The previous post probably erred in saying that it was Hank Moore of the USGS who was a big fan of the mineral jarosite. While it's conceivable that Moore was a jarosite fan, I think (thanks to a comment by Bruce) that the person they were talking about was in fact the pioneering mineralogist Roger Burns, who died ten years ago. There's a little about him and his legacy in the preface to this paper, which then goes on to review a lot of Mars mineralogy and discuss the possibility of sulphates such as those Opportunity has now discovered. Be warned, though, that venturing beyond the preface is probably best left to those who know their montmorillonite from their phyllosilicates. Sample: "The answer to the question "Why is Mars red?" is that sunlight incident on the surface is preferentially absorbed from the ultraviolet to about 0.6 um via a series of ligand field and cation pair transitions in ferric iron-bearing minerals".