講演情報
[PPS01-P02]Na2CO3への荷電粒子照射実験に基づくエウロパ表面でのCO2生成と炭酸塩枯渇の評価
*盧 清揚1、木村 智樹1、丹 秀也2、土屋 史紀3、仲内 悠祐4 (1.東京理科大学、2.国立研究開発法人海洋研究開発機構、3.東北大学大学院 理学研究科惑星プラズマ・大気研究センター、4.立命館大学)
キーワード：
エウロパ、氷衛星、照射実験
Jupiter’s moon Europa has a subsurface ocean beneath an icy crust and is a prime target for assessing habitability [Chyba et al. 2001]. Recent James Webb Space Telescope (JWST) observations detected carbon dioxide (CO2) absorption features at 4.25μm and 4.27μm concentrated in Tara Regio, a terrain recently resurfaced by the water plume that possibly facilitates material transport from the subsurface ocean on Europa, implying either delivery of carbon-bearing species from the ocean or in situ surface production driven by interaction with Jupiter’s magnetosphere [Villanueva et al. 2023]. CO2 detected on Europa’s surface could be produced from carbonate salts (e.g. Na2CO3, NaHCO3), yet carbonate bands have not been observed on Europa. One proposed explanation is that strong absorption by H2O and H2O2 can mask carbonate bands [Trumbo et al. 2023], whereas carbonates have been reported on other icy moons such as Ganymede and Enceladus [Tosi et al. 2023; Postberg et al. 2009]. This raises the question of whether irradiation of carbonates on Europa's orbit with Jupiter's magnetospheric plasma can sustain CO2 release over relevant fluence, or whether CO2 production rapidly decays as the carbonate is depleted.
Here we quantify volatile yields and time-evolving depletion signatures from Na2CO3 under high-fluence charged-particle irradiation. Vacuum-dried (150 °C for 24 h) Na2CO3 powder samples were irradiated at 300 K with 10 keV electrons and ions (H2+, O2+) using our plasma irradiation system [Kimura et al. 2023]. The volatile yields (molecules per incident particle/atom) were estimated from the pressure measured by a quadrupole mass spectrometer (QMS). Under the hydrogen irradiation with a fluence of 6.81 × 1018 atoms cm-2 and a particle flux of 6.31 × 1014 cm-2 s-1, the outgassing of H2O and CO2 decreased with time. The H2O partial pressure first increased then rapidly decreased to background level, suggesting that H2O signal mainly reflects the adsorbed water that desorbed early in the irradiation. CO2 was produced continuously, but its production rate slowly decreased over time. The yields calculated by integrating the production rate of the full irradiation interval were estimated to be 0.45 for H2O and 0.74 for CO2. Under the oxygen irradiation with a fluence of 2.49 × 1018 atoms cm-2 and a particle flux of 2.31 × 1014 cm-2 s-1, H2O also depleted rapidly, however, in contrast to the gradual decrease of CO2 under the hydrogen irradiation, CO2 depleted much more rapidly and dropped below the background at nearly the same time as H2O. The yields were estimated to be 0.98 for H2O and 0.41 for CO2. In contrast, the electron irradiation with a fluence of 7.18 × 1018 e- cm-2 and a particle flux of 6.68 × 1014 cm-2 s-1 showed nearly steady H2O and CO2 production, with estimated yields of 1.9 for H2O, and 0.95 for CO2. Production rate of CO2 increased toward the end of the irradiation, suggests that e- irradiation has the largest penetration depth. In contrast, under the hydrogen/oxygen irradiation the penetration depth was smaller, hydrogen/oxygen atoms and molecules penetrated only into the shallow surface layer of the sample. Consequently, CO2 production dropped below the background level during the irradiation.
To obtain the CO2 yield under more realistic surface conditions, we will perform simultaneous irradiation of e- and O2+, H2+ to model possible sample surface charging, and extend the experiments to 80–130 K to better reproduce Europa-like conditions. We will present the current status of our study in the presentation.
Here we quantify volatile yields and time-evolving depletion signatures from Na2CO3 under high-fluence charged-particle irradiation. Vacuum-dried (150 °C for 24 h) Na2CO3 powder samples were irradiated at 300 K with 10 keV electrons and ions (H2+, O2+) using our plasma irradiation system [Kimura et al. 2023]. The volatile yields (molecules per incident particle/atom) were estimated from the pressure measured by a quadrupole mass spectrometer (QMS). Under the hydrogen irradiation with a fluence of 6.81 × 1018 atoms cm-2 and a particle flux of 6.31 × 1014 cm-2 s-1, the outgassing of H2O and CO2 decreased with time. The H2O partial pressure first increased then rapidly decreased to background level, suggesting that H2O signal mainly reflects the adsorbed water that desorbed early in the irradiation. CO2 was produced continuously, but its production rate slowly decreased over time. The yields calculated by integrating the production rate of the full irradiation interval were estimated to be 0.45 for H2O and 0.74 for CO2. Under the oxygen irradiation with a fluence of 2.49 × 1018 atoms cm-2 and a particle flux of 2.31 × 1014 cm-2 s-1, H2O also depleted rapidly, however, in contrast to the gradual decrease of CO2 under the hydrogen irradiation, CO2 depleted much more rapidly and dropped below the background at nearly the same time as H2O. The yields were estimated to be 0.98 for H2O and 0.41 for CO2. In contrast, the electron irradiation with a fluence of 7.18 × 1018 e- cm-2 and a particle flux of 6.68 × 1014 cm-2 s-1 showed nearly steady H2O and CO2 production, with estimated yields of 1.9 for H2O, and 0.95 for CO2. Production rate of CO2 increased toward the end of the irradiation, suggests that e- irradiation has the largest penetration depth. In contrast, under the hydrogen/oxygen irradiation the penetration depth was smaller, hydrogen/oxygen atoms and molecules penetrated only into the shallow surface layer of the sample. Consequently, CO2 production dropped below the background level during the irradiation.
To obtain the CO2 yield under more realistic surface conditions, we will perform simultaneous irradiation of e- and O2+, H2+ to model possible sample surface charging, and extend the experiments to 80–130 K to better reproduce Europa-like conditions. We will present the current status of our study in the presentation.