11:15 AM - 11:30 AM
[PPS01-09] Evolution of satellite radius and internal structure of Ganymede due to post-accretion differentiation
Keywords:Ganymede, Thermal evolution
Ganymede is the only icy satellite in the solar system with an intrinsic magnetic field, indicating a conductive molten metal core with active convection. Its moment of inertial factor suggests a fully differentiated interior with a metal core, rocky mantle, and H2O layer. Furthermore, the existence of a subsurface ocean is suggested. In addition, its surface is covered with grooved terrain that is thought to have been formed by satellite expansion. These features represent the result of Ganymede's evolution. However, the evolutionary process in which the interior of Ganymede reached a high temperature to melt metal and generate convection in its metal core, leading to the formation of an intrinsic magnetic field, is not yet fully understood. Understanding this evolutionary process provides an important clue to addressing key questions in satellite formation and evolution, such as why Europa does not possess an intrinsic magnetic field, and why the internal structures and surface environments of other large icy satellites, such as Callisto and Titan, differ significantly from those of Ganymede. Furthermore, the potential existence of a subsurface ocean makes Ganymede an important target from the perspective of astrobiology.
Previous models of Ganymede's evolution assume that the initial state is complete or partial differentiation. However, considering only accretional heating and radiogenic heating, it is likely that Ganymede was undifferentiated at the end of accretion. If Ganymede is already differentiated after its formation, changes in the satellite radius are primarily caused by phase changes in the H2O layer. On the other hand, if Ganymede is undifferentiated at the end of formation, differentiation would occur during the post-accretional evolution process, leading to significant changes in the internal layer structure and density distribution, which in turn results in substantial changes in the satellite radius. The timing of these changes in satellite radius depends on when differentiation occurs, suggesting that the grooved terrain may have formed in the mid-stage of Ganymede's evolutionary process. Furthermore, if Ganymede remains undifferentiated at the end of accretion, the formation of a metal core and the generation of an intrinsic magnetic field would need to be reconsidered. In this study, we develop a 1D thermal and structural evolution model of Ganymede that incorporates previously unconsidered processes, including the rock-H2O differentiation, the hydration and dehydration of the rock, and the rock-metal differentiation, to investigate the evolutionary process of Ganymede.
Based on the calculations, if Ganymede was undifferentiated after formation, rock and liquid H2O differentiation would take ~1.5 billion years, increasing the satellite radius by ~2.5%. Around 1.5 billion years after accretion, dehydration would begin, causing the satellite radius to decrease by ~0.1%. Furthermore, the formation of a metal core would commence around 2.5 billion years after accretion. Moreover, the liquid H2O produced by the differentiation of rock and liquid H2O, dehydration and melting of ice at the boundary between the H2O layer and the rocky mantle would solidify quickly, and the subsurface ocean cannot exist for a long period. Additionally, continuous heat transport from the rocky mantle to the metal core results in a continuous increase in the core temperature, preventing the occurrence of thermal convection.
These results suggest grooved terrain formed ~1.5 billion years after accretion due to the expansion of the satellite radius caused by the differentiation of rock and liquid H2O. Additionally, maintaining the subsurface ocean requires tidal heating or efficient heat transport from the satellite's interior to the H2O layer through solid-state convection. Furthermore, solid-state convection may cool the metal core, drive thermal convection, and generate an intrinsic magnetic field.
Previous models of Ganymede's evolution assume that the initial state is complete or partial differentiation. However, considering only accretional heating and radiogenic heating, it is likely that Ganymede was undifferentiated at the end of accretion. If Ganymede is already differentiated after its formation, changes in the satellite radius are primarily caused by phase changes in the H2O layer. On the other hand, if Ganymede is undifferentiated at the end of formation, differentiation would occur during the post-accretional evolution process, leading to significant changes in the internal layer structure and density distribution, which in turn results in substantial changes in the satellite radius. The timing of these changes in satellite radius depends on when differentiation occurs, suggesting that the grooved terrain may have formed in the mid-stage of Ganymede's evolutionary process. Furthermore, if Ganymede remains undifferentiated at the end of accretion, the formation of a metal core and the generation of an intrinsic magnetic field would need to be reconsidered. In this study, we develop a 1D thermal and structural evolution model of Ganymede that incorporates previously unconsidered processes, including the rock-H2O differentiation, the hydration and dehydration of the rock, and the rock-metal differentiation, to investigate the evolutionary process of Ganymede.
Based on the calculations, if Ganymede was undifferentiated after formation, rock and liquid H2O differentiation would take ~1.5 billion years, increasing the satellite radius by ~2.5%. Around 1.5 billion years after accretion, dehydration would begin, causing the satellite radius to decrease by ~0.1%. Furthermore, the formation of a metal core would commence around 2.5 billion years after accretion. Moreover, the liquid H2O produced by the differentiation of rock and liquid H2O, dehydration and melting of ice at the boundary between the H2O layer and the rocky mantle would solidify quickly, and the subsurface ocean cannot exist for a long period. Additionally, continuous heat transport from the rocky mantle to the metal core results in a continuous increase in the core temperature, preventing the occurrence of thermal convection.
These results suggest grooved terrain formed ~1.5 billion years after accretion due to the expansion of the satellite radius caused by the differentiation of rock and liquid H2O. Additionally, maintaining the subsurface ocean requires tidal heating or efficient heat transport from the satellite's interior to the H2O layer through solid-state convection. Furthermore, solid-state convection may cool the metal core, drive thermal convection, and generate an intrinsic magnetic field.