3:30 PM - 3:45 PM
[PPS09-17] New Global Distribution of Lunar Pyroclastic Deposits Identified Using VIS-NIR Spectroscopy
★Invited Papers
Keywords:Remote sensing, VIS-NIR spectroscopy, Lunar volcanisms, Dark mantle deposits
Lunar volcanism is one of the possibilities for the source of water on the lunar surface. If only 0.1% of H2O vented from lunar volcanic eruptions remained in the lunar polar regions, all the water ice currently observed in the polar permanently shadowed regions could be explained (Needham & Kring, 2017). This estimate is based on the volatile quantitative analysis of the returned lunar volcanic materials (e.g. Saal et al., 2008; Hauri et al., 2011; 2015). However, there have been claims that the volcanic glasses returned by the Apollo missions might represent anomalous compositions and not be indicative of the lunar bulk composition (e.g. Albarende et al., 2013). Over 100 areas of Lunar Pyroclastic Deposits (LPDs) have been identified on the Moon with two areas of Apollo 15 and 17 landing sites. (e.g. Gaddis et al., 2003; Gustafson et al., 2012). Recently, A work by Milliken & Li (2017) suggested that a number of LPDs, near-infrared reflectance spectra acquired by the M3 exhibit absorptions consistent with enhanced OH- and/or H2O-bearing materials. It is interested that not all LPDs correspond to the water absorption signature. This may suggest the diversity of volatiles content within the LPD source lunar mantle.
The LPDs have been characterized by their low albedo and are often associated with morphologic vent depressions, however, there has been confusion from mare basalts or lunar cryptomare. In the last decade, the availability of global VIS-NIR spectroscopic data for the Moon has facilitated discussions on the spectral characteristics of LPDs in local regions (e.g. Jawin et al., 2015; Farrand et al., 2023; Henderson et al., 2023). Iron-bearing pyroclastic glasses exhibit characteristics of wide absorption 1μm band and shorter absorption peak in 2μm band (Horgan et al., 2014). Despite efforts in previous studies, a universal parameter of criteria for identifying glass spectra applicable to lunar global geology has not established. In this study, we organized the VIS-NIR spectral characteristics of glassy materials and developed a detection method for glassy LPDs across different lunar geologies. Utilizing VIS-NIR spectral data, we have created a global glass distribution map and updated the LPD distribution list across the entire Moon.
To quantify the glass materials, we used two parameters: the '1μm area width (1AW)' and the '2μm band center (2BC)'. The 1AW is obtained by dividing the normalized area of the 1μm band by the depth of the absorption peak using the data acquired by Kaguya Multiband Imager (MI). The 2BC is the wavelength of the absorption peak in the 2μm band. Glass materials tend to have a larger 1AW value due to the broadening of the 1μm band, and the 2BC shifts to shorter wavelengths.
We calculated the global lunar 1AW map from merged MI data with a resolution of 128pix/degree. The 2BC analysis utilized data from Moon Mineralogy Mapper (M3) onboard Chandrayaan-1, which is published in the LROC Quickmap. Most LPDs previously discovered can be identified by the characteristic parameters of 1AW > 400 and 2BC < 2100 nm. Additionally, we discovered more than 70 LPD candidates that were not listed in previous studies (e.g. Gaddis et al., 2003; Gustafson et al., 2012; Farrand et al., 2023), including regions of silicic volcanic domes without the feature of low albedo utilizing these criteria. Some regions reported with 3μm water absorption features may coincide with the distribution of several LPDs we discovered. This correlation is a interesting additional information for estimating the compositional diversity of lunar volcanisms. Our study has established an important method for accessing the older igneous activity of the Moon, which is invaluable for geological analysis and indispensable in formulating future lunar volcanology-targeted landing missions.
The LPDs have been characterized by their low albedo and are often associated with morphologic vent depressions, however, there has been confusion from mare basalts or lunar cryptomare. In the last decade, the availability of global VIS-NIR spectroscopic data for the Moon has facilitated discussions on the spectral characteristics of LPDs in local regions (e.g. Jawin et al., 2015; Farrand et al., 2023; Henderson et al., 2023). Iron-bearing pyroclastic glasses exhibit characteristics of wide absorption 1μm band and shorter absorption peak in 2μm band (Horgan et al., 2014). Despite efforts in previous studies, a universal parameter of criteria for identifying glass spectra applicable to lunar global geology has not established. In this study, we organized the VIS-NIR spectral characteristics of glassy materials and developed a detection method for glassy LPDs across different lunar geologies. Utilizing VIS-NIR spectral data, we have created a global glass distribution map and updated the LPD distribution list across the entire Moon.
To quantify the glass materials, we used two parameters: the '1μm area width (1AW)' and the '2μm band center (2BC)'. The 1AW is obtained by dividing the normalized area of the 1μm band by the depth of the absorption peak using the data acquired by Kaguya Multiband Imager (MI). The 2BC is the wavelength of the absorption peak in the 2μm band. Glass materials tend to have a larger 1AW value due to the broadening of the 1μm band, and the 2BC shifts to shorter wavelengths.
We calculated the global lunar 1AW map from merged MI data with a resolution of 128pix/degree. The 2BC analysis utilized data from Moon Mineralogy Mapper (M3) onboard Chandrayaan-1, which is published in the LROC Quickmap. Most LPDs previously discovered can be identified by the characteristic parameters of 1AW > 400 and 2BC < 2100 nm. Additionally, we discovered more than 70 LPD candidates that were not listed in previous studies (e.g. Gaddis et al., 2003; Gustafson et al., 2012; Farrand et al., 2023), including regions of silicic volcanic domes without the feature of low albedo utilizing these criteria. Some regions reported with 3μm water absorption features may coincide with the distribution of several LPDs we discovered. This correlation is a interesting additional information for estimating the compositional diversity of lunar volcanisms. Our study has established an important method for accessing the older igneous activity of the Moon, which is invaluable for geological analysis and indispensable in formulating future lunar volcanology-targeted landing missions.