A dual-role solid calibration strategy for Chang’E-7 LUWA: Onboard CO₂ referencing and ground-based H₂O/δD validation

Lunar polar water ice is a key target of the Chang’E-7 mission because its abundance, physical state, and origin remain insufficiently constrained. Within the Chang’E-7 multi-payload framework, the LUnar soil Water molecule Analyzer (LUWA) is designed to measure H₂O abundance and hydrogen isotopic composition in permanently shadowed regions. Reliable measurements require calibration compatible with high vacuum, low power consumption, limited sealing capability, and restricted heating temperatures, conditions under which conventional gas- and liquid-phase standards are poorly suited. Here, we develop and validate two complementary solid reference materials for LUWA. Vacuum thermogravimetric analysis showed that Ag₂CO₃ and Cu(OH)₂ decompose at approximately 100–120 °C, releasing CO₂ and H₂O, respectively. Spectroscopic and mass-spectrometric measurements identified the expected dominant signals, with no obvious interfering gaseous species detected within the measured ranges. In the current LUWA implementation, Ag₂CO₃ was selected as the onboard CO₂ reference material for in situ calibration, whereas traceable Cu(OH)₂ was developed primarily for ground-based H₂O and δD calibration, prototype testing, end-to-end validation, and controlled regolith-simulant experiments. Cu(OH)₂ synthesized from precursor waters with known δD values preserved the mean isotopic information of the water source, with absolute differences between precursor-water and solid-derived vapor means within 10‰. Independent Cr-EA/IRMS measurements further supported the isotope-measurement workflow. Beyond its current role, traceable Cu(OH)₂ can serve as a solid isotope standard for validating heating, gas release, transfer, pressure measurement, and spectroscopic or mass-spectrometric detection. It also provides a controllable H₂O and δD source for regolith simulants with known water contents and for future characterization of adsorption–desorption memory effects, baseline drift, and pressure-dependent isotope response. With further storage, contamination-control, and flight-qualification studies, Cu(OH)₂-type materials may support future planetary payloads requiring controlled H₂O and hydrogen-isotope references. Together, these results establish a dual-role solid calibration strategy for LUWA.