This is a review of
"Origin of acidic surface waters and the evolution of atmospheric chemistry on early Mars"
Joel A. Hurowitz, Woodward W. Fischer, Nicholas J. Tosca & Ralph E. Milliken
Nature Geoscience 3, 323 - 326 (2010) Published online: 4 April 2010 doi:10.1038/ngeo831
I've talked about the apparent paradox between the widely accepted view (it seems) that aqueous environments on Mars were predominately acidic and the fact that Mars is predominately a basaltic (and olivine -rich!) planet. The point being that basalt and olivine will act to neutralize any acidic solution they come into contact with. The other point being that there is a lot more rock on Mars than water or acid, so one would expect the rock to win. However, that is only true if there is a fair amount of water available to bring the acid in contact with said rock. This seems to be the case in the groundwater hypothesis for Meridiani which suggests that water was supplied from a global aquifer recharged thousands of miles away.
Anyhow, this paper is a possible solution to this paradox, and presents an interesting possibility.
My biggest issue is that the paper assumes a "neutral" Fe-, SO4-rich starting solution. Of course by doing this there is an assumption made that the water has already had acid added to it (How else would SO4 get there?). So the problem lies with charge balance. In their assumption, the authors presumably assume that the Fe2+ is charge balanced by SO4 -- although that is never indicated.
They conclude that acidity is produced by rapid oxidation of Fe2+ in the presence of potent atmospheric oxidants. While the system they describe is one possibility for the chemical environment of Meridiani Planum, Fe-oxidation cannot be the underlying source of the acidity because of charge balance in the solution. At equilibrium, the charge of an aqueous solution must be balanced, thus every positively charged cation must be balanced by a negatively charged anion. Therefore, what anions are balancing the Fe(2+) in the aqueous solution before it is oxidized? In order to answer this question, one must look back to how the Fe(2+) originally entered the solution. In this case Hurowitz et al. suggest the Fe(2+) is originated from anoxic basaltic weathering. The primary Fe-rich minerals in basalt are olivine and pyroxene which weather according to the following reactions:
Fe2Si2O6 + 2H2O --> 2Fe(2+) + 2SiO2 + 4 OH- (1)
Fe2SiO4 + 2H2O --> 2Fe(2+) + SiO2 + 4OH- (2)
The important thing to note is that the Fe(2+) is charge balanced by OH-. This is inescapable in this simple system because there are no other anions by which to balance the positive charge of the Fe(2+).
Likewise, simple oxidation of the Fe(2+) consumes H+ and in fact makes the aqueous solution more alkaline rather than more acidic (after eq 2 from Hurowitz et al.):
2Fe(2+) + 0.5O2 + 2H+ --> 2Fe(3+) + H2O (3)
Acidity is actually introduced by the hydrolysis of the aqueous Fe:
Fe(3+) + 3H2O --> Fe(OH)3 + 3H+ (4)
Fe(OH)3 --> FeO(OH) + H2O (5)
Fe(OH)3 is insoluble and precipitates from solution, subsequent dehydration of this species yields common iron phases such as goethite and hematite (eq. 5). Viewing equation 4 here in light of equations 1 and 2, it becomes clear that in precipitation, the iron has simply paired up with the OH- that was its original charge balance in the solution. So writing the complete chemical reaction including weathering, hydrolysis, and oxidation – looking only at iron:
2FeO(pyx, ol) + H2O + 0.5O2 --> 2FeO(OH) (6)
This reaction shows that the overall production of acid by weathering of Fe-rich minerals in basalt is in fact neutral. This is born out in the terrestrial environment where oxic weathering of basalt, which includes rapid Fe-oxidation, yields neutral to basic solutions.
In order to avoid this charge balance problem, Hurowitz et al. must provide some means for removal of OH- from the solution between the weathering of the basalt and the precipitation of the Fe at the surface. Charge balance plays an important role here as well, as one cannot simply add another cation to the solution without a anion to balance its charge. The only means that I can think of for getting rid of the OH- is to add some sort of acid to the solution. For example, this could be accomplished through addition of SO2 aerosols. Of course this means that the underlying source of the acid in the system is due to the sulfur and is not due to the iron as maintained by Hurowitz et al.
Another means for avoiding this problem would be to source the iron from something other than pyroxene and olivine. This has been proposed by others who suggest weathering of iron sulfides is the ultimate source of the acidity at Meridiani Planum. In this case, the underlying cause of the acidity would be weathering of iron sulfides while hydrolysis of iron would contribute to the acidity.
Another possibility maybe be chlorite formation, which is precipitated during seawater alteration of basalt and can produce acidic hydrothermal solutions. While this may not be applicable to Meridiani, chlorite has been identified on Mars and this process may be a means for creating acidic solutions on a basaltic planet.