POSSIBLE OCCURRENCE OF AMORPHOUS MAGNESIUM SULFATE HYDRATES IN SOME CEMENTED SOILS ON MARS

Evidence for Mg-sulfate salts on Mars dates back to recognition of an Mg-S correlation in 1976 X-ray fluorescence data from Viking landers at two widely separated landing sites. Highest Mg and S abundances were measured in indurated soil (“duricrust”). Data from Viking, Pathfinder, and the Mars Exploration Rovers Spirit and Opportunity indicate widespread presence of ~10 wt% Mg-sulfate salt (anhydrous weight basis) as a cementing agent in soil. Other cations (e.g. K, Fe) are likely associated with sulfate in other associations, as in the Opportunity rover’s observation of jarosite by Mössbauer analysis at Meridiani Planum. Nevertheless, the correlation between Mg and S in soils persists, as seen in alpha-proton X-ray spectrometry data collected from a shallow trench dug by the Spirit rover on sol 140 at Gusev Crater. Naturally occurring members of the MgSO4•nH2O series on Earth are epsomite (MgSO4•7H2O), hexahydrite (MgSO4•6H2O), and kieserite (MgSO4•H2O) but many other hydration states are known. Given the very different environment of Mars, the MgSO4•nH2O minerals common on Earth may not be common on Mars and forms rare on Earth may play a more important role on Mars. In particular, experiments indicate that amorphous MgSO4•nH2O may occur under very low RH conditions on Mars. We have examined products of magnesium sulfate brines allowed to crystallize at low pressure with and without admixed minerals and rocks to form simulated duricrust. Our experiments show that at low RH (<1%), brine crystallization proceeds from precipitation of epsomite through transformation to hexahydrite, but the stable phase, kieserite, does not form. Instead, hexahydrite becomes amorphous on timescales of hours to days at ~1-5 torr total pressure. The epsomite crystal morphologies persist despite dehydration and loss of crystal structure. When Mg-sulfate cements are formed under vacuum with synthetic regolith materials such as clays, zeolites, or palagonites, the product is a duricrust-like material in which Mg-sulfate salt forms initially as hexahydrite but soon becomes amorphous. If allowed to rehydrate the duricrust expands; the expanded form is retained even if the duricrust is again desiccated. These salt-cemented products may have mechanical properties similar to martian duricrust.