Comparative Analysis of Diurnal Dependency of Lunar Water Absorption Using Chandrayaan-1 and Chandrayaan-2 NIR Data

COMPARATIVE ANALYSIS OF DIURNAL DEPENDENCY OF LUNAR WATER ABSORPTION USING CHANDRAYAAN-1 AND CHANDRAYAAN-2 NIR DATA. M. Bhatt1, C. Wöhler2, S. Sathyan1,3 and K. Wohlfarth2, 1Physical Research Laboratory, Ahmedabad, India ([email protected]), 2TU Dortmund University, Dortmund, Germany, 3University of Kerala, Thiruvananthapuram, India. Introduction: Recent lunar missions over the past few decades have significantly improved our understanding of the Moon's origin and early evolution. The detection of OH/H2O ice on the lunar surface has sparked a new phase of lunar exploration, however, the compositional dependency on OH/H2O ice accumulation, retention and loss processes is not yet fully understood. The reflectance properties of the lunar regolith in the near infrared (NIR) are primarily a complex combination of minerals, volatile species, degree of compaction, viewing geometries, surface roughness and space weathering. The Moon Mineralogy Mapper (M3) onboard Chandrayaan-1 conducted the first ever systematic mapping of OH/H2O based on the detection of the spectral absorption band near 3.0 µm wavelength [1]. However, a complete characterization of the absorption band was limited due to its spectral coverage ending at 3.0 µm, restricting the discrimination between OH and H2O absorption bands. Additionally, for accurate estimation of the generally weak absorption band around 3.0 μm, a precise thermal component in this wavelength range is required to be estimated and removed [2-3]. The Imaging InfraRed Spectrometer (IIRS) on board the Chandrayaan-2 mission provides first-ever systematic coverage of the Moon surface in the wavelength range from 0.8 to 5.0 µm at a spatial resolution of 80 m [4]. This is the first experiment of its kind which can discriminate between OH and H2O absorption bands at a spectral resolution of ~20 nm by fully characterizing the 3.0 µm absorption band [5, 6], provided the data are properly calibrated and thermal emission effects are removed. In this work, we present our first-ever independent approach of IIRS data processing in parallel to M3 data reduction. This is our attempt of a comparative analysis for cross-calibrating the IIRS response to M3 and for quantifying the OH and H2O absorption bands to understand the time-of-day-dependent variability of hydration at the global scale. Data Set and Methodology: To evaluate the IIRS response, we selected regions with two or more overlapping IIRS observations. Among these, we also considered the landing sites of Apollo, Luna and Chang’e for cross-validation. Fig. 1 shows the IIRS overlapping coverage from December 2019 to June 2024, excluding the polar regions. The IIRS data were accessed from ISRO’s Pradan node [7]. We selected the IIRS data with multiple coverage by categorising the data into lunar local time; morning (6:00-9:00), midday (11:00-14:00), and evening (16:00-18:00). These strips are highlighted in ‘red’ in Fig. 1. We followed the same data processing routine for IIRS as developed for M3 [2, 8-9]. The IIRS radiance data (level-1 product) are dark-reduced and radiometrically corrected [6]. This response is further corrected for thermal emission using the physics-based model in [2]. Fig. 1. Overlapping IIRS coverages across equatorial and mid-latitude regions, shown as green polygons. Red-highlighted areas indicate overlaps between strips acquired at different local time combinations (morning/midday, morning/evening or midday/evening). Base image: LROC WAC. For a comparative approach, we converted the M3 response to the IIRS viewing geometry. Both datasets are corrected for topography, photometry, and thermal emission. Expected outcomes and future work: We will present newly derived integrated maps of the 3 µm absorption band depth from multiple overlapping regions (Fig. 1) and will compare the IIRS spectral characteristics with the outcomes derived from M³. This comparative analysis will establish cross-calibrated IIRS data products with respect to the M3 response, so that the information derived from two individual spectrometers can be combined for understanding the mineralogical association of surficial OH/H2O. The IIRS response is mainly considered for understanding the time-of-day dependency of the OH and H2O absorption bands. We rely on M3 for the mineralogical information of the region due to order sorting filter positions of IIRS around the 1 and 2 µm absorption bands [4]. The established methodology will be used at the global scale for understanding the spatial and temporal variations of the hydration absorption feature and its possible correlation with lunar surface composition for a more accurate estimation of the absolute water abundance. The approach will be further extended to the polar regions for identifying potential locations of surficial water ice exposures. References: [1] Pieters C. et al. (2009) Curr. Sci. 96(4), 500-505. [2] Wohlfarth, K. et al. (2023) Astron. Astrophys. 674, A69. [3] Bandfield, J. L., Poston, M. J., Klima, R. L., & Edwards, C. S. 2018, Nature geoscience, 11, 173. [4] Chowdhury A. R. et al. (2020) Current Science, 118(3), 368-375. [5] Chauhan, P. et al. (2021) Curr. Sci. 121(3), 391-401. [6] Verma P. A. et al. (2022) Icarus, 383, 115075. [7] https://pradan.issdc.gov.in/ [8] Wöhler, C. et al. (2014) Icarus 235, 86122. [9] Wöhler, C. et al. (2017) Sci. Adv. 3(9), e1701286.