Martian soil, on the rocks.
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."