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Pommier, A.  2018.  Influence of sulfur on the electrical resistivity of a crystallizing core in small terrestrial bodies. Earth and Planetary Science Letters. 496:37-46.   10.1016/j.epsl.2018.05.032   AbstractWebsite

Electrical experiments were performed on core analogues in the Fe-S system and on FeSi2 up to 8 GPa and 1850 degrees C in the multi-anvil apparatus. Electrical resistivity was measured using the four electrode method. For all samples, resistivity increases with increasing temperature. The higher the S content, the higher the resistivity and the resistivity increase upon melting. At 4.5 GPa, liquid FeS is up to >10 times more resistive than Fe-5 wt.% S and twice more resistive than FeSi2, suggesting a stronger influence of S than Si on liquid resistivity. Electrical results are used to develop crystallization resistivity paths considering both equilibrium and fractional crystallization in the Fe-S system. At 4.5 GPa, equilibrium crystallization, as expected locally in thin snow zones during top-down core crystallization, presents electrical resistivity variations from about 300 to 190 microhm-cm for a core analogue made of Fe-5 wt.%S, depending on temperature. Fractional crystallization, which is relevant to core-scale cooling, leads to more important electrical resistivity variations, depending on S distribution across the core, temperature, and pressure. Estimates of the lower bound of thermal resistivity are calculated using the Wiedemann-Franz law. Comparison with previous works indicates that the thermal conductivity of a metallic core in small terrestrial bodies is more sensitive to the abundance of alloying agents than that of the Earth's core. Application to Ganymede using core adiabat estimates from previous studies suggests important thermal resistivity variations with depth during cooling, with a lower bound value at the top of the core that can be as low as 3 Wim K. It is speculated that the generation and sustainability of a magnetic field in small terrestrial bodies might be favored in light element-depleted cores. (C) 2018 Elsevier B.V. All rights reserved.

Pommier, A.  2014.  Interpretation of magnetotelluric results using laboratory measurements. Surveys in Geophysics. 35:41-84.   10.1007/S10712-013-9226-2   AbstractWebsite

Magnetotelluric (MT) surveying is a remote sensing technique of the crust and mantle based on electrical conductivity that provides constraints to our knowledge of the structure and composition of the Earth's interior. This paper presents a review of electrical measurements in the laboratory applied to the understanding of MT profiles. In particular, the purpose of such a review is to make the laboratory technique accessible to geophysicists by pointing out the main caveats regarding a careful use of laboratory data to interpret electromagnetic profiles. First, this paper addresses the main issues of cross-spatial-scale comparisons. For brevity, these issues are restricted to reproducing in the laboratory the texture, structure of the sample as well as conditions prevailing in the Earth's interior (pressure, temperature, redox conditions, time). Second, some critical scientific questions that have motivated laboratory-based interpretation of electromagnetic profiles are presented. This section will focus on the characterization of the presence and distribution of hydrogen in the Earth's crust and mantle, the investigation of electrical anisotropy in the asthenosphere and the interpretation of highly conductive field anomalies. In a last section, the current and future challenges to improve quantitative interpretation of MT profiles are discussed. These challenges correspond to technical improvements in the laboratory and the field as well as the integration of other disciplines, such as petrology, rheology and seismology.

Davies, CJ, Pommier A.  2018.  Iron snow in the Martian core? Earth and Planetary Science Letters. 481:189-200.   10.1016/j.epsl.2017.10.026   AbstractWebsite

The decline of Mars' global magnetic field some 3.8-4.1 billion years ago is thought to reflect the demise of the dynamo that operated in its liquid core. The dynamo was probably powered by planetary cooling and so its termination is intimately tied to the thermochemical evolution and present-day physical state of the Martian core. Bottom-up growth of a solid inner core, the crystallization regime for Earth's core, has been found to produce a long-lived dynamo leading to the suggestion that the Martian core remains entirely liquid to this day. Motivated by the experimentally-determined increase in the Fe-S liquidus temperature with decreasing pressure at Martian core conditions, we investigate whether Mars' core could crystallize from the top down. We focus on the "iron snow" regime, where newly-formed solid consists of pure Fe and is therefore heavier than the liquid. We derive global energy and entropy equations that describe the long-timescale thermal and magnetic history of the core from a general theory for two-phase, two-component liquid mixtures, assuming that the snow zone is in phase equilibrium and that all solid falls out of the layer and remelts at each timestep. Formation of snow zones occurs for a wide range of interior and thermal properties and depends critically on the initial sulfur concentration, to. Release of gravitational energy and latent heat during growth of the snow zone do not generate sufficient entropy to restart the dynamo unless the snow zone occupies at least 400 km of the core. Snow zones can be 1.5-2 Gyrs old, though thermal stratification of the uppermost core, not included in our model, likely delays onset. Models that match the available magnetic and geodetic constraints have to xi(0) approximate to 10% and snow zones that occupy approximately the top 100 km of the present-day Martian core. (C) 2017 The Authors. Published by Elsevier B.V.