GP34A-01 INVITED
"Snowing" Core in Earth?
As a planet cools, an initially molten core gradually solidifies. Solidification occurs at shallow depths in the form of "snow", if the liquidus temperature gradient of the core composition is smaller than the adiabatic temperature gradient in the core. Experimental data on the melting behavior of iron-sulfur binary system suggest that the cores of Mercury and Ganymede are probably snowing at the present time. The Martian core is predicted to snow in the future, provided that the sulfur content falls into the range of 10 to 14 weight percent. Is the Earth's core snowing? If so, what are the surface manifestations? If the Earth's core snowed in the past, how did it affect the formation of the solid inner core and the geodynamo? Here, we evaluate the likelihood and consequences of a snowing core throughout the Earth's history, on the basis of mineral physics data describing the melting behavior, equation-of-state, and thermodynamic properties of iron-rich alloys at high pressures. We discuss if snowing in the present-day Earth can reproduce the shallow gradients of compressional wave velocity above the inner-core boundary, and whether or not snowing in the early Earth may reconcile the apparent young age of the solid inner core with a long-lived geodynamo.
GP34A-02 INVITED
Boundary Layer Control of Rotating Convection Systems
Rotating convection is ubiquitous in the natural universe, and is likely responsible for planetary processes
such magnetic field generation. Rapidly rotating convection is typically organized by the Coriolis force into
tall, thin, coherent convection columns which are aligned with the axis of rotation. This organizational effect
of rotation is thought to be responsible for the strength and structure of magnetic fields generated by
convecting planetary interiors. As thermal forcing is increased, the relative influence of rotation weakens,
and fully three-dimensional convection can exist. It has long been assumed that rotational effects will
dominate convection dynamics when the ratio of buoyancy to the Coriolis force, the convective Rossby
number, Roc, is less than unity.
We investigate the influence of rotation on turbulent Rayleigh-Benard convection via a suite of coupled
laboratory and numerical experiments over a broad parameter range: Rayleigh number, 103
GP34A-03 INVITED
Thermal and chemical diffusion within conduits of sinking metal-silicate plumes during core formation events.
Early and rapid core formation is suggested by recent isotopic studies. Accumulation of a short lived liquid metal pond at the base of a magma ocean during early impacts may provide a model for chemical diffusion of silicates and liquid metal to produce the observed abundances of siderophile elements in the Earth's mantle. Here we present results from laboratory fluid experiments of liquid gallium in high viscosity stratified corn syrup solutions to model the physical dynamics of core formation processes in the early Earth. Experiments are designed to consider the instability of a dense liquid metal pond as single droplets, Rayleigh-Taylor instability, and evolution of a liquid metal emulsion layer. We find that in all cases, a wide trailing conduit develops behind rapidly descending metallic plumes which entrains low density fluid to the base of the fluid box. We propose a model where the conduit itself provides a vehicle for thermal and chemical equilibration between metals and silicates at high pressures and temperatures during its path through the lower mantle. Diffusion processes contribute to the formation of this new entrained fluid layer at the base of the fluid box which is buoyant and evolves into a new type of thermo-chemical plume which subsequently rises. Using a range of viscosity and buoyancy ratios, experimental results will constrain the time scales for instability of a liquid metal pond, descent and upwelling times of this unique type of plume, as well as the nature and dynamics of conduit formation. This model provides a high pressure/temperature environment for metal- silicate equilibration consistent with petrologic and isotopic studies, is consistent with rapid core formation, and may also connect core formation to ancient hotspot activity on terrestrial planets.
GP34A-04 INVITED
Delivery System in the Earth's Mantle: Fluid Dynamic Modeling of Thermochemical Plumes
Present-day seismic mantle imaging reveals complex structures below hotspots. The classical plume model of a mushroom-shaped thermal anomaly rising from a steady localized heat source is unable to explain key observations such as the patchy nature of the seismic tomography images. Here we present a series of laboratory experiments on a thermochemical plume generated from a thermal boundary layer which is stratified in composition. For quantitative analysis, we invented a new measurement method to visualize temperature, composition, and velocity fields simultaneously. For the description of the dynamics of a laminar thermochemical plume out of a thin dense layer, four dimensionless parameters (Rayleigh number, Buoyancy ratio, thickness ratio, and viscosity ratio) should be addressed, and here we systematically varied initial Buoyancy ratio B0 which is the ratio of the stabilizing chemical buoyancy to the destabilizing thermal buoyancy at the onset of convection. When B0=0, a purely thermal plume which has a large plume head and a narrower conduit is produced. For large B0 (>1), the thermal density anomaly cannot counterbalance the compositional anomaly and convection develops above the compositional interface. For intermediate B0, the interplay between the thermal and compositional effects generates complicated morphologies. Because all plumes cool by thermal diffusion as they rise in a cooler mantle, a chemically composite thermal plume will eventually attain a level of neutral buoyancy, at which it will begin to collapse. Separation within the plume will occur, whereby the chemically denser material will start to sink back while the heated surrounding mantle keeps rising. Experimental scaling laws predict maximum height of the chemically composite plumes and could well explain the time-dependence and morphology of a melting anomaly such as Iceland hotspot. Our laboratory experiments more generally imply that 1) mantle plumes are not necessarily narrow and continuous throughout the mantle but can be fat and patchy, and 2) a hot mantle region may not be buoyant and rising, but on contrary may be sinking.