Martian Surface Mineralogy

We've been exploring that now for probably about seven of the nine years that we've beenon on sight there crater. The lake bed sediments are disappearing and being replaced now by sediments that were laid down in more dynamic environments, maybe at the shores of lakes or within rivers. This has caused people to wonder whether there was a planet wide transition from an environment that formed clay minerals to one that formed sulphates. Was it a gradual transition that occurred over millions of years? It probably wasn't just a black and white change. It probably came and went a few times, a wet, dry, wet, dry,. and then finally, dry, dry,dry.