Summary: Sulfur's impact on core evolution and magnetic field generation
Steven A. Hauck, II,1
Jonathan M. Aurnou ,2
and Andrew J. Dombard 3
Received 12 August 2005; revised 1 May 2006; accepted 13 June 2006; published 13 September 2006.
 Analysis of the melting relationships of potential core forming materials in Ganymede
indicate that fluid motions, a requirement for a dynamo origin for the satellite's magnetic
field, may be driven, in part, either by iron (Fe) ``snow'' forming below the core-
mantle boundary or solid iron sulfide (FeS) floating upward from the deep core. Eutectic
melting temperatures and eutectic sulfur contents in the binary Fe-FeS system decrease
with increasing pressure within the interval of core pressures on Ganymede (<14 GPa).
Comparison of melting temperatures to adiabatic temperature gradients in the core
suggests 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 iron snow or
floatation of solid iron sulfide, depending on whether the core composition is more or
less iron-rich than eutectic, is an inevitable consequence of cooling Ganymede's core.
These results are robust over a wide range of plausible three-layer internal structures and
thermal evolution scenarios. For precipitation regimes that include Fe-snow, we present