4:15 PM - 4:30 PM
[PPS08-22] Effect of porosity on moisture content measurement of lunar regolith samples using LIBS
Keywords:Luna, Laser-Induced Breakdown Spectroscopy(LIBS), regolish
Previous lunar missions indicate the presence of water on the Moon. Investigating its distribution provides insights into lunar formation, including the magma ocean, mantle volatiles, impact history, and solar wind interactions (Dhingra et al, (2009)). Identifying lunar water' s origin enhances understanding of planetary water delivery, including to Earth, and volatile migration in the early Solar System (Albarède et al, (2009)). Additionally, lunar water is a critical resource for space exploration.
Detecting hydrogen is crucial for lunar water studies. In the Moon' s vacuum, HO either volatilizes or stabilizes in minerals, often as OH. Remote sensing primarily relies on OH' s 2.7micrometer absorption band to indicate water.
Early missions suggested an extremely dry Moon (Gibson et al, (1971)). However, in 1998, neutron spectrometry from Lunar Prospector detected hydrogen in permanently shadowed regions (Feldman et al, (2001)). Later, the Moon Mineralogy Mapper (M3) on Chandrayaan-1 suggested surface water (Pieters et al, (2009)). In 2009, LCROSS provided direct evidence of water in shadowed regions (Colaprete et al, (2010)), while Apollo samples confirmed water in minerals (Saal et al, (2008)).
Remote sensing is the primary lunar water detection method, but in-situ measurements are limited. Spatial resolution constraints affect small-scale distribution and depth analysis. Laser-Induced Breakdown Spectroscopy (LIBS) has been proposed for in-situ hydrogen detection. LIBS offers high spatial resolution, minimal sample alteration, and subsurface probing (Yumoto et al, (2023)).
Previous studies showed that non-compacted samples exhibit stronger hydrogen spectral intensity than compacted ones, and 0.4 wt% H2O hydrogen emission is detectable in both high-porosity and rock-density samples. However, the quantitative relationship between density and hydrogen spectral intensity remains unclear. This study examines this relationship and evaluates LIBS for lunar missions, as JAXA' s detection limit (LOD) for H2O requires higher precision.
This study assesses hydrogen spectral intensity relative to density in LIBS measurements, particularly in low-density lunar environments, to improve detection accuracy. The findings support Artemis missions and planetary exploration. The approach involved mixing hydrated material with a basaltic lunar regolith simulant, using LIBS for hydrogen emission measurement, and assessing water quantification. Additionally, systematic porosity variations were introduced to evaluate LIBS' s applicability to lunar regolith.
Lunar Mare Simulant (LMS-1) was mixed with magnesium hydroxide to create 4 wt% H2O samples. The mixture was homogenized with a mortar and pestle, molded into 4 g pellets, and density was measured via microscopy. Four densities were prepared, ranging from 1280 to 1780 kg/m3, within error margins. This range was based on reported lunar mare densities of 1150~1820 kg/m3 (Matsushima et al,(2021)). The lowest-density sample was loosely packed, while the highest was adjusted to 1.82 g/cm3.
LIBS measurements were conducted in a vacuum chamber. Parameters: 500 microsecond exposure, 2 Hz repetition, 250 A current. Each measurement included 100 laser shots for signal stability. Camera gain was 24, and background noise level was 50. Each density sample was measured at five spots, and spectra were summed and normalized. Hydrogen peak intensity was extracted after baseline subtraction.
Results showed hydrogen spectral intensity varied systematically across the lunar density range. Lower-density samples exhibited stronger hydrogen emission, while higher-density samples showed reduced intensity. This suggests that low-density lunar regolith enhances LIBS hydrogen detection, improving trace water identification.
These findings indicate LIBS is effective for lunar regolith analysis, where density influences spectral intensity. Additionally, hydrogen spectral behavior differs between low-density regolith and rock, highlighting the need to consider density effects in future lunar water quantification studies.
Detecting hydrogen is crucial for lunar water studies. In the Moon' s vacuum, HO either volatilizes or stabilizes in minerals, often as OH. Remote sensing primarily relies on OH' s 2.7micrometer absorption band to indicate water.
Early missions suggested an extremely dry Moon (Gibson et al, (1971)). However, in 1998, neutron spectrometry from Lunar Prospector detected hydrogen in permanently shadowed regions (Feldman et al, (2001)). Later, the Moon Mineralogy Mapper (M3) on Chandrayaan-1 suggested surface water (Pieters et al, (2009)). In 2009, LCROSS provided direct evidence of water in shadowed regions (Colaprete et al, (2010)), while Apollo samples confirmed water in minerals (Saal et al, (2008)).
Remote sensing is the primary lunar water detection method, but in-situ measurements are limited. Spatial resolution constraints affect small-scale distribution and depth analysis. Laser-Induced Breakdown Spectroscopy (LIBS) has been proposed for in-situ hydrogen detection. LIBS offers high spatial resolution, minimal sample alteration, and subsurface probing (Yumoto et al, (2023)).
Previous studies showed that non-compacted samples exhibit stronger hydrogen spectral intensity than compacted ones, and 0.4 wt% H2O hydrogen emission is detectable in both high-porosity and rock-density samples. However, the quantitative relationship between density and hydrogen spectral intensity remains unclear. This study examines this relationship and evaluates LIBS for lunar missions, as JAXA' s detection limit (LOD) for H2O requires higher precision.
This study assesses hydrogen spectral intensity relative to density in LIBS measurements, particularly in low-density lunar environments, to improve detection accuracy. The findings support Artemis missions and planetary exploration. The approach involved mixing hydrated material with a basaltic lunar regolith simulant, using LIBS for hydrogen emission measurement, and assessing water quantification. Additionally, systematic porosity variations were introduced to evaluate LIBS' s applicability to lunar regolith.
Lunar Mare Simulant (LMS-1) was mixed with magnesium hydroxide to create 4 wt% H2O samples. The mixture was homogenized with a mortar and pestle, molded into 4 g pellets, and density was measured via microscopy. Four densities were prepared, ranging from 1280 to 1780 kg/m3, within error margins. This range was based on reported lunar mare densities of 1150~1820 kg/m3 (Matsushima et al,(2021)). The lowest-density sample was loosely packed, while the highest was adjusted to 1.82 g/cm3.
LIBS measurements were conducted in a vacuum chamber. Parameters: 500 microsecond exposure, 2 Hz repetition, 250 A current. Each measurement included 100 laser shots for signal stability. Camera gain was 24, and background noise level was 50. Each density sample was measured at five spots, and spectra were summed and normalized. Hydrogen peak intensity was extracted after baseline subtraction.
Results showed hydrogen spectral intensity varied systematically across the lunar density range. Lower-density samples exhibited stronger hydrogen emission, while higher-density samples showed reduced intensity. This suggests that low-density lunar regolith enhances LIBS hydrogen detection, improving trace water identification.
These findings indicate LIBS is effective for lunar regolith analysis, where density influences spectral intensity. Additionally, hydrogen spectral behavior differs between low-density regolith and rock, highlighting the need to consider density effects in future lunar water quantification studies.