Visible and Near-ir Reflectance Spectra for Smectite, Sulfate and Perchlorate under Dry Conditions for Interpretation of Martian Surface Mineralogy

Introduction: Visible and near-IR (VNIR) spectral data for the martian surface obtained from orbit by the MRO-CRISM and OMEGA instruments are interpreted as having spectral signatures of H 2 O/OH-bearing phases, including smectites and other phyl-losilicates, sulfates, and high-SiO 2 phases [e.g., 1-4]. Interpretations of martian spectral signatures are based on and constrained by spectra that are obtained in the laboratory on samples with known mineralogical compositions and other physicochemical characteristics under, as appropriate, Mars-like environmental conditions (e.g., temperature, pressure, and humidity). With respect to environmental conditions, differences in the absolute concentration of atmospheric H 2 O can effect the hydration state and therefore the spectra signatures of smectite phyllosilicates (solvation H 2 O) and certain sulfates (hydration H 2 O) [e.g., 5-7]. We report VNIR spectral data acquired under humid (laboratory air) and dry (dry N 2 gas) environments for two natural smectites (nontronite API-33A and saponite SapCa-1) to characterize the effect of solva-tion H 2 O on spectral properties. We also report spectral data for the thermal dehydration products of (1) melanterite (FeSO 4 .7H 2 O) in both air and dry N 2 gas and (2) Mg-perchlorate (Mg(ClO 4) 2 .6H 2 O) in dry N 2 environments. Spectral measurements for samples dehydrated in dry N 2 were made without exposing them to humid laboratory air. Methods: VNIR reflectivity spectra (0.35-2.50 µm) were acquired with Analytical Spectral Devices (ASD) fiberoptic spectrometers configured with Mug lights. A FieldSpec3 HiRes spectrometer was used for measurements in laboratory air at ~25°C. A Field-Spec3 spectrometer, a hot plate (ambient to 400°C), and a dewpoint meter (Vaisala DRYCAP DM70) were co-located in a one-atmosphere glove box that can be kept under continuous purge using dry N 2 gas (from liquid N 2) to provide a dry atmosphere. Spectral measurements on samples in the glove box were made in air (8000-12000 ppm H 2 O by volume) prior to starting the N 2 purge, after purging 24-650 hr at ~25°C (150-400 ppm H 2 O or 0.5-2.0% relative humidity), and, depending on the sample, after heating to 50-330°C on the hot plate and cooling to ~25°C. Because H 2 O evolves during heating, the time at temperature was governed by the time required for the H 2 O content of the gas at