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Acoustic and Electrical Properties of Fe-Ti Oxides with Application to the Deep Lunar Mantle
Acoustic and Electrical Properties of Fe-Ti Oxides with Application to the Deep Lunar Mantle
A. Pommier, M.J. Walter, M. Hao, J. Yang, R. Hrubiak (2024, EPSL).
The overturn of titanium-rich mantle cumulates has been invoked to explain the structure and dynamics of the Moon. These dense cumulates are stable at the core-mantle boundary (CMB) and could explain field observations inferred from geophysical studies. We report acoustic and electrical experiments on natural ilmenite-rutile aggregates up to 4.5 GPa and 1920 K. Seismic velocities show a weak pressure and temperature dependence, with Vs ~4.2 (+/-0.2) km/s and Vp ~ 8.0 (+/-0.2) km/s at the CMB conditions. Conductivity increases by a factor of 104 from 373-1920 K and is >103 S/m above 1573 K. Seismic and electrical models for the lunar mantle based on our results, considering mixtures of Fe-Ti oxides and olivine, indicate that field velocity and conductivity estimates are reproduced satisfactorily with 3-16 vol.% Fe-Ti oxides and 20 vol.% melt. Interactions between a Ti-rich, melt-bearing layer and the adjacent core likely affect the cooling and magnetic history of the Moon.
Support: the Carnegie Endowment.
Back-scattered electron (BSE) images of retrieved sample E2 (electrical experiment). TC=thermocouple wire.
Understanding core cooling processes
Understanding core cooling processes
A. Pommier (2020, American Mineralogist).
The thermal state and chemistry of the metallic core of terrestrial planets and moons governs their evolution and in particular, the intrinsic magnetic field. In Pommier (2020), the effect of nickel on the electrical properties of the core of small terrestrial bodies was investigated. Electrical resistivity experiments up to 8 GPa show that at a specified temperature, Fe-Ni(-S) alloys are more resistive than Fe by a factor of about 3. Fe-Ni alloys containing 5 and 10 wt% Ni present comparable electrical resistivity values. Based on these results, adiabatic heat fluxes were computed for both Ganymede’s core and the Lunar core, and heat flux values suggest a significant dependence to both core composition and the adiabatic temperature. Comparison with previous thermochemical models of the cores of Ganymede and the Moon suggests that some studies may have overestimated the thermal conductivity and hence, the heat flux along the adiabat in these planetary cores.
Phase relationships in Fe, Fe-Ni, and Fe-Ni-S systems and summary of experimental conditions in previous electrical works on Fe-Ni alloys and in this study. Phase diagrams for solid phases are from Huang et al. (1988). Fe-Ni alloys melting point (MP) temperature at 1 atm from Hansen (1958) and Fe melting curve from Ma et al. (2004) and Anzellini et al. (2013). Fe-S and Fe-Ni-S eutectic curves are from Fei et al. (2000); Li et al. (2001); Stewart et al. (2007); Zhang and Fei (2008); Morard et al. (2007). Fe-Ni-S eutectic melting temperature at 20 GPa comes from Zhang and Fei (2008) for a Ni/(Ni+Fe) ratio of 0.09. GH2015 = Gomi and Hirose 2015; H et al. 1983 = Ho et al. (1983). Comparison with the expected pressure and temperature conditions for the core of the Earth’s moon and Ganymede are also shown (green rectangles) (after Breuer et al. 2015).
Exploring Jupiter's Moon Io
Exploring Jupiter's Moon Io
A. Pommier & A. McEwen (Elements, 2022); Hamilton et al. (Planetary Science Journal, 2025).
Jupiter’s moon Io is the most volcanically active world in our Solar System. Eruptions on Io sustain its atmosphere, feed the Jovian magnetosphere, and contaminate neighboring moons. This unique volcanic and tectonic activity is powered by tidal heating, caused by its gravitational interactions with Jupiter and other moons. The silicate crust of Io is coated with sulfur compounds, and its interior—one that is exceptional for an outer-planet moon—is composed of a metallic core and a silicate mantle that may host a magma ocean. Such spectacular large-scale volcanism and high heat flow provide insights into the processes that shaped all terrestrial bodies. Future exploration of Io would answer key questions and herald a new era of discoveries about the evolution of terrestrial planets and moons within our Solar System and beyond.
With its 6 chapters, the Elements issue offers a journey to one of the most captivating worlds in our Solar System. There is a lot we know about Io, and even more that we do not. Several spacecraft flybys and many telescopic observations have revealed Io’s fascinating and intriguing characteristics, and we are now reaching the limit of what existing observations can teach us. These observations have raised fundamental questions about Io’s interior and evolution that only a space mission designed to study Io will be able to answer.
In Hamilton et al. (2025), scientific priorities for Io's exploration that could be addressed through near-term mission opportunities within NASA’s Discovery and New Frontiers (NF) programs are presented. Missions to Io would result in impactful measurements that could transform our scientific knowledge of tidal heating processes.
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