Formation Rate of Amorphous Magnesium Sulfates at Low Temperatures Approaching the Current Surface Conditions on Mars

Amorphous Mg-sulfates: The existence of hydrous Mg-sulfates in martian surface materials has been implied by all landed missions on Mars to date[17], and has been identified by orbital remote sensing (OMEGA and CRISM) [8-10]. Well-controlled laboratory experiments on hydrous sulfates [11-19] provide the fundamental underpinning to link the surface and orbital observations, as well as to address possible reasons for observational discrepancies (e.g., water contents). Among various Mg-sulfates, the existence of amorphous Mg-sulfate on Mars has been postulated since it can be readily formed in the laboratory from rapid dehydration of epsomite (MgSO4·7H2O) or hexahydrite (MgSO4·6H2O) [13,15,18], and it has been demonstrated to be stable under very dry conditions (5-10% RH) [19]. Thus, amorphous Mg-sulfate has been proposed to be one of the major forms of Mg-sulfate existing over broad regions of Mars’ surface [13,17]. We have previously reported on the formation of amorphous Mgsulfate within 2 hrs at room temperature (21±1 ̊C) by fast, vacuum dehydration of epsomite. We also showed that the hydration state of amorphous Mg-sulfate (i.e., number of water molecules per formula unit of MgSO4, WMPFU) can be determined by the position of the ν1 Raman peak near 1030 cm (despite its non-crystalline nature) [15,18]. We report here new results on the formation rates of amorphous Mg-sulfate at low temperatures. Low T experiments at current surface water-vapor pressure: Epsomite used in our experiments as a starting-phase was prepared at 21oC over a saturated aqueous NaCl solution (~75% RH), and was verified by multiple Raman measurements. The fast dehydration of epsomite was done by placing a number of open sample vials containing pre-weighed amounts of epsomite into a vacuum desiccator. The temperature of the desiccator was maintained at either 0 ̊C ± 0.5 ̊C using a water-ice bath, or -8 ̊C ± 3 ̊using a saturated aqueous KCl solution bath cooled by a water-ice bath and dry ice. The maximum vacuum attained in the desiccator was measured as 60 millitorr. With a 22– 36% RH range in our laboratory, the approximate water vapor pressures inside the desiccator during the period of our experiments were estimated to be within the range of annual water vapor pressures at equatorial regions on Mars, e.g., 0.04–0.15 Pa at the Opportunity site[20]. Sample vials were removed from the vacuum desiccator at scheduled times, sealed immediately; weighed, and analyzed in situ using the laser Raman spectrometer. Time for total amorphization at low T: A key result of these experiments is that at low temperatures approaching the current Mars surface temperature range, the time required for total amorphization of the same amount of epsomite increases tremendously. Raman spectra in Fig.1a show that at 21 ̊C, a ~50 mg epsomite sample totally converts into amorphous Mgsulfate in 2 hrs (light blue spectrum). The same amount of epsomite takes 50 hrs (Fig.1b, light blue spectrum) for total amorphization at 0 ̊C. At -8 ̊C, total amorphization was not reached after 208 hrs (Fig.1c, light blue spectrum). Maximum water content of amorphous Mg-sulfates: Our experiments indicate an upper limit for the WMPFU that an amorphous structure can hold. At both 21 ̊C and 0 ̊C, total amorphization (light blue spectra in Fig. 1 and large square symbols in Fig. 2a) occurred only after a WMPFU value ~3 was attained. These observations are in agreement with experimental results at 50 ̊C during the process of re-hydration of amorphous Mgsulfate in eight different relative humidity buffers, where the in situ Raman and weight-gain measurements indicated the persistence of amorphous structure until a WMPFU value ~3.1. Thereafter, crystalline starkeyite and epsomite began to appear from amorphous Mg-sulfate with a WMPFU value >3.3. Formation rate of amorphous Mgsulfate at low T: Figure 2a shows the 110