Hydration Features in Spectra of Lunar Analogs and Their Relationships to the Persistent and Diurnally Variable Hydration Features in M3 Spectra of the Moon

HYDRATION FEATURES IN SPECTRA OF LUNAR ANALOGS AND THEIR RELATIONSHIPS TO THE PERSISTENT AND DIURNALLY VARIABLE HYDRATION FEATURES IN M3 SPECTRA OF THE MOON. J. L. Bishop1, M. R. D. Gruendler1,2, C. Tokubo1,3, K. S. Wohlfarth4, K. A. Wilk5, M. Parente6, C. Wöhler4, J. Flahaut7, R. L. Klima8, M. Martinot7, A. M. Dapremont8, T. Panambur6, T. Sander4, S. Zorzan9, 1SETI Institute (Mountain View, CA, USA, [email protected]), 2University of California San Diego (La Jolla, CA, USA), 3University of California Davis (Davis, CA, USA), 4Technische Universität Dortmund (Dortmund, Germany), 5Brown University (Providence, RI, USA), 6University of Massachusetts Amherst (Amherst, MA), 7Université de Lorraine, CRPG (Vandœuvre-lès-Nancy, France), 8Johns Hopkins University Applied Physics Lab (Laurel, MD, USA), 9Luxembourg Institute of Science and Technology (Esch-sur-Alzette, Luxembourg). Introduction: Understanding the enigmatic hydration features on the Moon requires characterizing hydration features in the spectra of lunar analogs. For this study we evaluated reflectance spectra of multiple terrestrial volcanic glasses, feldspars, pyroxenes, and olivines. To compare these analog spectra to Moon Mineralogy Mapper (M3) spectra requires convolving the lab spectra to M3 channels. Global trends in changes in hydration on the Moon with time of day have been computed using an Integrated Band Depth (IBD) across the 2.7-3 µm region [1, 2] that includes components from OH and H2O [3]. More recently, lunar hydration maps were compared with mineralogy and elemental maps and observed two different types of hydrated components on the Moon [4]. This current project is focused on comparing differences in the hydration feature in analog spectra with these two forms of lunar hydration observed by M3. Methods: Lab Spectra. Reflectance spectra from 0.4-3 µm were selected from the Bishop spectral library, the USGS spectral library, and a current study by Wilk [5] (Fig. 1). The Bishop and Wilk spectra were measured at Brown University’s RELAB facility after purging the samples in the chamber overnight to remove adsorbed moisture. Spectral Subsampling. To optimize spectral comparisons, we first convolved each lab spectrum using an algorithm that averaged the lab data at high scpectral resolution to each of the M3 targeted datapoints from 0.4460 to 2.9912 µm at ~10 nm intervals. Following this, we converted the lab data to the lower spectral resolution M3 global channels. Empirical testing indicates that the transformation from M3 targeted channels to M3 global channels involves averaging 4 channels of data, with variable step sizes at certain points in the data to maximize the mineralogy data achieved with the global dataset. Step sizes for the channel averaging began at 4 channels, then changed to 3 at 0.71551, then 2 at 0.73547, then back to 3 at 1.5639, then 4 at 1.6038, then 5 at 1.6537, then back to 4 at 1.6936 until the end. M3 Global Dataset. Spectral parameter maps [4] were derived from recently calibrated M3 global images [2] and correlated with lunar elemental abundance maps [6] and mineral abunance maps [7]. Analyses of these 20 pixel per degree spectral parameter maps from different times of day indicate that there are two distinct hydration components – one that varies with composition across the surface of the Moon, and another one that varies with time of day [4]. Spectra were collected from 5 regions of the moon (Fig. 1) where data were available for both the morning and midday times (Fig. 2). Fig. 1. Locations of 5 Lunar Regions where M3 spectra were collected. Regions 1, 3 and 5 are from highalbedo areas, while Regions 2 and 4 are from darker areas associated with stronger pyroxene signatures [4]. Fig. 2. M3 spectra from 5 regions. Top) Ratios of spectra collected in the lunar morning to spectra collected at midday from 5 locations in Fig. 1. Bottom) Individual spectra from both morning and afternoon times for Regions 1 and 2. Grey lines mark selected M3 channels in the hydration region. Results – Analog Spectra: The convolved spectra in our study illustrate variations in the hydration band shape and intensity for nominally anhydrous minerals and materials (Fig. 3). Stretching vibrations for hydroxyl groups bound to metals in the mineral structure typically occur near 2.7-2.9 µm, while bands due to H2O vibrations connected to minerals generally occur near 2.8-3.2 µm [8]. Many minerals thought to be anhydrous actually contain vibrational bands due to OH and H2O [9] that are either trapped in the mineral structure or present as alteration or to compensate for charge imbalances. Fig. 3. Reflectance spectra of selected analog materials convolved to M3 global channels. Gray lines mark selected M3 channels in the hydration region to illustrate differences among these spectral types. Results – Hydration Bands in Lunar Spectra: Spectral comparisons indicate a steeper drop in reflectance near 2.7-2.8 μm and a band minimum near 2.862.93 μm in the morning to midday ratio spectra for brighter regions studied here (Fig. 2) that are more consistent with the spectra of augite and pigeonite near 2.7-3 µm (Figs. 3-4). In contrast, the morning to midday ratio spectra for darker regions exhibit stronger hydration bands overall that continue to drop in reflectance to the end of the M3 range. These hydration bands are more consistent with anorthite, Mgrich olivine, and many other analog materials. Implications: Spectra of the darker lunar regions more consistent with pyroxene (Fig. 1) tend to have a stronger hydration band, as observed previously [4]. However, the change in shape of this hydration band in ratio spectra from morning to midday is more consistent with the hydration band in spectra of other materials. This could imply that the diurnally mobile hydration component is not associated with pyroxene but with other materials. We will continue investigating this trend with additional locations on the lunar surface and a wider collection of lunar analogs. Fig. 4. Comparison of M3 global spectra to lunar analogs spectra. A) 0.5 to 3 µm region: M3 ratio spectra are shown at the top (brighter regions in blue, darker regions in green) and analog spectra are shown below. B) 2.4 to 3 µm region: M3 ratio and analog spectra illustrating hydration differences. Acknowledgments: Support from NASA LDAP grant #80NSSC21K1480, the SETI Institute, and the FDL program are much appreciated. Assistance form T. Hiroi with the RELAB spectral measurements is also greatly appreciated. References: [1] Wöhler C. etal. (2017) Science Advances, 3, e1701286. [2] Wohlfarth K.S. etal. (2023) A&A, 674, A69. [3] Pieters C.M. etal. (2009) Science, 326, 568-572. [4] Wohlfarth K.S. etal. (2024) AGU Fall Mtg, #P41C-02. [5] Wilk K. A. et al. (2024) LPS LIII, #1666. [6] Bhatt M. et al. (2019) A&A, 627, A155. [7] Arnaut M. et al. (2020) LPS LI, #3008. [8] Bishop J.L. et al. (1994) CCM, 42, 702716. [9] Wilk K.A. et al. (2024) Icarus, 411, 115945.