The viscosity of the Earth’s inner core

Theme: Solid Earth Dynamics

Primary Supervisor:

Lidunka Vočadlo

Earth Sciences, UCL

Lidunka Vočadlo's Profile Picture

Secondary Supervisor:

John Brodholt

Earth Sciences, UCL

John Brodholt's Profile Picture

Project Description:

The transport properties of fluid materials (either solid or liquid) are determined by their viscosity, which defines the resistance of the material to fluid flow (i.e., how ‘runny’ it is). While the viscosity of the Earth’s liquid outer core is relatively well understood (runny like water), that of the inner core is not. High-temperature experiments on solid iron at ambient pressure lead to estimates for viscosities of ~1013 Pas; however this is likely to be a lower limit as the value may increase at higher pressures. Seismological and geodetic observations have led to a number of estimates for inner core viscosity ranging from 1011 to 1020 Pas.

Quantifying the viscosity of the phase(s) present in the inner core at a microscopic level is a very difficult problem because of the strong dependence of the viscosity expressions on the completely unknown quantities of grain size and dislocation density. Grain sizes in the inner core could be anything from 10-3-103 m, resulting in diffusion viscosities in the range 1015-1027 Pas; dislocation densities could be as low as 106 m-2 or nearer to the dislocation melting limit of 1013 m-2, resulting in dislocation driven viscosities of 109-1016 Pas.

The aim of this project is to place clear constraints on inner core viscosity by ab initio simulation methods. The student will apply new methods to determine the viscosity of iron and iron alloys at inner core conditions, thereby providing a greater understanding of one of the most important quantities in deep Earth physics.

Policy Impact of Research:

Placing numerical constraints on the viscosity of the inner core is fundamental to understanding important core processes such as differential inner core rotation, inner core oscillation and inner core anisotropy.


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