The H2o Sorption Properties of a Martian Dust Analog

Introduction: Palagonitic tephra (basaltic tephra containing hydrated and hydroxylated volcanic glass of basaltic composition) has received widespread attention in the planetary literature because many are basaltic spectral and magnetic analogs of bright Martian surface materials [1, 2]. Here we present the H2O adsorption properties of two samples of palagonitic tephra (HWMK919, PA6-7) from the Island of Hawaii and compare their behaviour with that of two smectites (nontronite, NG-1, and montmorillonite, STx-1) similar to those that have been spectroscopically identified on Mars by the Mars Express/OMEGA Team [3]. One tephra sample (HWMK919) is the <5 m size fraction of a phyllosilicate-poor palagonitic tephra and is thus a size analog for bright Martian dust [4]. The results are important for understanding the speciation of adsorbed H2O, hydrated and hydroxylated phase on the nearequatorial Martian surface as detected by the presence of the 3 μm hydration feature across the entire planet (Mars Express/OMEGA) [5]. The work here builds on previous published work on the interaction of Martian atmospheric H2O with Mars analog samples under simulated Martian environmental conditions [e. g. 6-9]. Experimental: The sorption properties of H2O in HWMK919<5μm, Pahala Ash PA5-6, montmorillonite (STx-1), nontronite (NG-1) were investigated by means of isotherm measurements, thermogravimetry (TG) and differential scanning calorimetry (DSC). Sorption isotherms were measured gravimetrically from 255-313 K with a McBain quartz spring balance equipped with two MKS Baratron pressure sensors covering a range of 10 -5 -10 mbar. TG/DSC measurements were performed on a SETARAM TG-DSC 111 apparatus with a heating rate of 3 K/min to 673 K. Prior to the experiments the samples were loaded at a relative H2O pressure of 0.3. Scanning electron microscopy (SEM, JOEL JSM640), X-ray powder diffraction (XRD, Bruker D8, Cu K radiation) and N2 adsorption (Quantachrome Autosorb-1) were applied to characterize the solids. Before each adsorption experiment, about 150 mg of sample was degassed at 383 K and p<10 -5 mbar for several hours. At these conditions, however, not all strongly adsorbed H2O was removed (especially for HWMK919, which retained about one third of the total H2O compared with the total mass loss). We did not heat the samples to higher temperatures because low degassing temperatures are crucial to prevent possible modifications to the solids. Results and Discussion: Table 1 and Figure 1 show the results of a general characterization of the samples to get information about surface are, pore diameter and particle size. As can be seen, significant differences in the N2 BET surface area of the palagonites and the smectites were found because N2, as opposed to H2O, is unable to penetrate into the interlayer spaces of the smectites. A formal application of the BET equation on the water adsorption isotherms (Table 1) supports this behavior. XRD analysis of HWMK919 characterized it as phyllosilicate-poor in contrast to PA6-7. Furthermore, HWMK919 contains a considerable amount of allophane which is generally microporous and hydrophilic.