June 20, 2008—Before-and-after photos taken by NASA's Mars Phoenix Lander show "perfect evidence" of water ice on Mars, according to Peter Smith, the mission's principal investigator, in a statement released Thursday.

The dice-size crumbs of bright material seen in the bottom left corner of the so-called Snow White trench in the left image, taken June 15, appear to have vanished by the time the right image was taken, on June 19.

Scientists are convinced the material was frozen water that vaporized after the lander's robotic arm dug up the material.

"There had been some question whether the bright material was salt," Smith, of the University of Arizona, said. "Salt can't do that."

Phoenix is studying whether the Mars's arctic region could be suitable for life as we know it—a key ingredient of which is water.

More recently, Phoenix's arm encountered a hard surface on Thursday while digging another trench. The arm went into a "holding position" after three attempts to dig further, which is expected when the arm reaches a hard surface, NASA said.

Scientists, who hope to uncover an icy layer, have dubbed the newer trench Snow White 2, in keeping with the Phoenix team's practice of using names from fairy tales and mythology to designate features at the lander's "Wonderland" site.

In 2002 the Mars Odyssey orbiter had detected hints of a vast store of ice below the surface of Mars's 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.

(Related photos: "Mars Lander Takes First 'Self Portraits'" [May 27, 2008].)

—John Antczak, Associated Press