Geodynamo Conductivity Limits

Driscoll, Peter; Du, Zhuofei

doi:10.1029/2019gl082915

Cited by 20 publications

(11 citation statements)

References 47 publications

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“…Both thermal and compositional convections play an equally important role in driving the presentday geodynamo. In addition, our determined thermal conductivity at the topmost core (∼70 W m −1 K −1 ) supports a geodynamo driven by thermal convection over Earth's history [36] and therefore solved the "core paradox" raised by Olson [82].…”

Section: Fig 2 Measured and Calculated Resistivity Of Hcp Fe Atsupporting

confidence: 65%

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Reconciliation of Experiments and Theory on Transport Properties of Iron and the Geodynamo

et al. 2020

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We measure the electrical resistivity of hcp iron up to ∼170 GPa and ∼3000 K using a four-probe van der Pauw method coupled with homogeneous flattop laser heating in a DAC, and compute its electrical and thermal conductivity by first-principles molecular dynamics including electron-phonon and electronelectron scattering. We find that the measured resistivity of hcp iron increases almost linearly with temperature, and is consistent with our computations. The results constrain the resistivity and thermal conductivity of hcp iron to ∼80 AE 5 μΩ cm and ∼100 AE 10 Wm −1 K −1 , respectively, at conditions near the core-mantle boundary. Our results indicate an adiabatic heat flow of ∼10 AE 1 TW out of the core, supporting a present-day geodynamo driven by thermal and compositional convection.

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Section: Fig 2 Measured and Calculated Resistivity Of Hcp Fe Atsupporting

confidence: 65%

“…[7,8,14,17,20,[32][33][34]). Uncertain thermal conductivity of the core may result in a big difference in our understanding of the thermal history of the core and its geodynamo [35,36]. Thus, further improved and integrated experimental measurements and theoretical studies on the transport properties of Fe at high P-T are needed.…”

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confidence: 99%

Reconciliation of Experiments and Theory on Transport Properties of Iron and the Geodynamo

et al. 2020

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“…Earth’s core is thought to be marginal for thermal convection today (i.e. the CMB heat flow is similar to the conductive requirement to drive convection) 41 . Compared to Earth, therefore, the cores of more massive planets tend to cool more slowly due to the increased mantle viscosity and have a higher core thermal conductivity, implying that thermal convection is less likely.…”

Section: Resultsmentioning

confidence: 99%

Melting and density of MgSiO3 determined by shock compression of bridgmanite to 1254GPa

et al. 2021

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The essential data for interior and thermal evolution models of the Earth and super-Earths are the density and melting of mantle silicate under extreme conditions. Here, we report an unprecedently high melting temperature of MgSiO3 at 500 GPa by direct shockwave loading of pre-synthesized dense MgSiO3 (bridgmanite) using the Z Pulsed Power Facility. We also present the first high-precision density data of crystalline MgSiO3 to 422 GPa and 7200 K and of silicate melt to 1254 GPa. The experimental density measurements support our density functional theory based molecular dynamics calculations, providing benchmarks for theoretical calculations under extreme conditions. The excellent agreement between experiment and theory provides a reliable reference density profile for super-Earth mantles. Furthermore, the observed upper bound of melting temperature, 9430 K at 500 GPa, provides a critical constraint on the accretion energy required to melt the mantle and the prospect of driving a dynamo in massive rocky planets.

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“…Solidification of an inner core can drive compositional core convection by the release of light elements and assist the generation of a magnetic field. Inner core solidification does not guarantee core convection because the total buoyancy may still be subcritical if the core is thermally stratified (Driscoll & Du, 2019). Although T CMB plays a minor role on the mineralogy of the lowermost mantle (see above), it plays a major role in the core , since it determines the structure and thermal state of the core.…”

Section: Resultsmentioning

confidence: 99%

Super‐Earth Internal Structures and Initial Thermal States

2020

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The presence of a planetary magnetic field is an important ingredient for habitability. The coexistence of a solid and a liquid core can facilitate the maintenance of a compositionally driven dynamo; however, the likelihood of such a configuration in super-Earths is unknown. Recently, shock experiments and ab initio calculations have constrained the stability, equations of state, and melting properties of ultrahigh pressure core and mantle phases. Here, we investigate the internal structure of super-Earth exoplanets with a range of total masses and core mass fractions ranging from that of Mars (0.2) to Mercury (0.68), including an Earth-like bulk composition. We examine the effect of the initial core-mantle boundary temperature (T CMB ) on their internal structure and identify regimes with coexisting solid and liquid cores, and deep mantle melting. We find that the range of T CMB for which an inner core is growing increases with the total planet mass and even more with the core mass fraction. Therefore, our results suggest that super-Earths should have a crystallizing core over a large temperature range. We also find that the presence of a growing inner core is likely to be accompanied by a partially liquid lower mantle, which will likely influence planetary thermal evolution. We estimate the initial CMB temperature after super-Earth accretion by assuming an accretional heat retention efficiency similar to Earth. We find that massive super-Earths are expected to have an initial internal temperature consistent with a partially liquid core, allowing for the possibility of thermal and compositional dynamo action. Plain Language SummaryThe presence of a magnetic field in a planetary body is an important criterion for habitability, since it may reduce atmospheric loss and shield planetary surfaces from high-energy charged particles. In terrestrial planets, when an inner iron-rich core crystallizes, it releases light elements in the residual liquid outer core, which can facilitate liquid iron alloy convection and contribute to the maintenance of a magnetic field. In this study, we investigate the optimal conditions for the presence of a crystallizing core in super-Earths (i.e., large rocky exoplanets), by modeling their internal structure and using recent data on the melting properties of materials at ultrahigh pressure. We find that cores of super-Earths of large sizes have a greater likelihood to be partially molten, which may aid in maintaining a magnetic field. In addition, the lowermost mantle of massive super-Earths are likely to experience prolonged partial melting. Increasing the size of their core relative to their mantle increases even more the probability of a growing inner core. In addition, we calculate the initial heat retained during planetary formation, which confirms that large super-Earths are likely to have an initial crystallizing core.

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Geodynamo Conductivity Limits

Cited by 20 publications

References 47 publications

Reconciliation of Experiments and Theory on Transport Properties of Iron and the Geodynamo

Reconciliation of Experiments and Theory on Transport Properties of Iron and the Geodynamo

Melting and density of MgSiO3 determined by shock compression of bridgmanite to 1254GPa

Super‐Earth Internal Structures and Initial Thermal States

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