Demystifying the ice giants’ puzzling poles

Like our own planet, Jupiter and Saturn have magnetic and geographic poles that are closely, but not perfectly, aligned; each planet’s magnetic dipole is angled just a few degrees off its rotational axis. But the magnetic fields of Uranus and Neptune, the so-called ice giants, are dramatically tilted—by 59° and 46°, respectively. New experiments by Eric King (University of California, Berkeley) and Jonathan Aurnou (UCLA) may help to explain why. Planetary magnetic fields are thought to be generated by dynamos—turbulent, convective flows of electrically conducting fluid in the planet’s interior (see the article by Daniel Lathrop and Cary Forest in Physics Today, July 2011, page 40) . Coriolis forces due to planetary rotation can influence those convective flows and thereby orient the dynamo’s field. To probe that effect in the lab, King and Aurnou did what’s known as a rotating Rayleigh–Bénard experiment: They heated a 20-cm-wide spinning drum of liquid gallium from below while simultaneously cooling it from above. Using conditions that, through appropriate scaling, mimic those of planetary interiors, the researchers found that the rotation–convection coupling is especially pronounced in fluids with high thermal conductivity. Therein may lie the secret of the ice giants’ skewed poles: Based on trends seen in the experimental data, the researchers estimate that Earth, Jupiter, and Saturn, whose dynamos comprise high-conductivity molten metal, fall within the strongly coupled regime, whereas Uranus and Neptune, whose dynamos comprise moderate-conductivity aqueous solution, do not. (E. M. King, J. M. Aurnou, Proc. Natl. Acad. Sci. USA 110, 6688, 2013, doi:10.1073/pnas.1217553110 .)