Yet Water Didn't Show Up In The Chemical Test

Mars must be an unusual place. The topsoil must be nearly-impermeable, in order to keep the surface dessicated in the presence of ground ice:

June 19, 2008 -- 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."