Dynamos in the Inner Solar System

Dynamo magnetic fields are primarily generated by thermochemical convection of electrically conductive liquid metal within planetary cores. Convection can be sustained by secular cooling and may be bolstered by compositional buoyancy associated with core solidification. Additionally, mechanical stirring of core fluids and external perturbations by large impact events, tidal effects, and orbital precession can also contribute to sustaining dynamo fields. Convective dynamos cease when the core-mantle heat flux becomes subadiabatic or if specific crystallization regimes inhibit core fluid flows. Therefore, exploring the histories of magnetic fields across the Solar System provides a window into the thermal and chemical evolution of planetary interiors. Here we review how recent spacecraft-based studies of remanent crustal magnetism, paleomagnetic studies of rock samples, and planetary interior models have revealed the magnetic and evolutionary histories of Mercury, Earth, Mars, the Moon, and several planetesimals, as well as discuss avenues for future exploration and discovery. ▪ Paleomagnetism and remanent crustal magnetism studies elucidate the magnetic histories of rocky planetary bodies. ▪ Records of ancient dynamo fields have been obtained from 3 out of 4 terrestrial planets, the Moon, and several planetesimals. ▪ The geometries, intensities, and longevities of dynamo fields can provide information on core processes and planetary thermal evolution. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 50 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Electronic transition in liquid Fe at P, T conditions: Implication for double-diffusive convection in Mercury’s dynamo.

A double-diffusive model demonstrated that Mercury's dynamo is driven by thermochemical convection with thermal energy source deep in its core. Although radioactive elements concentrated in the core has been suggested as the source of the energy, arguments were made that there is no geochemical or geophysical validation for this. We report the temperature-dependent resistivity of solid and liquid Fe measured up to 21 GPa in multi anvil. With increasing pressure, we observe resistivity transition with a value change of about 35 µΩ-cm at the melting boundary on the liquid side at ~18 GPa. This change in resistivity corresponds to a change in thermal conductivity of about 30 Wm-1K-1 that would generate ~0.94 TW change in heat flux. This indicates that transition in Fe properties is responsible for thermal buoyancy in Mercury's dynamo and not radioactive materials. From the entropy balance, we estimate a thermal dynamo power of about 0.1 TW.

Numerical simulation of asthenospheric flow around cratonic keels

<p>In this work, we studied the mantle flow around cratonic keels using numerical models to simulate the thermochemical convection in the terrestrial mantle taking into account the relative displacement between the lithosphere and asthenosphere. The numerical simulations were performed using the finite element code developed by Sacek (2017) to solve the Stokes Flow for an incompressible Newtonian fluid. Several synthetic models in 2D and 3D were constructed considering different keel geometries and different regimes of relative displacement between the lithosphere and asthenosphere. In the present numerical experiments, we adopted a rheology in which the viscosity of the mantle is controlled by temperature, pressure and composition, assuming that the cratonic keel is compositionally more viscous than the surrounding asthenosphere, using a factor f to rescale the lithospheric viscosity compared to the asthenospheric one. We tested different f values, reference viscosity for the asthenosphere, and relative velocity between the lithosphere and the base of the upper mantle, quantifying the amount of deformation of the cratonic keel in each scenario. In general, we conclude that for a relatively low compositional factor (f < 20), the lithospheric keel can be significantly deformed in a time interval of few tens of million years when the lithosphere is moving horizontally relative to the base of the upper mantle, does not preserving its initial geometry. The synthetic models can be helpful for a better understanding of the interaction in the lithosphere-asthenosphere interface such as the deformation and flow patterns in the mantle around the keels, the rate of erosion of the root of the continental lithosphere due to the convection in the upper mantle and how it affects the thermal flow to the surface.</p><p>Sacek, V. (2017). Post-rift influence of small-scale convection on the landscape evolution at divergent continental margins. Earth and Planetary Science Letters, 459, 48-57.</p>

The history of the core dynamos of Mars and the Moon inferred from their crustal magnetization: a brief review

The core dynamos of Mars and the Moon have distinctly different histories. Mars had no core dynamo at the end of accretion. It took ∼100 Myr for the core to create a strong dynamo that magnetized the martian crust. Giant impacts during 4.2–4.0 Ga crippled the core dynamo intermittently until a thick stagnant lithosphere developed on the surface and reduced the heat flux at the core–mantle boundary, killing the dynamo at ∼3.8 Ga. On the other hand, the Moon had a strong core dynamo at the end of accretion that lasted ∼100 Myr and magnetized its primordial crust. Either precession of the core or thermochemical convection in the mantle or chemical convection in the core created a strong core dynamo that magnetized the sources of the isolated magnetic anomalies in later times. Mars and the Moon indicate dynamo reversals and true polar wander. The polar wander of the Moon is easier to explain compared to that of Mars. It was initiated by the mass deficiency at South Pole Aitken basin, which moved the basin southward by ∼68° relative to the dipole axis of the core field. The formation of mascon maria at later times introduced positive mass anomalies at the surface, forcing the Moon to make an additional ∼52° degree polar wander. Interaction of multiple impact shock waves with the dynamo, the abrupt angular momentum transfer to the mantle by the impactors, and the global overturn of the core after each impact were probably the factors causing the dynamo reversal.