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Dynamo maker ready to roll

Two 3-metre-tall rotating spheres encased within a sturdy steel box aim to mimic Earth’s interior. Ten years in the making, the $2-million project is nearly ready for its inaugural run. Early next year, the sphere will begin whirling around while loaded with 13,000 kilograms of molten sodium heated to around 105 °C.

Researchers hope that the churning, electrically conducting fluid will generate a self-sustaining electromagnetic field that can be poked, prodded and coaxed for clues about Earth’s dynamo, which is generated by the movement of liquid iron in the outer core. If it works, it will be the first time that an experiment that mirrors the configuration of Earth’s interior has managed to recreate such a phenomenon.

Dynamos are easy to generate in nature, but the same is not true for the lab. Simulating this process will guide our understanding of processes that take place 3,000 kilometres below ground, at depths that can be probed only indirectly — by analysing seismic waves that have travelled through Earth’s deep interior, for example.

The University of Maryland set-up consists of two concentric spheres. The inner sphere, at 1 metre across, stands in for Earth’s solid inner core; the outer sphere the edge of Earth’s mantle. The space between the two is filled with liquid sodium, mimicking the liquid outer core.

Each sphere is powered by a separate motor so that it can rotate independently of the other. By spinning the spheres across a range of matched and unmatched velocities — up to 4 revolutions per second for the outer sphere and 12 for the inner — scientists will study how heat and rotation might affect the movement of the molten iron in Earth’s core.

The experiment will use Earth’s natural magnetism as a ‘seed field’ to kick-start the process. As this field is dragged and stretched by the spinning, conducting liquid it will generate electric currents. Those currents will then create additional magnetic fields that, when sufficiently twisted around, can amplify themselves and drive the process forward.

nature.com »