J
JonClarke
Guest
Opportunity has detected the presence of gypsum, Mg-sulphate and jarosite (Fe-Al sulphate at Meridiani, as well as haematite. Jarosite forms only under fairly acidic conditions (pH <4) Most modelling I have seen on salts formed on Mars have ignored pH as a factor as the dominant chloride and sulphate salts are not considered pH dependent (http://www.geology.yale.edu/~ajs/1999/07-09.1999.10Morse.pdf). As a result the co-occurrence of these sulphates is generally taken as confirmation of acid brine models for Martian water chemistry. <br /><br />However, my limited knowledge of chemistry (largely rule of thumb) suggests this assumption is not correct. Mg-sulphates on earth are formed from near neutral marine-derived waters in evaporitic ponds. They are also found in neutral to alkaline saline soils. Epsomite is known to dissolve in acid mine leachate when the pH falls to <5, this is consistent with its use to remediate acid sulphate soils (pH <4) where its addition buffers the pH to more acceptable levels ( />4).<br /><br />This is important with respect to mars in that I suspect that Mg-sulphates are not likely to coexist with jarosite in any large water body. As the Ca and Mg sulphates at Meridiani are clearly very early, to me it seems logical to assume that the waters that deposited these sediments was of relatively low acidity. The higher acidity associated with the jarosite must therefore be a later feature, perhaps associated with weathering. This is supported by the formation of jarosite in modern weathering in the Antarctic.<br /><br />Despite this I have not been able to find any diagram or table that shows the stability of Mg-sulphates with respect to pH. Does anyone have such a table or diagram? Am I onto something here, or simply full of it?<br /><br />Thanks<br /><br />Jon<br /> <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em> Arthur Clarke</p> </div>