EPSS Colloquium spring-2018

Core crystallization is a crucial ingredient in the evolution of terrestrial bodies and is controlled primarily by chemistry and temperature. Crystallization within a metallic core releases latent heat and gravitational energy, influencing significantly the processes responsible for the presence of a magnetic field. The diversity of magnetic fields observed in small terrestrial bodies, such as Mars, Mercury or Ganymede suggests different core cooling history. Past missions have observed that Mars does not currently possess an internally-generated magnetic field but likely had one early in its history, while Mercury currently possesses a weak magnetic field and Ganymede is characterized by a strong one. The origin of this diversity is not well understood and seems to depend highly on the onset, depth, and rate of crystallization. This presentation will focus on the effect of chemistry on core crystallization and its implications for the magnetic field. Phase equilibria and electrical experiments on core and core-mantle analogues have been conducted in the Fe, Fe-S, Fe-S-O and Fe-S-Si (+/- olivine) systems using the multi-anvil apparatus at conditions relevant to the core of small terrestrial bodies (pressure up to 20 GPa and temperature up to 1850°C). The effect of sulfur and oxygen on core crystallization from melting relations will be discussed in terms of light element distribution and core compositional stratification. Laboratory-based electrical resistivity-crystallization models will be used to provide insight regarding the insulating properties of chemical heterogeneities that result from core crystallization or mantle-core interactions. All results will be compared to the magnetic history and available observational constraints on the core structure, temperature and composition of Mars, Mercury and Ganymede.