence of solid hexagonal close-packed iron up to pressure of at least 170 GPa along the principal Hugoniot, corresponding to a temperature of 4,150 K. This is confirmed by the agreement between the pressure obtained from the measurement of the iron volume in the sample and the inferred shock strength from velocimetry deductions. Results presented in this study are of the first importance regarding pure Fe phase diagram probed under dynamic compression and can be applied to study conditions that are relevant to Earth and super-Earth cores.

Laser-driven quasi-isentropic compression experiments and numerical studies of the iron alpha-epsilon transition in the context of planetology

The iron alpha-epsilon transition is one of the most studied solid-solid phase transition. However, for quasi-isentropic compression, the dynamic and the influences of this transition on the high-pressure states of iron are still unknown. We present experimental results and numerical simulations to study these effects. Experiments performed at LULI2000 and the Janus laser facility (LNLL), using two different ramp shapes and different compression rates allowed to study the dynamic of the alpha-epsilon transition. We have observed the transition at particle velocity ranging from 0.25 km/s to 0.52 km/s depending on the compression rate. Depending on the ramp, either a shock formation was observed (high compression rate) at the transition or a flat plateau whose duration is function of compression rate. Increasing the compression rate leads to a smaller plateau duration. These results are important for reproducing Earth and Super-earth core conditions (2-15Mbar, 5- 15000K) on laboratory where the quasi-isentropic compression is the most promising experimental scheme.

Dynamic fragmentation as a possible diagnostic for high pressure melting in laser shock-loaded iron

High pressure melting of iron conditions the understanding of the Earth core constitution. Shock compression has been used for many years to complement the data obtained under quasi‐static loading. Still, shock‐induced melting is not easy to detect. Here, we investigate how dynamic fragmentation of laser shock‐loaded iron can be affected by melting. Iron samples are irradiated by a high power pulsed laser. The motion of the fragments ejected from the free surface is recorded by transverse shadowgraphy and soft recovery of the ejecta is performed in a low density gel. At low laser intensity, spalled layers can be seen in the shadowgraphs and solid fragments of mm‐dimensions are recovered. At higher intensity, wide debris clouds are observed to expand from the free surface, and tiny spherical fragments are recovered in the gel. This evolution is qualitatively consistent with the predictions of one‐dimensional hydrodynamic simulations accounting for laser‐matter interaction and pulse decay during propagation...

Characterization of laser-driven ultrafast shockless compression using gold targets

Indirect laser-driven shockless compression experiments on gold targets were performed to characterize pressure loading processes and target states. Free surface velocities of the gold target under ramped pressure loading were measured using line-imaging velocity interferometers. From the velocity data and the equation of state, the maximum pressure and strain rate attained under compression were estimated to be ∼50u2009GPa and ∼4u2009×u2009107u2009s−1, respectively. Optical reflectivity was measured simultaneously with the velocity, the result suggesting no significant or unexpected temperature increases in the ultrafast shockless compression process.