The viscosity of liquid iron at the physical conditions of the Earth's core

The Earth's magnetic field is powered from energy supplied by slow cooling and freezing of the liquid iron core. Core thermal history calculations have been hindered in the past by poor knowledge of the properties of iron alloys at the extreme pressures and temperatures pertaining in the core. This obstacle is now being overcome by developments in high pressure experiments and computational mineral physics. Here we review the relevant properties of iron alloys at core conditions and discuss their uncertainty and geophysical implications.Powerful constraints on core evolution are now possible, due partly to recent factor 2-3 upward revisions of the all-important electrical and thermal conductivities. This has dramatic implications for the thermal history of the entire Earth, not just the core: the inner core is very young, the core is cooling quickly, and was so hot in the past that the lowermost mantle

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“…Recent ab initio estimates 25,57 of ν are given in Table 1 for the present core chemistry model; they are in line with older values 61 .…”

Section: Materials Properties For Earth's Coresupporting

confidence: 49%

Constraints from material properties on the dynamics and evolution of Earth’s core

et al. 2015

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“…In our model the viscosity of both the silicate and the iron melt is chosen to be 1 Pa s, similar to e.g. Sramek et al (2012), although smaller viscosities for the iron melt (η Fe = 0.015 Pa s) have been suggested (de Wijs et al 1998). We have tested the influence of a variation in the iron melt viscosity.…”

Section: Summary and Discussionmentioning

confidence: 99%

Differentiation and core formation in accreting planetesimals

Neumann

Breuer

Spohn

2012

A&A

110

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Aims. The compositions of meteorites and the morphologies of asteroid surfaces provide strong evidence that partial melting and differentiation were widespread among the planetesimals of the early solar system. However, it is not easily understood how planetesimals can be differentiated. To account for significantly smaller radii, masses, gravity and accretion energies early, intense heat sources are required, e.g. the short-lived nuclides 26 Al and 60 Fe. Here, we investigate the process of differentiation and core formation in accreting planetesimals taking into account the effects of sintering, melt heat transport via porous flow and redistribution of the radiogenic heat sources. Methods. We use a spherically symmetric one-dimensional model of a partially molten planetesimal consisting of iron and silicates, which considers the accretion by radial growth. The common heat conduction equation has been modified to consider also melt segregation. In the initial state, the planetesimals are assumed to be highly porous and consist of a mixture of Fe,Ni-FeS and silicates consistent to an H-chondritic composition. The porosity change due to the so called hot pressing is simulated by solving a corresponding differential equation. Magma segregation of iron and silicate melt is treated according to the flow in porous media theory by using the Darcy flow equation and allowing a maximal melt fraction of 50%. Results. We show that the differentiation in planetesimals depends strongly on the formation time, accretion duration, and accretion law and cannot be assumed as instantaneous. Iron melt segregation starts almost simultaneously with silicate segregation and lasts between 0.4 and 10 Ma. The degree of differentiation varies significantly and the most evolved structure consists of an iron core, a silicate mantle, which are covered by an undifferentiated but sintered layer and an undifferentiated and unsintered regolith -suggesting that chondrites and achondrites can originate from the same parent body.

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“…The low viscosity of the core (eg Poirier, 1988;de Wijs et al, 1998;Perrillat et al, 2010) makes convection currents very efficient at mixing all the extensive variables, and in particular entropy and composition. A good evidence of that efficiency comes from the observation of the secular variation of the magnetic field which implies a flow velocity of the order of 10 −4 m s −1 (eg.…”

Section: Radial Structure Of the Corementioning

confidence: 99%

Thermal evolution of the core with a high thermal conductivity

Labrosse

2015

Physics of the Earth and Planetary Interiors

183

271

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The rate at which heat is extracted across the core mantle boundary (CMB) is constrained by the requirement of dynamo action in the core. This constraint can be computed explicitly using the entropy balance of the core and depends on the thermal conductivity, whose value has been revised upwardly. A high order model (fourth degree polynomial of the radial position) for the core structure is derived and the implications for the core cooling rate and thermal evolution obtained, using the recent values of the thermal conductivity. For a thermal conductivity increasing with depth as proposed by some of these recent studies, a CMB heat flow equal to the isentropic value (13.25TW at present) leads to a 700 km thick layer at the top of the core where a downward convective heat flow is necessary to maintain an isentropic and well mixed average state. Considering a CMB heat flow larger than the well mixed isentropic value leads to an inner core less than 700 Myr old and the thermal evolution of the core is largely constrained by the conditions for dynamo action without an inner core. Analytical calculations for that period show that a CMB temperature larger than 7000 K must have prevailed 4.5 Gyr ago if the geodynamo has been driven by thermal convection for that whole time. This raises questions regarding the onset of the geodynamo and its continuous operation for the last 3.5 Gyr. Implications regarding the evolution of a basal magma ocean are also considered.

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The viscosity of liquid iron at the physical conditions of the Earth's core

Cited by 270 publications

References 24 publications

Constraints from material properties on the dynamics and evolution of Earth’s core

Constraints from material properties on the dynamics and evolution of Earth’s core

Differentiation and core formation in accreting planetesimals

Thermal evolution of the core with a high thermal conductivity

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