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Observations and Modeling of the Adsorbed Water in Montmorillonite with Reflectance Spectroscopy

Introduction: Understanding the role of water in surface and atmospheric processes is a focus of future Mars missions. NASA’s Mars Reconnaissance Orbiter and ESA’s Mars Express will carry VIS-NIR spectrometers (CRISM and OMEGA, respectively) capable of retrieving spectral data at wavelengths of 0.4-4.0 μm [1]. Characteristic water and hydroxyl absorptions at ≈1.4, 1.9, 2.2, and 3 μm provide the opportunity for these instruments to detect aqueous or hydrothermally altered surface materials. It is anticipated that seasonal variations in the amount of adsorbed H2O [2] might be detected as well. Though Mars is known to have a strong 3 μm absorption band due to water [3], the different forms of water (adsorbed, structural, bound) and the structure of the host mineral (clays, hydroxides, etc.) affect the shape and strength of the absorption, making a quantitative assessment of this water difficult. Previous work has focused on the strength of water absorptions as a function of particle size [4] and inter-layer cations [5]. Bishop et al. [5] were able to show the different forms of water (adsorbed and bound) are observed in reflectance spectra of clays, but did not try to quantify the amount of water present. Yen et al. [6] modeled the percentage of water in materials as a function of apparent absorbance. Their results showed a correlation between percent water and apparent absorbance, but the uncertainties in estimating the percent water were ±2%. Given that the water content of the Martian soil is <2 wt. % [7], this method of apparent absorbance may not be sensitive enough to quantify small amounts (or changes in) adsorbed water. Here we model the loss of adsorbed water in reflectance spectra of montmorillonite (SWy-1) as a function of the scattering efficiency (QS) and absorption efficiency (QA) of water. This method is capable of detecting and modeling changes in reflectance spectra due to small changes (<0.1 wt. %) of adsorbed water. Procedure: All samples were measured over the range of 1.5 25 μm using a Nicolet FTIR spectrometer in RELAB at Brown University. Multiple samples of SWy-1 montmorillonite were measured for each time-series to test the precision and reproducibility of the results. All samples were first measured under ambient conditions; the sample chamber was then closed and purged by passing dry air (CO2 and H2O filtered) across the samples. The samples were then measured at time-steps of 1, 2, 3, 4, 6, 9, and 12 hours while under purged conditions to track spectral changes due to loss of adsorbed water. Samples were weighed immediately before the first measurement (ambient) and after the last measurement (purged 12 hours) to determine the weight loss due to adsorbed water. A gold reference standard was measured at the beginning of each timestep measurement. Four replicates of SWy-1 were measured and showed similar spectral changes and weight losses. The reproducibility of the observations indicates the loss of adsorbed water in SWy-1 samples should be equivalent at any given time within the same timeseries. Six SWy-1 samples were then placed in the chamber, where two samples were for spectral measurements while the other four were removed after 1, 2, 3, and 4 hours of purging to determine the weight loss at that time-step. The results are several series of spectra of SWy-1 measured under ambient (“0 hours”) and purged (1, 2, 3, 4, 6, 9, and 12 hours) conditions with known weight losses corresponding to the 1, 2, 3, 4, 6, and 12 hour spectra. One of these series is presented in Fig. 1 from 1.8-5.0 μm (a region covered by CRISM and OMEGA) and the weight loss for each time-step is listed in Table 1.