2.1. Spectral Data Sets

Hydroxyl groups and water molecules exhibit vibrational absorption features near 3 μm associated with the symmetric stretch of the O‐H bond. The bonding environment (bond strength and length) determines the precise vibrational frequency at which the absorption occurs. However, the ability to accurately constrain the physical and chemical state of the O‐H and speciation (OH vs. H₂O) from the 3 μm feature alone is not clear (Dyar et al., ²⁰¹⁰) with the combined OH + H₂O abundance referred to as “total water.” Total water may refer to structural OH, OH or H₂O within glasses, adsorbed H₂O, and surficial OH (terminal hydroxyl), all of which exhibit 3 μm absorption bands.

We simulated lunar surface reflectance spectra from 1 to 4 μm under varying levels of hydration by using a Monte Carlo method to generate intimate single scattering albedo mixtures using laboratory reflectance data of Apollo samples from the NASA Reflectance Experiment Laboratory (RELAB) at Brown University and reflectance data from step‐wise heating experiments of water‐bearing mid‐ocean‐ridge basalt (MORB) glasses (S. Li, ²⁰¹⁶; Shimizu et al., ²⁰¹⁶).

We introduced varying levels of hydration to the reflectance simulations using hydrated MORB glass spectra. We chose the hydrated MORB glasses based on their water content range, their visual homogeneity, and having as little alteration as possible (S. Li, ²⁰¹⁶). Crucially, the absolute water content was measured independently by secondary‐ion mass spectrometry (SIMS) (Shimizu et al., ²⁰¹⁶), enabling us to verify the accuracy of our spectroscopic water retrieval. The MORB glasses were heated in a step‐wise fashion to remove water allowing for a correlation between the strength of the 3 μm band and water content from SIMS measurements (S. Li & Milliken, ²⁰¹⁷). These hydrated MORB glasses have been used to estimate the abundance of water in the lunar 3 μm band of remote sensing datasets (Honniball et al., ²⁰²⁰; S. Li & Milliken, ²⁰¹⁷). The Supplementary Material contains spectral comparisons and accompanying discussion of the hydrated MORB glasses used here to the 3 μm band shape from lunar remote sensing data from the InfraRed Telescope Facility (IRTF, Figure S1 in Supporting Information S1) and from M³ (Figure S2 in Supporting Information S1). The shape of the MORB 3 μm band can also be compared to the hydroxyl feature in laboratory spectra of proton‐irradiated Apollo soils (Ichimura et al., ²⁰¹²; McLain et al., ²⁰²¹). Ubiquitous (but unquantified) adsorbed surface water is present in the laboratory reference spectra of Apollo samples we used (Figure S3 in Supporting Information S1) due to the high reactivity of lunar soil samples, with similar band shape to the hydrated MORB glasses. Based on these comparisons and its prior use in interpreting lunar remote sensing datasets we believe that the MORB glass spectra represents the best quantitative method of introducing hydration signatures into our spectral mixtures.

In our simulations we separated highlands and mare datasets to test the effects of terrain type on the multispectral reflectance retrieval. For both the highlands and mare datasets, each spectrum was generated from a random mixture of mature and immature Apollo sample spectra, an Apollo pyroxene spectrum, and a MORB spectrum. Representative mature and immature mare sample spectra were selected from Taylor et al. (²⁰⁰¹), and mature and immature highlands sample spectra were selected from Taylor et al. (²⁰¹⁰). The mature mare sample had 15.7% pyroxene abundance and the immature mare sample had 20.5% pyroxene abundance in the 20–45 μm size fraction (Taylor et al., ²⁰⁰¹). The mature highlands sample had 5.1% and pyroxene abundance and the immature highlands sample had 7.4% pyroxene abundance in the 20–45 μm size fraction (Taylor et al., ²⁰¹⁰). We also included varying amounts of additional pyroxene in the mixtures to test whether its presence affects the 3 μm band depth determination. Pyroxenes exhibit an absorption band that can extend from 1.4 to 2.6 μm corresponding to a crystal field transition in Fe²⁺ (Burns, ¹⁹⁹³; Cloutis, ²⁰⁰²; Sunshine & Pieters, ¹⁹⁹³). This broad absorption may coincide with the onset edge of the hydration band, affecting the continuum from which the band depth is determined. The list of Apollo soil samples, spectra labels, and abundance ranges for the mixtures are given in Table 1. The abundance ranges for the glass and overall pyroxene were chosen to mimic the heterogeneity of the lunar surface based on data from Lunar Sourcebook (Table 5.1, Papike et al., ¹⁹⁹¹).

Information on Endmember Spectra Used to Generate Intimate Mixtures

Note. The main simulations used mixtures of the top six endmembers. Additional limited simulations were performed using the synthetic lunar glasses.

Finally, we endeavored to test our multispectral lidar retrieval method on a mixture containing a non‐MORB glass, also with precisely known water content, to assess how differences in the 3 μm band shape affected the retrieval. For this we used RELAB spectra of orange and yellow synthetic lunar glasses. These glasses have the same bulk chemical composition as green and orange glasses found in Apollo samples. Importantly, the water abundance in the glasses was measured via SIMS by Wetzel and co‐authors (Wetzel et al., ²⁰¹⁵), which allows us to use it to create spectral mixtures with a known total water amount. The orange synthetic lunar glass had a total water content of 390 ppm, and the yellow synthetic lunar glass had a total water content of 305 ppm as measured by SIMS.