In March 2011, nearly seven years after its launch, NASA’s MESSENGER probe settled into orbit around Mercury (pictured here) and began a detailed survey of the planet’s geochemistry, topography, and space environment. (See the article by Sean Solomon, Physics Today, January 2011, page 50.) Within months, the spacecraft delivered a surprise finding: Mercury’s magnetic field is roughly three times as strong in the northern hemisphere as in the southern one. A new paper by Hao Cao, Jonathan Aurnou, Christopher Russell (all at UCLA), and coauthors explains how such a top-heavy field could arise. The researchers simulated the planet’s dynamo—the churning, magnetic-field-inducing flows of the spinning molten core. Such dynamos can be driven in part by the buoyancy of light elements released as the core solidifies. In typical simulations, that solidification is assumed to occur from the inside out, but Cao and company considered a scenario in which precipitates form—and light elements are released—throughout the core’s volume. That scenario, considered plausible for Mercury due to the chemical complexity and relatively low pressure of its core, resulted in lopsided magnetic fields similar to the one MESSENGER observed. Curiously, the asymmetric solutions were stable only when heat escaped the core faster near the equator than at high latitudes. That suggests an opportune test of the model’s validity: If nonuniform heat flows do in fact prevail on Mercury, they should leave subtle traces in the planet’s density distribution, which is also being measured by MESSENGER. (H. Cao et al., Geophys. Res. Lett., in press.)—Ashley G. Smart