A nice article and one worth thinking about as the MER rover heads for the haematite area.

One incomplete factor in the article is the lack of consideration of CO2 as an oxygen donor. CO2 is easier to reduce than H2O and is clearly available to some extent on Mars. A geothermal system on Mars might involve carbonated water or pure CO2 fluids.

Certainly, basalts exposed to Mars' CO2 atmosphere in experimental rigs have been shown to oxidise to a thin skin of silvery haematite, just as flows exposed to Earth's atmosphere do.

I'm corresponding with David Catling on this issue at present. Any results, I'll feed back to the group...