Doctor of Philosophy
Secco, Richard A.
Experimental investigations of materials at high pressures (P) and temperatures (T) provide insight into the properties and behaviours expected within the inaccessible interiors of planetary bodies. Using a four-wire electrical resistance technique, the electrical resistivity (ρ) of 4d transition metal (Ag) and 3d transition metal alloys (Fe-S) were measured in the solid and molten states at high P. The thermal conductivity (κ) of these materials is inversely proportional to ρ, as described by the Wiedemann-Franz Law. When applied to planetary cores, κ is an important parameter that regulates heat transport mechanisms and magnetic field production.
A hypothesis of ‘resistivity invariance’ suggested that for pure d-band filled metals the magnitude of ρ along the P- and T-dependent melting boundary is constant. This implied that investigations at low P can provide a singular constraint value of ρ and κ at more extreme P and T conditions expected for planetary cores, such as the inner-outer core boundary of Earth which is a solidification boundary. The ρ of silver (Ag) was measured at P up to 5 GPa and T up to ~1650 K. The results showed a decrease in ρ along the P-dependent melting boundary, contrary to prediction, and were discussed in terms of increasing energy separation between the Fermi level and 4d-band as a function of increasing P.
The ρ of solid and molten iron sulfide (FeS) and Fe-FeS were measured at T up to ~1750 K and ~1350 K, respectively, and P up to 5 GPa. These material compositions are relevant to the sulphur (S)-rich core of Ganymede, with the experimental P and T approximating the conditions at the top, or outer-most portion, of the core. The dipolar magnetic field of Ganymede may be generated by an internal dynamo, implying a molten core that may transport heat by thermal convection. The κ and adiabatic conductive heat flow for molten FeS and Fe-FeS core models of Ganymede were calculated from the measured ρ. The results showed that heat transport by thermal convection is permissible in the core models and may act as an energy source to power a dynamo-produced magnetic field.
Summary for Lay Audience
The cores of planetary bodies are inaccessible to direct measurement of their transport properties because of extreme pressure (P) and temperature (T) conditions and kilometers-thick surrounding rock. However, laboratory experiments at high P and T are capable of replicating interior conditions and the results of these investigations can be used to estimate the properties and behaviours of the cores of these bodies. Electrical resistivity (ρ) and thermal conductivity (κ) are important transport properties to estimate for planetary cores because they affect thermal evolution and production of magnetic fields. For metals and alloys, ρ and κ are related by the Wiedemann-Franz Law, where one transport property can be calculated if the other property is known.
It was hypothesized that ρ of pure metals will have the same value at the melting T at any P. This implied that laboratory measurements at low P and high T could be used to indirectly determine ρ at significantly higher P and T conditions expected for large planetary cores, such as Earth. The ρ of silver (Ag) was measured at P up to 5 GPa and T up to ~1650 K. The results showed a decrease in ρ along the melting boundary, contrary to prediction, and were discussed in terms of effects on electron energy states with increasing P.
Ganymede, a moon of Jupiter, is known to have a magnetic field and is expected to have a core made predominantly of iron (Fe) with some sulphur (S). The ρ of solid and molten iron sulfide (FeS) and Fe-FeS were measured at T up to ~1750 K and ~1350 K, respectively, and P up to 5 GPa. These experimental P and T approximate the conditions at the top, or outer-most portion, of the core. The κ and adiabatic conductive heat flow for molten FeS and Fe-FeS core models of Ganymede were calculated from the measured ρ. The results showed that a molten core could transport heat by thermal convection. If the molten core is thermally convecting, this may act as an energy source to power and generate the magnetic field of Ganymede.
Littleton, Joshua A.H., "Heat Flow in the Core of Ganymede: High Pressure-Temperature Electrical Resistivity Measurements of Solid and Liquid Ag and Fe-S Alloys" (2021). Electronic Thesis and Dissertation Repository. 8030.
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