11:00 〜 13:00
[PPS07-P16] 氷天体内部の多相固体氷層の対流ダイナミクス
In the interior of icy bodies, several ice layers in various solid phases appear depending on the size of the body, the temperature and the pressure conditions. Surficial icy shell consists of low-pressure phase ice Ih, and additionally in a deeper region of large icy bodies, such as Jovian moon Ganymede and Saturnian moon Titan, multi-layered ice mantle which consists of various high-pressure phases exist. Convection in these ice layers with solid-solid phase transitions is likely to play an important role in thermal evolution, surface tectonics and material transport. For examples, methane in the atmosphere of Saturn's moon Titan is estimated to convert into a complex hydrocarbons within several 10s Myrs through photochemical reactions, suggesting that methane can be continuously replenished to the surface possibly from the rocky core through thick solid ice layers. Also, the bottom of the high-pressure ice layer (the boundary with the rocky core) can be molten according to the heat flow from the core, and the rate at which molten water with lower density than surrounding ice rises through the high-pressure ice layer could have a significant effect on the thermal evolution and the material transport.
In such ways, the solid-state convection of ice associated with solid phase transition controls the heat and material transports from the rocky core to the surface. For the rocky mantle of the Earth, many numerical works focusing the effect of 660 km discontinuity have been performed. However, multi-layered icy mantle within a large icy body has not been well investigated. Although the effect of the phase transition is characterized by the Clapeyron slope, various different values for the high-pressure water ice have been provided throughout many previous experimental studies. Thus here we examine quantitative investigation of convective dynamics and transport efficiency considering such uncertainty of phase boundaries. We perform numerical simulations of solid-state convection in a multilayered ice mantle with solid phase transitions to investigate the flow structure and heat transport rate.
Our calculations assume the layered structure of the ice I - III, and ice I - III - V, and perform in a two-dimensional region with various Rayleigh number (105∼9) and Clapeyron slope (0 ∼ −0.3 MPa/K). As a general trend, the convective structure changes from one to two layers with a smaller (larger absolute value in negative) Clapeyron slope, and the Nusselt number significantly decreases. On the other hand, very large Rayleigh number shows that the two-layered structure unsteadily collapses and the transport efficiency intermittently increases even in case of smaller Clapeyron slope.
We evaluate the upward timescale through the ice layers based on the velocity of upwelling plume, and compare the CH4 conversion timescale for the Titan's atmosphere (10s Myrs). For a relatively smaller Rayleigh number (105∼6), the upward timescale is about 100 Myrs or more for a larger (smaller absolute value in negative) Clapeyron slope (0 ~ -0.1 MPa/K) implying difficult to maintain a steady amount of atmospheric methane, and similar orders of timescale for a smaller (larger absolute value in negative) Clapeyron slope (-0.2 ~ -0.3 MPa/K). For a larger Rayleigh number (107 or more), the upward timescale is 1 ~ 2 orders of magnitude smaller than the CH4 conversion timescale in any value of the Clapeyron slope, suggesting that a sustainable supply of CH4 from the rocky core to the atmosphere is possible.
In such ways, the solid-state convection of ice associated with solid phase transition controls the heat and material transports from the rocky core to the surface. For the rocky mantle of the Earth, many numerical works focusing the effect of 660 km discontinuity have been performed. However, multi-layered icy mantle within a large icy body has not been well investigated. Although the effect of the phase transition is characterized by the Clapeyron slope, various different values for the high-pressure water ice have been provided throughout many previous experimental studies. Thus here we examine quantitative investigation of convective dynamics and transport efficiency considering such uncertainty of phase boundaries. We perform numerical simulations of solid-state convection in a multilayered ice mantle with solid phase transitions to investigate the flow structure and heat transport rate.
Our calculations assume the layered structure of the ice I - III, and ice I - III - V, and perform in a two-dimensional region with various Rayleigh number (105∼9) and Clapeyron slope (0 ∼ −0.3 MPa/K). As a general trend, the convective structure changes from one to two layers with a smaller (larger absolute value in negative) Clapeyron slope, and the Nusselt number significantly decreases. On the other hand, very large Rayleigh number shows that the two-layered structure unsteadily collapses and the transport efficiency intermittently increases even in case of smaller Clapeyron slope.
We evaluate the upward timescale through the ice layers based on the velocity of upwelling plume, and compare the CH4 conversion timescale for the Titan's atmosphere (10s Myrs). For a relatively smaller Rayleigh number (105∼6), the upward timescale is about 100 Myrs or more for a larger (smaller absolute value in negative) Clapeyron slope (0 ~ -0.1 MPa/K) implying difficult to maintain a steady amount of atmospheric methane, and similar orders of timescale for a smaller (larger absolute value in negative) Clapeyron slope (-0.2 ~ -0.3 MPa/K). For a larger Rayleigh number (107 or more), the upward timescale is 1 ~ 2 orders of magnitude smaller than the CH4 conversion timescale in any value of the Clapeyron slope, suggesting that a sustainable supply of CH4 from the rocky core to the atmosphere is possible.