When Opportunity first found the interesting mineral “veins” recently, the question of course was what were they made of? Were they the same as similar-looking veins on Earth? It turns out they are indeed similar and are composed of calcium sulfate – gypsum, specifically.
What does this mean? Basically, it is further evidence, and in the opinion of scientists involved, the best evidence to date for past water on Mars that was less acidic (more neutral) than water in some other areas. Such locations would have been more favourable for life.
According to Steve Squyres, principal investigator, “This tells a slam-dunk story that water flowed through underground fractures in the rock. This stuff is a fairly pure chemical deposit that formed in place right where we see it. That can’t be said for other gypsum seen on Mars or for other water-related minerals Opportunity has found. It’s not uncommon on Earth, but on Mars, it’s the kind of thing that makes geologists jump out of their chairs.”
The vein deposit, nicknamed Homestake, probably formed from rising groundwater dissolving calcium out of volcanic rocks. Dune fields of wind-blown gypsum have been seen before on Mars, but this deposit is believed to have formed right where it is. This and other similar veins have been found in the debris apron surrounding Endeavour crater which Opportunity is still studying, and is the first time they have ever been seen by either rover.
See also this excellent update from The Planetary Society.