"It must be ice. 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."
Source: Peter Smith of the University of
Arizona, Tucson, Principal Investigator for the Mars Phoenix Lander
mission. Dice-size crumbs of bright material have vanished from
inside a trench where they were photographed by Phoenix four days
ago... convincing scientists that the material was frozen water
that vaporized after digging exposed it.