Climate
From the MSNBC article David mentions
Squyres took pains to distinguish between the earlier round of "water on Mars" findings and the new results. "It's like the difference between water you can draw from a well and water you can swim in," he told journalists.
Bruce Jakosky, a planetary scientist at the University of Colorado who was not involved in the latest research, said the distinction is subtle but important. "The difference is that this tells us about the climate at the time that the deposits were formed," he told MSNBC.com.
Subsurface water could have been present on a localized basis even if conditions at the surface were barren and dry — but the evidence for water at the surface itself tells scientists that "conditions must have been conducive to support water in a very widespread way," he said.