Master of Science
Planetary Science and Exploration
Secco, Richard A.
Investigation of energy sources in an early dynamo in Vesta has been carried out using high pressure-temperature experimental studies. Electrical resistivity of Fe‐10wt%Ni was measured at 2-5 GPa up to 2082 K in the liquid state and compared to previous results of pure Fe and pure Ni. Thermal conductivity was calculated from electrical resistivity to determine adiabatic core heat flux. The results are applied to determine whether thermal convection could be responsible for the putative dynamo in early Vesta’s core. An adiabatic core heat flux of ~300 MW at the top of Vesta’s core is estimated from this study and compared to a range of estimates of heat flux through the CMB of 1.5-78 GW. It is concluded that thermal convection would have occurred, playing an important role as an energy source of dynamo action that generated a magnetic field for tens of millions of years in Vesta’s early history.
Summary for Lay Audience
The electrical resistivity of the metal core of a small, Earth-like asteroid body was experimentally studied using a sample with matching conditions of pressure, temperature, and composition. The electrical resistivity of the top of a body’s core can tell us its thermal conductivity and therefore the rate at which heat passes via conduction through the outermost portion of the core. If the conducted heat is less than the total heat flowing out of the core into the mantle, then the missing heat is due to thermal convection of the liquid core. Convection of a liquid iron alloy has the potential to generate the magnetic fields observed on Earth and similar bodies, although heat transfer is not the only cause for convection.
In the experiments, the electrical resistivity of a small sample of an alloy of 90% iron and 10% nickel by weight (Fe10Ni) was measured at high temperatures (a few hundred Kelvin above melting) and high pressures. A 1000-ton cubic anvil press compressed a small sample wire inside a larger (~3.2 cm) cubic pressure cell to a target pressure in the range 2 to 5 GPa. Wires contacting each side of the Fe10Ni wire formed both thermocouples and electrodes on each end. The thermocouples measured temperature, while the electrodes completed a circuit, allowing voltage across the sample to be measured. These measurements gave the resistance of the sample, the electrical resistivity, and, by simple calculation, the thermal conductivity of Fe10Ni at specific temperatures and pressures.
The results are applied to the second-largest asteroid, Vesta. This asteroid, like Earth, is differentiated into a core, mantle, and crust. Meteorites known to have originated on Vesta show that early Vesta had a magnetic field generated internally. Thermal conductivity estimates of the alloy Fe10Ni from this study allow an estimate the amount of heat flow, 2 mW/m2, conducted through the outermost region of Vesta’s core early in its history. Because these results are lower than existing estimates of total heat flow from Vesta’s core to its mantle, Vesta’s core likely experienced thermal convection early in its history, which could have generated its magnetic field.
Lenhart, Eric M., "Investigation of the Energy Source for an Early Dynamo in Vesta from Experiments on Electrical Resistivity of Liquid Fe-10wt%Ni at High Pressures" (2021). Electronic Thesis and Dissertation Repository. 8101.