09:30 〜 09:45
[PPS06-03] Oxygen ion irradiation experiments on Phobos candidate materials: Implications on the unique space weathering environment of the Martian moons
★Invited Papers
キーワード：火星、フォボス、宇宙風化、大気進化
Reconstructing the history of atmospheric loss on Mars is crucial to understanding when and how the aqueous environment was lost in the past. Recent observations by MAVEN have revealed that oxygen ions (O+) derived from the escaping Martian atmosphere are actively precipitating on Martian moons (e.g., Dong et al., 2015). This suggests that the surface material of Martian moons may have undergone space weathering by the oxygen ion, unique to the Martian system. The largest moon, Phobos, is a target of the MMX (Martian Moons eXploration) mission, and a sampling campaign is planned together with spectroscopic observations, which will provide clues to reconstruct the Martian atmospheric loss history (Kuramoto et al., 2022). However, no experimental studies have investigated the effect of space weathering by oxygen ions on materials related to the Martian system. In this study, we conducted laboratory experiments to investigate the effects of oxygen ions on candidate materials of Phobos and measured changes in the spectrum by reflectance spectroscopy.
To simulate the oxygen ions precipitating on Phobos with energies at 1–1000 eV (Nenon et al., 2021), we employed a 2450 MHz microwave induced plasma, which provides an electron temperature of approximately ~10 eV. The experimental setup is a vacuum system with a plasma irradiation chamber of 30 cm quartz glass. Oxygen was supplied from a gas cylinder and was continuously pumped to achieve a steady state with an oxygen plasma at a range of 10–100 Pa. A current probe was installed at downstream of the plasma chamber to measure oxygen ion flux by applying a negative voltage. Irradiation of oxygen plasma was conducted for olivines (Pakistan, ~Fo90), Alkalic olivine basalts (Hawaii, MacDonald and Katsura, 1964), and a Phobos simulant (Miyamoto et al., 2021). Reflectance spectroscopy in the range of 400–900 nm (Cho et al., 2021) was performed on the samples before and after irradiation to examine changes in the reflectance.
After the irradiation for 30 hours, which is equivalent to 104 years on Phobos considering the flux at the sub-Martian point (Nenon et al., 2021), the reflectance spectra of the samples show various changes. We observed an increase in reflectance (i.e., brightening) and a decrease in spectral slope (i.e., bluing) over the 400–900 nm range for the olivine and olivine basalt. Phobos simulant changed from the original dark black color to gray after irradiation. The reflectance increased from its initially low value (~3%) to >15%.
The changes in reflectance for olivine, basalt, and a simulant due to irradiation of oxygen ions suggest the possibility that Martian atmospheric oxygen might be contributing to the space weathering on Phobos. The observed bluing in olivine and olivine basalt are similar to the spectral changes reported in previous studies which examined the spectra of olivine-magnetite and olivine-hematite mixture (Bou-Orm et al., 2022), indicating the potential formation of iron oxides. Furthermore, the significant increase in reflectance observed in the Phobos simulant may be attributed to the oxidation and volatilization of nano-carbon, which is added for 5 wt.% to match the observed low reflectance. These results imply that the organic content on the uppermost Phobos surface may be low. Although the timescale for resurfacing on Phobos remains unknown, estimates for Itokawa and Ryugu range from 103 to 106 years (Nagao et al., 2011; Takaki et al., 2022; Okazaki et al., 2022). Investigating the vertical profile of oxygen on the irradiated surface of the experimental samples will provide insights into the relationship between irradiation dose and vertical distribution. Together with a surface age constrained by other techniques (e.g., cosmic ray exposure ages) through the analysis of MMX's return samples, our results would contribute to reconstructing Martian atmospheric loss history.
To simulate the oxygen ions precipitating on Phobos with energies at 1–1000 eV (Nenon et al., 2021), we employed a 2450 MHz microwave induced plasma, which provides an electron temperature of approximately ~10 eV. The experimental setup is a vacuum system with a plasma irradiation chamber of 30 cm quartz glass. Oxygen was supplied from a gas cylinder and was continuously pumped to achieve a steady state with an oxygen plasma at a range of 10–100 Pa. A current probe was installed at downstream of the plasma chamber to measure oxygen ion flux by applying a negative voltage. Irradiation of oxygen plasma was conducted for olivines (Pakistan, ~Fo90), Alkalic olivine basalts (Hawaii, MacDonald and Katsura, 1964), and a Phobos simulant (Miyamoto et al., 2021). Reflectance spectroscopy in the range of 400–900 nm (Cho et al., 2021) was performed on the samples before and after irradiation to examine changes in the reflectance.
After the irradiation for 30 hours, which is equivalent to 104 years on Phobos considering the flux at the sub-Martian point (Nenon et al., 2021), the reflectance spectra of the samples show various changes. We observed an increase in reflectance (i.e., brightening) and a decrease in spectral slope (i.e., bluing) over the 400–900 nm range for the olivine and olivine basalt. Phobos simulant changed from the original dark black color to gray after irradiation. The reflectance increased from its initially low value (~3%) to >15%.
The changes in reflectance for olivine, basalt, and a simulant due to irradiation of oxygen ions suggest the possibility that Martian atmospheric oxygen might be contributing to the space weathering on Phobos. The observed bluing in olivine and olivine basalt are similar to the spectral changes reported in previous studies which examined the spectra of olivine-magnetite and olivine-hematite mixture (Bou-Orm et al., 2022), indicating the potential formation of iron oxides. Furthermore, the significant increase in reflectance observed in the Phobos simulant may be attributed to the oxidation and volatilization of nano-carbon, which is added for 5 wt.% to match the observed low reflectance. These results imply that the organic content on the uppermost Phobos surface may be low. Although the timescale for resurfacing on Phobos remains unknown, estimates for Itokawa and Ryugu range from 103 to 106 years (Nagao et al., 2011; Takaki et al., 2022; Okazaki et al., 2022). Investigating the vertical profile of oxygen on the irradiated surface of the experimental samples will provide insights into the relationship between irradiation dose and vertical distribution. Together with a surface age constrained by other techniques (e.g., cosmic ray exposure ages) through the analysis of MMX's return samples, our results would contribute to reconstructing Martian atmospheric loss history.