Advancing Hydrogen Detection on Airless Planetary Bodies through Laser-Induced Breakdown Spectroscopy

Researchers from the University of Tokyo in Japan have made significant progress in accurately measuring hydrogen on airless planetary bodies, such as the Moon and asteroids, using the laser-induced breakdown spectroscopy (LIBS) technique (1). These findings, published in Spectrochimica Acta Part B: Atomic Spectroscopy, provide valuable insights into the spatial distributions and depth profiles of hydrogen-bearing materials, addressing previous uncertainties in remote-sensing observations.

Hydrogen-bearing materials have been detected on the surfaces of airless bodies through remote-sensing observations. However, determining their precise spatial distributions at small scales and depth profiles has been challenging. In this study, the researchers employed LIBS to conduct in situ analyses of hydrogen-bearing materials, leveraging its small ablation footprint and ability to penetrate into the subsurface.

To overcome calibration challenges and determine the measurement accuracy of hydrogen with LIBS on airless and hydrous planetary bodies, the researchers conducted two experiments. First, they measured compressed powder samples of rocks using LIBS under a vacuum condition. Various concentrations of hydroxyls were added to geostandards, enabling the preparation of hydrogen-rich samples. The researchers successfully calibrated the hydrogen concentration accurately and achieved a measurement accuracy of ±0.9 wt% H₂O in the 0–12 wt% H₂O range. This level of accuracy is crucial for planetary exploration applications, including water mining on the Moon.

In the second experiment, the researchers demonstrated the capability of LIBS to distinguish the molecular structure of hydrogen. By employing heating lasers in tandem with LIBS analysis, they were able to differentiate between ice and hydroxyl. The Hα lines of ice-bearing samples decreased after laser heating due to sublimation, while those of hydroxyl-bearing samples remained unchanged. Additionally, the study revealed an enhancement of hydrogen emission from loose powders, suggesting the potential for higher sensitivity measurements of hydrogen in lunar soils.

The results of this study highlight the versatility and power of LIBS as a stand-off measurement tool for accurately assessing hydrogen-bearing materials on airless planetary bodies. These findings contribute to the advancement of our understanding of these celestial bodies and have implications for future space exploration missions, resource utilization, and the development of hardware and operations for extracting water resources on the Moon.

Reference

(1) Yumoto, K.; Cho, Y.; Kameda, S.; Kasahara, S.; Sugita, S. In-situ measurement of hydrogen on airless planetary bodies using laser-induced breakdown spectroscopy. Spectrochim. Acta, Part B 2023, 205, 106696. DOI: 10.1016/j.sab.2023.106696