The rover was digging a trench nicknamed Dodo-Goldilocks with its robotic arm when it hit some hard, refelective material. The scientists back on Earth who control Phoenix halted the digging, and spent the next couple of days taking photographs of the hole, trying to figure out what they were looking at in the ditch. Was the whitish material a kind of salt? But over those days of photography and scrutiny, something interesting happened to the marble-sized chunks. They evaporated. Long entombed beneath the iron-oxide surface of the red planet, the substance turns out to be part of a frozen layer of water just below the ground covered by Phoenix.