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Compositional instability of Earth's solid inner core
Author(s) -
Gubbins D.,
Alfè D.,
Davies C. J.
Publication year - 2013
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1002/grl.50186
Subject(s) - inner core , convection , outer core , stratification (seeds) , thermodynamics , rayleigh number , thermal conduction , instability , temperature gradient , oxygen , mechanics , thermal conductivity , thermal diffusivity , silicon , materials science , natural convection , geophysics , geology , chemistry , physics , meteorology , germination , botany , organic chemistry , dormancy , seed dormancy , biology , metallurgy
All models that invoke convection to explain the observed seismic variations in Earth's inner core require unstable inner core stratification. Previous work has assumed that chemical effects are stabilizing and focused on thermal convection, but recent calculations indicate that the thermal conductivity at core temperatures and pressures is so large that the inner core must cool entirely by conduction. We examine partitioning of oxygen, sulfur, and silicon in binary iron alloys and show that inner core growth results in a variable light element concentration with time: oxygen concentration decreases, sulfur concentration decreases initially and increases later, and silicon produces a negligible effect to within the model errors. The result is a net destabilizing concentration gradient. Convective stability is measured by a Rayleigh number, which exceeds the critical value for reasonable estimates of the viscosity and diffusivity. Our results suggest that inner core convection models, including the recently proposed translational mode, can be viable candidates for explaining seismic results if the driving force is compositional.