LOS ANGELES, California (AP) -- Scientists believe that NASA's Phoenix Mars lander exposed bits of ice while recently digging a trench in the soil of the Martian arctic, the mission's principal investigator said Thursday.
Trenches excavated by the lander's robotic arm have turned up white material mixed in with the reddish dirt.
Crumbs of bright material initially photographed in the trench later vanished, meaning they must have been frozen water that vaporized after being exposed, said Peter Smith of the University of Arizona, Tucson, 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.
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.
Engineers also have prepared a software patch to send up to Phoenix to fix a problem that surfaced Tuesday in the use of its flash memory.
NASA said that because Phoenix generated a large amount of duplicative file-maintenance data that day, the mission team has been avoiding storing science data in the flash memory and is instead transmitting it to Earth at the end of each day.
"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 May 25. The $420 million mission is planned to last 90 days.
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