Crumbs of bright material initially photographed in the trench later vanished, meaning they must have been frozen water that vaporized after being exposed, Peter Smith of the University of Arizona, Tucson, said in a statement.
"These little clumps completely disappearing over the course of a few days, that is perfect evidence that it's ice," Smith said. "There had been some question whether the bright material was salt. Salt can't do that."
Phoenix Mars is studying whether the arctic region of the Red Planet could be habitable. The probe is using its robotic arm to dig up soil samples, and scientists hope it will find frozen water.
However, an initial soil sample heated in a science instrument failed to yield evidence of water.
"Ice has been the goal of this mission from day one," says CBS News space consultant Bill Harwood. "Confirming the ice is here, I think, is a big shot in the arm for Phoenix and all the science team because it's one thing to think it's there, it's another thing to get there and actually find it. So it's a big step forward for this mission."
The bright material was seen in the bottom of a trench dubbed "Dodo-Goldilocks" that Phoenix enlarged on June 15. Several of the bright crumbs were gone when the spacecraft looked into the trench again early Thursday, NASA said.
Phoenix's arm, meanwhile, encountered a hard surface while digging another trench Thursday and scientists were hopeful of uncovering an icy layer, the space agency said. That trench is called "Snow White 2."
The arm went into a "holding position" after three attempts to dig further, which is expected when it the reaches a hard surface, NASA said.
Scientists have been using names from fairy tales and mythology to designate geologic features around Phoenix and the trenches it has been digging.
In 2002, the orbiting Mars Odyssey detected hints of a vast store of ice below the surface of Mars' polar regions. The arctic terrain where Phoenix touched down has polygon shapes in the ground similar to those found in Earth's permafrost regions. The patterns on Earth are caused by seasonal expansion and shrinking of underground ice.
"We now understand what happened, and we can fix it with a software patch," said Barry Goldstein, the Phoenix project manager at NASA's Jet Propulsion Laboratory in Pasadena.
Phoenix landed near Mars' north pole on May 25. The $420 million mission is planned to last 90 days.