Presentation Information
[PPS01-P01]Estimating the Depletion Timescale of Ocean-Derived Sulfates on Europa’s Surface Based on Charged-Particle Irradiation Experiments Reproducing Europa’s Surface Charging Environment
*masaya kudo1, Tomoki Kimura1, Qingyang Lu1, Osawa Kai1, Shuya Tan2, Masahisa Kato4, Fuminori Tsuchiya3 (1.Tokyo university of science, 2.Japan Agency for Marine-Earth Science and Technology, 3.Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, 4.Kyoto University)
Keywords:
Europa,Planetary Science,Radiation Chemistry,Surface Charging
Jupiter’s moon Europa is thought to possess a subsurface ocean beneath its icy crust, and constraining the ocean’s chemical composition is essential for assessing its habitability. Two main hypotheses have been proposed for the origin of Europa’s surface materials: an exogenic source from Io's volcanic gas influx and micrometeoroid impacts (Cooper et al., 2001), and an endogenic source via transport from the subsurface ocean (Kargel et al., 2000). Determining the origin and composition of surface materials would provide key constraints on the subsurface ocean’s composition. However, magnetospheric particle bombardment drives radiolysis that alters surface materials, complicating the identification of their source and pre-alteration composition.
Otsuki (master’s thesis, Tokyo University of Science, 2024) reproduced radiolysis and resultant depletion by irradiating magnesium sulfate (MgSO4), a candidate Europa's surface material, with electrons, oxygen ions, and hydrogen ions. From the inferred lifetime, that study estimated a surface age of 6.2E+2 years and suggested an endogenic origin for the sulfate-related materials in linear features reported by Carlson et al. (2009). However, Otsuki (2024) used single-species irradiation, which may have charged the sample more strongly than under Europa's conditions, likely producing a surface potential of up to 10 kV, and thereby electrostatically shielding the surface from subsequent particle flux. In Europa's environment, simultaneous impacts of positive and negative particles can mitigate surface charging; the surface potential is theoretically estimated to be typically -14 to -52 V (Reddy et al., 2024). Accordingly, it is necessary to evaluate whether the radiolysis and depletion time reported by Otsuki (2024) realistically reproduce Europa’s surface conditions.
Here, we reproduce the radiolytic process and re-evaluate the depletion time by simultaneously irradiating MgSO4 with hydrogen ions and electrons, reproducing Europa's surface charging environment for the first time. In our experiments, MgSO4 was irradiated for approximately 30 minutes with simultaneous electron and hydrogen-ion fluxes of 6.3E+14(/cm^2/s) and 2.4E+14(/cm^2/s), respectively. Sulfur dioxide (SO2), hydrogen sulfide (H2S), sulfuric acid (H2SO4), sulfur tetroxide (SO4), and sulfur allotrope (S8) were detected in experiments of Otsuki (2024), however, H2SO4 and SO4 were not detected in our experiments, and SO2, H2S, and S8 were identified as the primary products. From gas production rates measured with a mass spectrometer and infrared spectra of the irradiated sample surface, the corresponding yield for SO2, H2S, and S8 were estimated to be 7.9E+0, 3.2E-1, 5.9E+1, respectively. These rates correspond to 59, 3.4, and 9.9 times greater than the rates estimated by Otsuki (2024) that were weighted by the yields of H-only and e-only irradiation for SO2, H2S, and S8, respectively.
The MgSO4 depletion time was estimated at 8.3E+1 years on Europa from our experiments, indicating dissociation and degassing of the surface material about 7.5 times faster than the estimate by Otsuki (2024). This likely reflects reduced sample surface charging under simultaneous ion and electron bombardments. The surface potential is calculated to be 1.3 V based on the Mott-Smith and Langmuir theory (1926), allowing more particles to reach the surface than in single-species irradiation. The depletion time of 8.3E+1 years implies that sulfates can persist on the surface only if they are replenished on shorter timescales than the depletion time. These results suggest that sulfate-like materials observed by the Galileo NIMS in the linea (Carlson et al., 2009) were supplied by geological resurfacing—i.e., water plume from the subsurface ocean—within the past 8.3E+1 years.
We plan to introduce an electrostatic potential probe to measure the sample surface potential and to quantify charging effects on the charged particle incident flux and production rates of radiolysis products. These measurements will tightly constrain the physical model, enabling more accurate estimates of surface age based on the reproduction of Europa’s electromagnetic and chemical environments. This presentation reports these results and the current status of our ongoing work.
Otsuki (master’s thesis, Tokyo University of Science, 2024) reproduced radiolysis and resultant depletion by irradiating magnesium sulfate (MgSO4), a candidate Europa's surface material, with electrons, oxygen ions, and hydrogen ions. From the inferred lifetime, that study estimated a surface age of 6.2E+2 years and suggested an endogenic origin for the sulfate-related materials in linear features reported by Carlson et al. (2009). However, Otsuki (2024) used single-species irradiation, which may have charged the sample more strongly than under Europa's conditions, likely producing a surface potential of up to 10 kV, and thereby electrostatically shielding the surface from subsequent particle flux. In Europa's environment, simultaneous impacts of positive and negative particles can mitigate surface charging; the surface potential is theoretically estimated to be typically -14 to -52 V (Reddy et al., 2024). Accordingly, it is necessary to evaluate whether the radiolysis and depletion time reported by Otsuki (2024) realistically reproduce Europa’s surface conditions.
Here, we reproduce the radiolytic process and re-evaluate the depletion time by simultaneously irradiating MgSO4 with hydrogen ions and electrons, reproducing Europa's surface charging environment for the first time. In our experiments, MgSO4 was irradiated for approximately 30 minutes with simultaneous electron and hydrogen-ion fluxes of 6.3E+14(/cm^2/s) and 2.4E+14(/cm^2/s), respectively. Sulfur dioxide (SO2), hydrogen sulfide (H2S), sulfuric acid (H2SO4), sulfur tetroxide (SO4), and sulfur allotrope (S8) were detected in experiments of Otsuki (2024), however, H2SO4 and SO4 were not detected in our experiments, and SO2, H2S, and S8 were identified as the primary products. From gas production rates measured with a mass spectrometer and infrared spectra of the irradiated sample surface, the corresponding yield for SO2, H2S, and S8 were estimated to be 7.9E+0, 3.2E-1, 5.9E+1, respectively. These rates correspond to 59, 3.4, and 9.9 times greater than the rates estimated by Otsuki (2024) that were weighted by the yields of H-only and e-only irradiation for SO2, H2S, and S8, respectively.
The MgSO4 depletion time was estimated at 8.3E+1 years on Europa from our experiments, indicating dissociation and degassing of the surface material about 7.5 times faster than the estimate by Otsuki (2024). This likely reflects reduced sample surface charging under simultaneous ion and electron bombardments. The surface potential is calculated to be 1.3 V based on the Mott-Smith and Langmuir theory (1926), allowing more particles to reach the surface than in single-species irradiation. The depletion time of 8.3E+1 years implies that sulfates can persist on the surface only if they are replenished on shorter timescales than the depletion time. These results suggest that sulfate-like materials observed by the Galileo NIMS in the linea (Carlson et al., 2009) were supplied by geological resurfacing—i.e., water plume from the subsurface ocean—within the past 8.3E+1 years.
We plan to introduce an electrostatic potential probe to measure the sample surface potential and to quantify charging effects on the charged particle incident flux and production rates of radiolysis products. These measurements will tightly constrain the physical model, enabling more accurate estimates of surface age based on the reproduction of Europa’s electromagnetic and chemical environments. This presentation reports these results and the current status of our ongoing work.