5:15 PM - 6:45 PM
[PPS07-P04] Liquid regions inside Europa and Electrofreezing effect
Keywords:Europa, Electrofreezing effect, Internal ocean and Partially melted regions within the ice shell, H2O-MgSO4 system
Three of Jupiter's Galilean satellites, Europa, Ganymede, and Callisto, are icy satellites with most of their surfaces covered by ice, and their internal structures are inferred from Galileo spacecraft surveys (Fig. 1). Near-infrared spectroscopic observations also infer the presence of salt hydrates on Europa's surface, mainly MgSO4 hydrates (J. B. Dalton et al. 2007). Furthermore, it has been suggested that these salt hydrates may be derived from the internal ocean(K. M. Soderlund et al. 2019).If Europa's inner ocean is conductive due to salt dissolution, orbiting within Jupiter's magnetosphere generates an electric field in the internal ocean (D. S. Colbern et al. 1986).
When liquid water is present in an electric field environment, the freezing temperature after the supercooled state is affected by the application of an electric field (electrofreezing effect). This effect is theoretically understood to be caused by a combination of various factors, including a decrease in the critical nuclear radius associated with a change in Gibbs free energy (T. Kang et al. 2020). Experimentally, the electrofreezing effect has been investigated by many previous studies for H2O single component system (e.g., S. Wei et al. 2008). However, few previous studies (Yahong Ma et al. 2010 : NaCl solution, L. F. Javitto et al. 2023 : nitrate solutions) have investigated the electrofreezing effect on salt solutions inferred in Europa's internal ocean or in the partially melted regions within the ice shell, the magnitude of the electrofreezing effect measured was a freezing temperature decrease of -1.5°C in the former and a freezing temperature increase of +17.5°C in the latter, which is an opposing trend.
The above discussion indicates that if Europa's internal ocean and partially melted regions in the ice shell experience global cooling of icy satellites over time and become supercooled, the phase stability of these liquid regions may be affected by the electrofreezing effect. The electrofreezing effect can modify the thermal evolution model of icy satellites (H. Hussmann et al. 2002) by suppressing or enhancing solidification near the ice shell-internal ocean boundary in this process. However, as mentioned above, few previous studies have investigated the electrofreezing effect on salt solutions, and the results vary widely with each study. Furthermore, the electrofreezing effect of MgSO4 solution, which is a representative candidate for the environment of the internal sea and the partially melted region in the ice shell, has not yet been investigated.
In my study, I will set up an experimental apparatus (Fig. 2) to investigate the electrofreezing effect in MgSO4 solutions, and conduct solidification experiments using 5wt.% to 15wt.% MgSO4 solutions as samples. The following is a summary of the experimental method. I Cool the sample sealed in the sample room at 0.4[°C/min] while applying an electric field of 0.0~1.1×103[V/m]. II Record the temperature just before the sample solidifies (hereinafter referred to as the solidification temperature). Measurements and observations are carried out by measuring the temperature using a K-type thermocouple, measuring the current value, and observing under an optical microscope. In this study, especially when 15 wt.% MgSO4 solution was used as the sample in this study, the freezing temperature increased from -10.3±1.0[°C] (0.0[V/m]) to -7.5±1.6[°C] (1.1×103[V/m]), an increase of 2.8°C due to the electrofreezing effect. Applying these results to the interior of an ice satellite (with relatively high MgSO4 concentrations) suggests that the internal ocean and partially melted regions within the ice shell may freeze earlier with cooling. However, the current experimental conditions are conducted under higher electric field conditions than the actual conditions inside the icy satellites, so lower electric field conditions and a different salt candidate (NaCl, Na2SO4) will be used for additional verification of the electrofreezing effect.
When liquid water is present in an electric field environment, the freezing temperature after the supercooled state is affected by the application of an electric field (electrofreezing effect). This effect is theoretically understood to be caused by a combination of various factors, including a decrease in the critical nuclear radius associated with a change in Gibbs free energy (T. Kang et al. 2020). Experimentally, the electrofreezing effect has been investigated by many previous studies for H2O single component system (e.g., S. Wei et al. 2008). However, few previous studies (Yahong Ma et al. 2010 : NaCl solution, L. F. Javitto et al. 2023 : nitrate solutions) have investigated the electrofreezing effect on salt solutions inferred in Europa's internal ocean or in the partially melted regions within the ice shell, the magnitude of the electrofreezing effect measured was a freezing temperature decrease of -1.5°C in the former and a freezing temperature increase of +17.5°C in the latter, which is an opposing trend.
The above discussion indicates that if Europa's internal ocean and partially melted regions in the ice shell experience global cooling of icy satellites over time and become supercooled, the phase stability of these liquid regions may be affected by the electrofreezing effect. The electrofreezing effect can modify the thermal evolution model of icy satellites (H. Hussmann et al. 2002) by suppressing or enhancing solidification near the ice shell-internal ocean boundary in this process. However, as mentioned above, few previous studies have investigated the electrofreezing effect on salt solutions, and the results vary widely with each study. Furthermore, the electrofreezing effect of MgSO4 solution, which is a representative candidate for the environment of the internal sea and the partially melted region in the ice shell, has not yet been investigated.
In my study, I will set up an experimental apparatus (Fig. 2) to investigate the electrofreezing effect in MgSO4 solutions, and conduct solidification experiments using 5wt.% to 15wt.% MgSO4 solutions as samples. The following is a summary of the experimental method. I Cool the sample sealed in the sample room at 0.4[°C/min] while applying an electric field of 0.0~1.1×103[V/m]. II Record the temperature just before the sample solidifies (hereinafter referred to as the solidification temperature). Measurements and observations are carried out by measuring the temperature using a K-type thermocouple, measuring the current value, and observing under an optical microscope. In this study, especially when 15 wt.% MgSO4 solution was used as the sample in this study, the freezing temperature increased from -10.3±1.0[°C] (0.0[V/m]) to -7.5±1.6[°C] (1.1×103[V/m]), an increase of 2.8°C due to the electrofreezing effect. Applying these results to the interior of an ice satellite (with relatively high MgSO4 concentrations) suggests that the internal ocean and partially melted regions within the ice shell may freeze earlier with cooling. However, the current experimental conditions are conducted under higher electric field conditions than the actual conditions inside the icy satellites, so lower electric field conditions and a different salt candidate (NaCl, Na2SO4) will be used for additional verification of the electrofreezing effect.