DYNAMIC AND ELASTIC EFFECTS OF LARGE THERMAL EXPANSIONS OF CORE-FORMING ALLOYS: IMPLICATIONS FOR CONVECTIVE STYLE OF PLANETARY CORES

Increasing evidence has accumulated over the last decade that molten iron alloys have large, ambient pressure thermal expansions: values in excess of 1.2 x 10^-4/K are dictated by data derived from levitated and sessile drop techniques. These large values of thermal expansion (α) in turn imply that the adiabatic gradients within early planetesimals and present day moons that have comparatively low-pressure, iron-rich cores are steep (typically greater than ~35 K/GPa at low pressures): these values, at low pressures, exceed the slope of the melting curve, and hence show that the cores of small solar system objects probably crystallize from the top-down.

A separate manifestation of these large values of thermal expansion is that the adiabatic and isothermal bulk moduli differ very significantly from one another, as imposed by the proportionality of 1 + αγT between the two moduli, where γ is the Gruneisen parameter (these moduli differ for iron alloys by 20-30% different near the melting point, and are more divergent at higher temperatures). This difference between adiabatic and isothermal moduli for iron liquids is dramatically larger than that associated with solids. This work probes how elastic data, from both ultrasonic (and hence adiabatic) measurements on liquid molten iron alloys can be deployed in tandem with compression data on liquids measured under isothermal conditions to determine over what pressure and compositional ranges the anomalously high thermal expansions of liquid iron alloys are “squeezed out” by increasing pressure. Elastic data, derived from different probes, can thus be utilized to constrain one of the most fundamental characteristics of the dynamics of planetary core solidification: the size and compositional ranges of planetary bodies over which cores solidify from the top-down or bottom-up.