Ne-Ar separation using a permeable membrane to measure Ne isotopes for future planetary explorations

Abstract

We propose a method using a permeable membrane to separate Ne from Ar for Ne isotope analyses in a future planetary exploration, in particular, for the Martian surface. The results of simulation based on permeation data and theoretical formulas show that significant amounts of 20Ne without 40Ar permeate through Viton (fluoropolymer elastomer) and Kapton (polyimide) with appropriate thicknesses. Considering the simulation results, the amounts of 20Ne and 40Ar that permeate through 1 ​mm-thick Viton and 125 ​μm-thick Kapton were determined experimentally. The amounts of permeated 20Ne agree with those of 20Ne obtained by the theoretical calculation within experimental uncertainties. The amounts of permeated 40Ar are as small as those of the background, which is also consistent with the fact that the calculated amounts of 40Ar through those membranes are several orders of magnitude lower than the background 40Ar. In the present experiments, 20Ne/40Ar ratios increased up to 1 for Viton and 200 for Kapton from the terrestrial atmospheric ratio of 1.8 ​× ​10−3 after permeation for 30 and 60 ​min, respectively. A Kapton sheet efficiently enhances the 20Ne/40Ar ratio and enables measurement of the Ne isotope composition without the significant 40Ar++ interference in planetary exploration. Assuming the Martian atmospheric pressure (7 ​hPa) and composition (2.3 ​ppm for 20Ne and 1.9% for 40Ar), the amounts of 20Ne and 40Ar that permeate through Kapton were calculated. The 20Ne/40Ar ratio can be increased up to ca. 1 using 75 ​μm-thick Kapton with a duration time of 30–60 ​min, where the contribution of 40Ar++ is approximately 10% of 20Ne+ and the uncertainty after the 40Ar++ correction is estimated to be a few percent. An effective area of 50 ​cm2 is required if the sensitivity of the QMS used in this study is assumed.