Sulfur's impact on core evolution and magnetic field generation on Ganymede

Analysis of the melting relationships of potential core forming materials in Ganymede indicate that convective motions capable of generating the satellite's magnetic field may be driven, in-part, either by iron "snow" forming below the core-mantle boundary or solid iron sulfide floating upward from the deep core. Eutectic melting temperatures in the binary Fe-FeS system decrease with increasing pressure over the interval of core pressures on Ganymede (<14 GPa). Comparison of melting temperatures to adiabatic temperature gradients in the core suggest that solid iron is thermodynamically stable at shallow levels for bulk core compositions more iron-rich than eutectic (i.e., <21 wt % S). Calculations based on high-pressure solid-liquid phase relationships in the Fe-FeS system indicate that Fe snow or floatation of solid FeS, depending on whether the core composition is more or less Fe-rich than eutectic, is an inevitable consequence of cooling Ganymede's core. Our results demonstrate that these conclusions are robust over a wide-range of plausible three-layer internal structures and thermal evolution scenarios. Using scaling arguments based on recent experimental work we estimate core Rossby and magnetic Reynolds numbers plausibly consistent with a dynamo being generated in Ganymede's core via Fe-snow. Depending on core composition, either shallow formation of Fe snow or deep precipitation and subsequent floatation of FeS is an important mechanism for driving the moon's strong internally-generated magnetic field.