A dual-role solid calibration strategy for Chang’E-7 LUWA: Onboard CO2 referencing and ground-based H2O/δD validation
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GRAPHICAL ABSTRACT
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PUBLIC SUMMARY
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Chang’E-7’s LUnar soil Water molecule Analyzer (LUWA) will measure lunar polar water and hydrogen isotopes.
Solid materials were developed to release CO2 or H2O under vacuum with modest heating.
Ag2CO3 enables repeated onboard CO2 referencing through controlled partial gas release.
Traceable Cu(OH)2 provides an H2O and hydrogen-isotope reference for ground calibration.
These solid references support calibration of LUWA water and hydrogen-isotope measurements.
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ABSTRACT
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. -
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Chen R., Li X., Lu W., et al. (2026). A dual-role solid calibration strategy for Chang’E-7 LUWA: Onboard CO2 referencing and ground-based H2O/δD validation. The Innovation Informatics 2:100062. https://doi.org/10.59717/j.xinn-inform.2026.100062
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Chen R., Li X., Lu W., et al. (2026). A dual-role solid calibration strategy for Chang’E-7 LUWA: Onboard CO2 referencing and ground-based H2O/δD validation. The Innovation Informatics 2:100062. https://doi.org/10.59717/j.xinn-inform.2026.100062
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Figure 1.
Ground calibration platform and LUWA prototype
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Figure 2.
Time-resolved δD measurements of H2O released from two commercial analytical-grade Cu(OH)2 samples obtained from the same batch.
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Figure 3.
Solid-isotope measurement platform based on the LUWA prototype.
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Figure 4.
Averaged vacuum thermogravimetric curves obtained from two independent measurements of CaCO3, MgCO3, Ag2CO3, and Cu(OH)2.
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Figure 5.
Spectroscopic and mass-spectrometric signals of gases evolved from Ag2CO3 and Cu(OH)2
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Figure 6.
SEM images of synthesized Cu(OH)2 at scale bars of (A) 100 μm, (B) 10 μm, and (C) 2 μm.
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Figure 7.
XRD pattern of the synthesized material compared with the reference diffraction pattern of Cu(OH)2 (PDF#99-000-
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Figure 8.
Time-resolved δD measurements of H2O released from synthesized Cu(OH)2 samples prepared using three precursor waters with different hydrogen isotopic compositions.
