Dice-size crumbs of bright material have vanished from inside a trench where they were photographed by NASA's Phoenix Mars Lander four days ago, convincing scientists that the material was frozen water that vaporized after digging exposed it.
"It must be ice," said Phoenix Principal Investigator Peter Smith of the University of Arizona, Tucson. "These little clumps completely disappearing over the course of a few days, that is perfect evidence that it's ice. There had been some question whether the bright material was salt. Salt can't do that."
I saw it first from Phoenix's Twitter feed, but here's the full story. (including the pictures that show the sublimation)
So now we know for sure that Mars has water, which has a lot of implications for possible life there as well fairly significant impact in theories of planet formation and the like.