Calibration of the Mass Spectrometer of China's Chang'E-7 Lunar Soil Water Molecule Analyzer

The existence of water ice at the lunar south pole has become a major topic in contemporary deep-space exploration. China's Chang'e-7 lunar mission is scheduled for launch in 2026. One of its primary scientific objectives is to conduct an in situ investigation of water ice within a permanently shadowed crater at the lunar south pole. The candidate landing site is located near the Shackleton crater. During the exploration, lunar regolith samples will be drilled to a depth of approximately 1 m within the permanently shadowed crater at the lunar south pole and transferred to a furnace to be heated to 200 °C. The H2O and other volatile components released during heating will be introduced into the LSWMA, Lunar Soil Water Molecule Analyzer, which will measure the composition and amount of gases released from the lunar regolith. We developed the LSWMA and established a ground-based mass spectrometer calibration device capable of calibrating gas-liquid mixtures. The flight model of the mass spectrometer was calibrated using CO2 and H2O under different temperatures and inlet pressures. The sensitivity of CO2, S*(I/p)CO2, and the sensitivity of H2O, S*(I/p)H2O, were obtained under different pressures and temperatures, and the relative sensitivity of CO2 to H2O, Sr, was then calculated. Under molecular-flow inlet conditions, the mass spectrometer's relative sensitivity remains constant. Based on this principle, the sensitivity of H2O can be extrapolated from the sensitivity of CO2 under on-orbit conditions. Ultimately, this allows for the accurate inversion of the partial pressure of H2O gas in the tubing assembly and the H2O content of the lunar soil.