16:30 〜 16:45
[PCG18-17] Development of a method to separate Ne from Ar using a polyimide sheet for future planetary missions
キーワード：Planetary mission、Martian atmosphere、Mars、Neon、Noble gas
The elemental and isotopic compositions of planetary atmospheres provide knowledge on their origin and evolution, including atmospheric escape, degassing from interior, and chemical processes in the surface environment. Since noble gases are inert in chemical reaction, they reflect only physical processes in planetary evolution. Among noble gases, neon is a useful element to trace the degree of loss rate and degassing for its high volatility (sensitive to physical phenomena) and its moderate atomic masses (20, 21, and 22). Because it has multi (three) isotopes, a variety of isotopic analysis can be made using two isotopic ratios. Kurokawa et al. (2021) [1] applied model calculation using currently available Ne data for Mars. They concluded that Ne has to be continuously supplied to the atmosphere and that the present atmospheric Ne on Mars does not reflect primordial trapped atmosphere and is most likely dominated by mantle Ne supplied recently (< 1 Ga) via volcanic degassing.
Despite its importance, however, isotopic measurement of Ne has been difficult. Only 22Ne has been measured in previous Mars missions [2]. The abundance of the dominant isotope (i.e., 20Ne) could not be measured by interference of double-charged-ions of 40Ar++ to single-charged-ions of 20Ne+. The isotope of 21Ne has not been measured due to its low abundance; 21Ne/20Ne is ~0.003 for the terrestrial atmospheric and Solar Ne. Furthermore, CO2++ interferes with 22Ne+. The 20Ne/22Ne ratio of the Martian atmosphere was estimated from Ne trapped in Martian meteorites so far, for which somewhat different ratios, ranging from ~7 to ~10, have been reported (e.g., [3-5]). These variable results may suffer from cosmogenic isotopes generated in the pathway of the meteorites from Mars to Earth. Thus, in-situ measurements on Mars or gas sample return from Mars have been argued for [6].
For accurate Ne isotopic measurements, the abundances of Ar and CO2 must be reduced before gas sample is introduced to a mass spectrometer. Carbon dioxide can be removed chemically using a getter. Because Ar is also an inert gas, however, we cannot use such a chemical method. Therefore, we recently proposed a new method using a permeable membrane to separate Ne from Ar [7]. The previous study demonstrated the fundamental principle that a polyimide sheet with about 100 μm in thickness can reasonably separate Ne from Ar. Nevertheless, we did not examine practical details of this method, such as how to achieve a stable vacuum seal between a polyimide sheet and metal flanges in high vibration and shock conditions expected in launch and landing. In this study, we developed a measurement system in which a polyimide sheet is stably sealed with metal flanges to withstand vibration and shock conditions for future planetary missions. Using the new system, we also analyze the amounts of its background gases and permeated Ne throughout the polyimide sheet under the terrestrial atmosphere. The present method uses a metal sealing gasket and VF-VG flanges to fix the polyimide sheet as shown in the figure. Our preliminary results show that the amounts of 20Ne and 40Ar permeated through a 100-µm-thick polyimide sheet with 28 cm2 in area during 40 min. are ~1 x 10-7 cm3 STP and not a detectable amount beyond the present system-background level (~5 x 10-9 cm3STP), respectively. This Ne/Ar separation capability is consistent with the results obtained in the previous study [7]. We plan to reduce background Ar and to examine temperature dependence of permeation and shock resisting.
References: [1] Kurokawa et al. (2021) Icarus 370, 114685. [2] Owen et al. (1977) J. Geophys. Res. 82, 4635-4639. [3] Swindle et al. (1986) Geochim. Cosmochim. Acta 50, 1001-1015. [4] Garrison and Bogard (1988) Meteorit. Plant. Sci. 33, 721-736. [5] Park et al. (2017) Lunar Planet. Sci. XLVIII, abstract #1157. [6] Swindle et al. (2021) Astrobiology, doi: 10.1089/AST.2021.0107. [7] Miura et al. (2020) Planet. Space Sci. 193, 105046.
Despite its importance, however, isotopic measurement of Ne has been difficult. Only 22Ne has been measured in previous Mars missions [2]. The abundance of the dominant isotope (i.e., 20Ne) could not be measured by interference of double-charged-ions of 40Ar++ to single-charged-ions of 20Ne+. The isotope of 21Ne has not been measured due to its low abundance; 21Ne/20Ne is ~0.003 for the terrestrial atmospheric and Solar Ne. Furthermore, CO2++ interferes with 22Ne+. The 20Ne/22Ne ratio of the Martian atmosphere was estimated from Ne trapped in Martian meteorites so far, for which somewhat different ratios, ranging from ~7 to ~10, have been reported (e.g., [3-5]). These variable results may suffer from cosmogenic isotopes generated in the pathway of the meteorites from Mars to Earth. Thus, in-situ measurements on Mars or gas sample return from Mars have been argued for [6].
For accurate Ne isotopic measurements, the abundances of Ar and CO2 must be reduced before gas sample is introduced to a mass spectrometer. Carbon dioxide can be removed chemically using a getter. Because Ar is also an inert gas, however, we cannot use such a chemical method. Therefore, we recently proposed a new method using a permeable membrane to separate Ne from Ar [7]. The previous study demonstrated the fundamental principle that a polyimide sheet with about 100 μm in thickness can reasonably separate Ne from Ar. Nevertheless, we did not examine practical details of this method, such as how to achieve a stable vacuum seal between a polyimide sheet and metal flanges in high vibration and shock conditions expected in launch and landing. In this study, we developed a measurement system in which a polyimide sheet is stably sealed with metal flanges to withstand vibration and shock conditions for future planetary missions. Using the new system, we also analyze the amounts of its background gases and permeated Ne throughout the polyimide sheet under the terrestrial atmosphere. The present method uses a metal sealing gasket and VF-VG flanges to fix the polyimide sheet as shown in the figure. Our preliminary results show that the amounts of 20Ne and 40Ar permeated through a 100-µm-thick polyimide sheet with 28 cm2 in area during 40 min. are ~1 x 10-7 cm3 STP and not a detectable amount beyond the present system-background level (~5 x 10-9 cm3STP), respectively. This Ne/Ar separation capability is consistent with the results obtained in the previous study [7]. We plan to reduce background Ar and to examine temperature dependence of permeation and shock resisting.
References: [1] Kurokawa et al. (2021) Icarus 370, 114685. [2] Owen et al. (1977) J. Geophys. Res. 82, 4635-4639. [3] Swindle et al. (1986) Geochim. Cosmochim. Acta 50, 1001-1015. [4] Garrison and Bogard (1988) Meteorit. Plant. Sci. 33, 721-736. [5] Park et al. (2017) Lunar Planet. Sci. XLVIII, abstract #1157. [6] Swindle et al. (2021) Astrobiology, doi: 10.1089/AST.2021.0107. [7] Miura et al. (2020) Planet. Space Sci. 193, 105046.