PLANETARY MUSINGS

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

Recently, we have been working to understand whether Ganymede may have a relatively exotic way of driving convection in a fluid portion of a  metallic core that would in-turn generate its magnetic field.  In the Earth, convection in the outer core is driven by some combination of thermally-generated buoyancy as the core cools and compositional buoyancy from the release of some light element at the inner core outer core boundary and then rising to shallower levels.  The latter is energetically efficient because it isn't subject to the inefficiency of a heat engine.  On Ganymede the same process may also hold, however the phase diagram of a potential core alloy (Fe-FeS) is different at the low pressures in Ganymede's core compared to the much higher ones deep in the Earth.  This difference allows for the possibility that solid iron might precipitate shallow in the core and fall through the core and potentially driving the magnetic field.  It is also possible that if the core is very sulfur rich solid FeS might float up from deep in the core and also drive core convection.  Working Jonathan Aurnou (UCLA) and Andrew Dombard (APL) we have outlined the potential consequences of these relatively unique modes of core evolution in a recently published paper:

Hauck, Steven A., II, Jonathan M. Aurnou, Andrew J. Dombard, Sulfur's
impact on core evolution and magnetic field generation on Ganymede, J. Geophys. Res., 111, E09008, doi: 10.1029/2005JE002557 (2006). Article

Abstract

Analysis of the melting relationships of potential core forming materials in Ganymede indicate that fluid motions, a requirement for adynamo 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 scaling arguments that give typical Rossby and magnetic Reynolds numbers consistent with dynamo action occurring in Ganymede's core. Furthermore, by applying recently derived scaling relationships relating magnetic field strength to buoyancy flux, we obtain estimates of surface magnetic field strength comparable with observed values.