000007404 001__ 7404
000007404 005__ 20250122194501.0
000007404 037__ $$aCTALK-2025-0044
000007404 100__ $$aRivoldini, Attilio
000007404 245__ $$aTop-down crystallization in small planetary bodies: The effect of non-equilibrium and core composition
000007404 260__ $$c2024
000007404 269__ $$c2024-04-16
000007404 520__ $$aUnderstanding the crystallization of metallic cores is necessary to constrain the structure and thermal evolution of terrestrial bodies in our solar system and beyond. Core cooling is also closely related to the generation and sustainability of a magnetic field. The core crystallization regime depends primarily on the depth of intersection of the core temperature with the liquidus ([1], and refs therein). Core composition, pressure, and thermal profile are the major parameters controlling the depth of intersection. If the temperature gradient across the core is steeper than that of the liquidus, solidification starts at the top, the “top-down” crystallization regime. At low pressure (≤10 GPa) relevant to small terrestrial planets, moons, and possibly some asteroids, the eutectic temperature decreases with increasing pressure (e.g., [2] for the Fe-S system),  favoring  an  onset  of  crystallization  at  the  top  of  the  core. Top-down crystallization has been proposed to exist in several planets and moons in the Solar System, such as Mercury [2], [3], Mars ([4], [5]), and Ganymede [6], [7], [8], [9]. In this study, which was performed by the International Space Science Institute (ISSI) Team “A new non-equilibrium model of iron snow in planetary cores”, we investigate the effect of non-equilibrium as well as the effect of the core composition on top-down crystallization. We find that the time scale of phase relaxation is significantly shorter than the time scales usually employed in one-dimensional evolution models. Consequently, the assumption of equilibrium in these models remains valid. Nevertheless, the time scales associated with crystallization, melting, and crystal settling may be similar to the phase relaxation time scale, which warrants a closer investigation. Additionally, if the amount of supercooling required to initiate nucleation is large [11], non equilibrium could play a much larger role. In terms of core chemistry we studied two different core alloys (Fe-S and Fe-C) motivated by silicate-metal partitioning experiments (reviewed by [12]) at various concentrations in the framework of the equilibrium top-down crystallization model. We find that the time scales of growing either the snow zone (iron-rich compositions) or the flotation crust (iron-poor compositions) can vary significantly between the Fe-S and Fe-C system. Furthermore, the exact concentration of sulfur or carbon has an impact on the thermodynamic parameters, subsequently affecting the entropy available to the dynamo.
000007404 536__ $$a3PRODPALNINT/$$c3PRODPALNINT/$$f3PRODPALNINT
000007404 594__ $$aNO
000007404 700__ $$aRückriemen-Bez, Tina
000007404 700__ $$aAnders, Sten
000007404 700__ $$aDavies, Chris
000007404 700__ $$aEckert, Sven
000007404 700__ $$aHuguet, Ludovic
000007404 700__ $$aPommier, Anne
000007404 773__ $$tEGu 2024, Vienna, Austria
000007404 8560_ $$fattilio.rivoldini@ksb-orb.be
000007404 85642 $$ahttps://doi.org/10.5194/egusphere-egu24-20752
000007404 906__ $$aContributed
000007404 980__ $$aCTALKCONT