The Chemistry of Mars

One of the reasons why NASA believes that there is water on Mars, is due to the high salt concentration found on the surface. This salt is composed of sulfur, known as sulfates. According to Steven Squyres, in order to have that much sulfates present in one location it would require lots of water. (Unfortunately, this article dismisses the need to discuss how sulfates are formed). The observations of sulfate concentrations were performed by space craft Opportunity by using alpha particle X-ray spectrophotometer. Apparently, Opportunity also came across a hydrated a jarosite, a hydrated iron sulfate, which is said to only be found in areas with high liquid water.
http://www.space.com/scienceastronomy/marswater_chemistry_040303.html