Presentation Information
[PPS01-P03]Constraints on the Na/K Ratio in Europa’s Surface and Subsurface Ocean by Charged-Particle Irradiation Experiments and Observations of Europa’s Tenuous Atmosphere
*Osawa Kai1, Tomoki Kimura1, Fuminori Tsuchiya2, Shuya Tan3, Kazuya Yoshioka1, Shoki Morioka1, Qingyang Lu1, Go Sato1, Masaya Kudo1 (1.Tokyo University of Science , 2.Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, 3.Japan Agency for Marine-Earth Science and Technology)
Europa, a satellite of Jupiter, is thought to contain a liquid ocean beneath its global iceshell. This subsurface ocean makes Europa one of the most promising environments forpotential life in the solar system. Surface reflectance spectra from the Hubble Space Telescope suggested that salts such as NaCl and KCl may be present on Europa's surface [Trumbo et al., 2019; Volwerk et al., 2001]. Observations with the Kuiper and Keck telescopes have detected Na and K in Europa's tenuous atmosphere [Brown & Hill, 1996; Brown, 2001]. These elements are thought to originate from the surface salts desorbed and sputtered by charged particles in Jupiter's magnetosphere. It remains unknown whether the surface salts originate from the endogenous material supplied by the subsurface ocean or from the exogenous sources, Iogenic plasma in Jupiter's magnetosphere. Determining the origin of these materials is important for understanding the chemistry of Europa's subsurface ocean.
The Na/K ratio on Europa is a crucial indicator for investigating the origin of surface salts, as it varies depending on the source of the salts and the release processes into the tenuous atmosphere. Observations of Europa's atmosphere estimated the Na/K ratio of 25 +/- 3, while the surface Na/K ratio was suggested to vary with its source, about 20 for the endogenous material and about 10 for the exogenous material [Brown, 2001; Zolotov & Shock, 2001]. The previous studies evaluated whether these surface ratios can accountfor the atmospheric Na/K ratio using charged-particle irradiation experiments on single-component salts or metallic Na and K atoms deposited on ice, along with numerical simulations. These studies suggested that a surface Na/K ratio of 10 cannot explain the atmospheric ratio of 25 [Johnson et al., 2002]. However, these studies focused only on the single-component salts or metal atoms deposited on ice. Moreover, realistic irradiation experiments on NaCl-KCl mixed samples, which are expected to coexist on Europa's surface, have not been carried out. Thus, it is still unclear how salts such as NaCl and KCl on Europa's surface change their Na/K ratio under charged-particle irradiation and how they contribute to the Na/K ratio in the tenuous atmosphere.
Here, we irradiated NaCl-KCl mixed samples simulate Europa's surface with 10 keV electrons modeling Jupiter's magnetospheric plasma (10 keV, 7.96E+13-1.99E+14 /cm^2/s, 90 min) at room temperature and evaluated the degassing yields of Na and K. We used NaCl-KCl mixed samples with initial Na/K ratios of 10 and 20, measured the partial pressures of the degassed particles with a mass spectrometer, and determined their Na/K ratios. As a result, the Na/K ratios of the degassed particles in the steady state were significantly lower than the initial compositions for both samples. For the Na/K = 10 sample, the degassing Na/K ratio ranged from 0.10-0.27, and for the Na/K = 20 sample, it ranged from 0.28-0.46. These results indicate that K is preferentially degassed relative to Na, leading the Na/K ratios to be far below 1.
Johnson et al. (2002) reported that the metallic Na and K atoms deposited on ice exhibited similar kinetic energy distributions and comparable desorption efficiencies at 200 eV electron irradiation, suggesting that the Na/K ratio immediately after desorption should match the surface composition. In contrast, our measurements showed the first experimental evidence that the atmospheric Na/K ratio does not correspond to the surface Na/K ratio. The Na/K ratios obtained in this study (0.10-0.27 and 0.28-0.46) deviate greatly from the initial values. This discrepancy may be attributed to differences in electron energy and sample form, which affect the desorption behavior of Na and K. We will perform ion-irradiation experiments at 80-100 K closer to Europa's environment to evaluate the temperature dependence of degassing and the effects of knock-on sputtering.
The Na/K ratio on Europa is a crucial indicator for investigating the origin of surface salts, as it varies depending on the source of the salts and the release processes into the tenuous atmosphere. Observations of Europa's atmosphere estimated the Na/K ratio of 25 +/- 3, while the surface Na/K ratio was suggested to vary with its source, about 20 for the endogenous material and about 10 for the exogenous material [Brown, 2001; Zolotov & Shock, 2001]. The previous studies evaluated whether these surface ratios can accountfor the atmospheric Na/K ratio using charged-particle irradiation experiments on single-component salts or metallic Na and K atoms deposited on ice, along with numerical simulations. These studies suggested that a surface Na/K ratio of 10 cannot explain the atmospheric ratio of 25 [Johnson et al., 2002]. However, these studies focused only on the single-component salts or metal atoms deposited on ice. Moreover, realistic irradiation experiments on NaCl-KCl mixed samples, which are expected to coexist on Europa's surface, have not been carried out. Thus, it is still unclear how salts such as NaCl and KCl on Europa's surface change their Na/K ratio under charged-particle irradiation and how they contribute to the Na/K ratio in the tenuous atmosphere.
Here, we irradiated NaCl-KCl mixed samples simulate Europa's surface with 10 keV electrons modeling Jupiter's magnetospheric plasma (10 keV, 7.96E+13-1.99E+14 /cm^2/s, 90 min) at room temperature and evaluated the degassing yields of Na and K. We used NaCl-KCl mixed samples with initial Na/K ratios of 10 and 20, measured the partial pressures of the degassed particles with a mass spectrometer, and determined their Na/K ratios. As a result, the Na/K ratios of the degassed particles in the steady state were significantly lower than the initial compositions for both samples. For the Na/K = 10 sample, the degassing Na/K ratio ranged from 0.10-0.27, and for the Na/K = 20 sample, it ranged from 0.28-0.46. These results indicate that K is preferentially degassed relative to Na, leading the Na/K ratios to be far below 1.
Johnson et al. (2002) reported that the metallic Na and K atoms deposited on ice exhibited similar kinetic energy distributions and comparable desorption efficiencies at 200 eV electron irradiation, suggesting that the Na/K ratio immediately after desorption should match the surface composition. In contrast, our measurements showed the first experimental evidence that the atmospheric Na/K ratio does not correspond to the surface Na/K ratio. The Na/K ratios obtained in this study (0.10-0.27 and 0.28-0.46) deviate greatly from the initial values. This discrepancy may be attributed to differences in electron energy and sample form, which affect the desorption behavior of Na and K. We will perform ion-irradiation experiments at 80-100 K closer to Europa's environment to evaluate the temperature dependence of degassing and the effects of knock-on sputtering.