Measuring lunar water more reliably by accounting for vented vapor
image: The ground-based experimental platform: (a) system schematic, (b) autosampler, and (c) laboratory setup. Controlled water-vapor injection and pressure monitoring were used to characterize vapor loss during the venting process. Source: Fig. 2 of the article.Figure 1. The ground-based experimental platform: (a) system schematic, (b) autosampler, and (c) laboratory setup. Controlled water-vapor injection and pressure monitoring were used to characterize vapor loss during the venting process. Source: Fig. 2 of the article.
Credit: HIGHER EDUCATION PRESS
Accurately measuring water in lunar regolith can be challenging, particularly for samples with relatively high water contents. Heating releases water vapor, but the resulting pressure buildup inside the measurement chamber can saturate the spectroscopic signal. Controlled venting prevents excessive pressure buildup but also removes some of the water being measured. Correcting for this loss is therefore critical to accurately determining the original water contents in the measured samples.
In a research article published in Planet, Rui Chen and colleagues from Macau University of Science and Technology, the Chinese Academy of Sciences, and collaborating institutions investigate this challenge in the context of the Chang’E-7 Lunar Soil Water Molecule Analyzer (LSWMA). They develop and experimentally validate a method that accounts for water vapor lost during dynamic venting, improving the accuracy of water-content estimates.
Using an experimental platform designed to reproduce the relevant geometry of the instrument, the team established an empirical relationship between chamber pressure and the rate of pressure decrease during venting. By integrating this relationship over time, they quantified cumulative vapor loss and incorporated it into the calculation of the initial water content of the measured sample.
The method was further evaluated using simulated lunar regolith in experiments covering sample transfer, thermal extraction, and dynamic venting. Combined with previously established corrections for other sources of water loss, the method yielded estimation error within ±20% for the tested samples, a substantial improvement over the original ±50% accuracy specification of the instrument.
A key contribution of this study is the quantitative link established between instrument pressure regulation and water loss during measurement. By correcting for vapor released through venting, the method improves water-content estimates for relatively water-rich samples and provides a useful basis for future lunar water resource investigations. Further validation remains necessary to assess the method under more realistic measurement conditions. The venting calibration used pure water vapor, whereas heated lunar regolith may release mixed gases with different flow behavior. Non-equilibrium adsorption and desorption processes could introduce additional uncertainties. Further experiments and physical modeling will be important for evaluating the method under more complex conditions and determining its applicability to other instrument configurations.