11:00 AM - 11:15 AM
[PPS06-02] The NIR spectroscopy of frosted mineral powders for lunar polar exploration: Effect of difference in mineral species and particle size on ice absorption intensity
Keywords:NIR spectroscopy, remote sensing, Lunar polar exploration
Lunar explorations since the 1990s have discovered the evidence of the existence of water in the polar permanently shadowed regions (PSRs) of the Moon. Since all previous studies are based on in-orbit remote sensing and the estimation model from observation data is uncertain, information on the distribution range of water and the phase of water, such as whether it is water ice or hydrous minerals, is unclear.
To detect the water in the lunar PSRs, Japan Aerospace Exploration Agency and Indian Space Research Organization have planned Lunar Polar Exploration Mission (LUPEX). For LUPEX a near-infrared spectrometer, developed by a group including the author, is adopted as one of the apparatuses to detect the water. In preparation for this mission, we have developed a frosting system (Ogishima and Saiki, 2021). This equipment simulates the states of cold trapped water ice particles attached to the lunar regolith in a cryogenic environment. As lunar regolith simulants, three species of minerals (diopside, plagioclase, and olivine) were crushed and sieved to produce 75 – 125 um and 180 – 250 um powders. The frosted sample of mineral powder was made with the frosting system and observed by a Near-Infrared imaging spectrometer developed by us (Saiki et al., 2019) in the range of 900 - 1640 nm under the conditions of an incident angle of 30°and an emission angle of 0°.
To quantify the correlation between absolute water content and water-related absorption at a wavelength of 1500 nm, we used the band depth parameter used in previous studies (e.g. Milliken & Mustard, 2005; Ogishima and Saiki, 2021) and obtained the calibration lines of the band depth with respect to the ice abundance ratio. As a result of the experiments, it was found that the slope of the calibration line becomes large when the particle size of the powder is large for the same mineral. Comparing samples of the same particle size, the slope of the calibration line increases in the order of olivine < plagioclase < diopside. This order is the same as the order of reflectance of these mineral powders in the dry state at the wavelength of 1500 nm.
Because the band depth strength is affected by the powder composition and particle size distribution, it is not easy to estimate the absolute water content in a few observation data available. We tried to estimate the absolute water content with high accuracy using only the dry spectrum.
We found that the six plots in 3D axis space (parameters of the slope of calibration lines, mineral particle size, and dry powder reflectance) distributes on a single plane. If this feature applies to common minerals, the absolute water content in the regolith may be determined with high accuracy by using only two parameters, particle size and dry reflectance at 1500 nm. In addition, we investigated whether a similar tendency can be seen in the model spectrum by using a mixing powder spectrum model (Hapke, 1993) to create frosted mineral powder spectrum. As a result, it was suggested that various minerals with a wide range of reflectance also tend to be plotted on a single plane.
To detect the water in the lunar PSRs, Japan Aerospace Exploration Agency and Indian Space Research Organization have planned Lunar Polar Exploration Mission (LUPEX). For LUPEX a near-infrared spectrometer, developed by a group including the author, is adopted as one of the apparatuses to detect the water. In preparation for this mission, we have developed a frosting system (Ogishima and Saiki, 2021). This equipment simulates the states of cold trapped water ice particles attached to the lunar regolith in a cryogenic environment. As lunar regolith simulants, three species of minerals (diopside, plagioclase, and olivine) were crushed and sieved to produce 75 – 125 um and 180 – 250 um powders. The frosted sample of mineral powder was made with the frosting system and observed by a Near-Infrared imaging spectrometer developed by us (Saiki et al., 2019) in the range of 900 - 1640 nm under the conditions of an incident angle of 30°and an emission angle of 0°.
To quantify the correlation between absolute water content and water-related absorption at a wavelength of 1500 nm, we used the band depth parameter used in previous studies (e.g. Milliken & Mustard, 2005; Ogishima and Saiki, 2021) and obtained the calibration lines of the band depth with respect to the ice abundance ratio. As a result of the experiments, it was found that the slope of the calibration line becomes large when the particle size of the powder is large for the same mineral. Comparing samples of the same particle size, the slope of the calibration line increases in the order of olivine < plagioclase < diopside. This order is the same as the order of reflectance of these mineral powders in the dry state at the wavelength of 1500 nm.
Because the band depth strength is affected by the powder composition and particle size distribution, it is not easy to estimate the absolute water content in a few observation data available. We tried to estimate the absolute water content with high accuracy using only the dry spectrum.
We found that the six plots in 3D axis space (parameters of the slope of calibration lines, mineral particle size, and dry powder reflectance) distributes on a single plane. If this feature applies to common minerals, the absolute water content in the regolith may be determined with high accuracy by using only two parameters, particle size and dry reflectance at 1500 nm. In addition, we investigated whether a similar tendency can be seen in the model spectrum by using a mixing powder spectrum model (Hapke, 1993) to create frosted mineral powder spectrum. As a result, it was suggested that various minerals with a wide range of reflectance also tend to be plotted on a single plane.