Thermal Convection Can Power the Geodynamo
New high-pressure experiments on fluid iron suggest thermal convection without compositional buoyancy is sufficient to drive the dynamo generating Earth's magnetic field.
SOURCE: Geophysical Research Letters
A graph showing total conductive heat flow in the core as a function of radius for pure iron (black line) and compositional models containing Fe-Ni-S (blue line) and Fe-Ni-Si (red line). Total conductive heat flow in the core as a function of radius for pure iron (black line) and compositional models containing Fe-Ni-S (blue line) and Fe-Ni-Si (red line). The inner core boundary (ICB) is shown by the vertical dashed line and values for the heat flow at the core-mantle boundary are represented along the right side of the diagram (radius, 3400 km). The grey shaded area shows the range of values from geophysical estimates (5-15 TW), with most recent values given by the grey and dash-shaded area (10-15 TW). The new values of 8-9 TW support thermal convection over compositional buoyancy as the primary source of energy powering the geodyamo. Credit: Yong et al. [2019], Figure 3c
The Earth's magnetic field shields humanity from the ionizing radiation of solar wind, and it also remains critical to modern navigation systems. The field is produced in the core by a dynamo that requires the fluid motion of liquid iron alloy and electrical conductivity.
However, the driving forces behind the geodynamo have been debated, since estimates for the total heat flow across the core-mantle boundary (CMB) range from 5-15 TW. Most recent estimates are on the high end of this range (10-15 TW), which call into question the timing of core formation and the relative importance of thermal versus chemically driven convection in the outer core.
Experimental studies of the properties of liquid iron at high pressures and temperatures remain extremely challenging. Yong et al. [2019] measured the electrical resistivity of liquid and solid iron up to the highest pressures yet (24 GPa) for a large-volume apparatus, and they found that resistivity is constant with pressure along the melting boundary. These results were used to estimate a total heat flow of 8-9 TW across the CMB.
The results imply that outer-core convection can be thermally driven even before the solid inner core formed, which is consistent with paleomagnetic data that suggest the geomagnetic field existed as early as 4.2 billion years ago.
Citation: Yong, W., Secco, R. A., Littleton, J. A. H., & Silber, R. E. [2019]. The iron invariance: Implications for thermal convection in Earth's core. Geophysical Research Letters, 46. https://doi.org/10.1029/2019GL084485
—Steven D. Jacobsen, Editor, Geophysical Research Letters
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