Mars: A new core-crystallization regime

The evolution of the martian core is widely assumed to mirror the characteristics observed for Earth’s core. Data from experiments performed on iron-sulfur and iron-nickel-sulfur systems at pressures corresponding to the center of Mars indicate that its core is presently completely liquid and that it will not form an outwardly crystallizing iron-rich inner core, as does Earth. Instead, planetary cooling will lead to core crystallization following either a “snowing-core ” model, whereby iron-rich solids nucleate in the outer portions of the core and sink toward the center, or a “sulfide inner-core ” model, where an iron-sulfide phase crystallizes to form a solid inner core. For Mars, the presence of a metallic coredominated by Fe, Ni, and S is well es-tablished by martian meteorite geo-chemistry (1–3). However, because of the absence of seismic data and direct samples from the martian mantle, the composition, thermal evolution, and physical state of the core are hotly debated. The Mars Global Surveyor mission has allowed detailed refinement of the geophysical data available for the planet. Precise determina