Bright crumbs that were visible earlier this week in one of the trenches dug by NASA's Phoenix Mars Lander are no longer there, according to images released yesterday by the space agency. Mission scientists said the disappearing act leaves them with only one conclusion: the material was ice that vaporized directly from the solid form in the dry, frigid Martian polar atmosphere.

Phoenix's mission is to seek out and study the polar ice believed to be buried up to a few inches beneath the Martian soil. The Phoenix team had seen hints of ice in the form of shiny patches underneath the probe, apparently uncovered during landing, and in "Dodo-Goldilocks," the first set of trenches dug by the machine's robotic arm.

But they couldn't rule out salt as an explanation. They first noticed the shiny grains last week as they were preparing to expand the initial trench, Dodo. Although the objects—about eight of them, measuring up to two-thirds of an inch—seemed to be shrinking after a day, the definitive evidence came in early yesterday, when they were gone entirely.

"It must be ice," chief Phoenix scientist Peter Smith of the University of Arizona at 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. There had been some question whether the bright material was salt. Salt can't do that."

Solids sublimate or vaporize without passing through a liquid stage when they are heated at low enough pressure. The process can also happen to snow on Earth on a cold, dry sunny day. Mars has a thin atmosphere and frigid temperatures that dip below minus 100 degrees Fahrenheit (minus 73 Celsius) in the summer.

Sublimation would prevent ice from lasting very long on the surface, Smith said during a news conference today. The ice was exposed about two inches (five centimeters) below the surface. "If you were to get a big broom and sweep this dirt off, we're on a big ice sheet," Smith said.

Researchers expected Mars to have ice under its north pole based on scans by the Mars Odyssey orbiter as well as imaging that showed the polar surface to be riddled with troughs forming a polygonal or roughly honeycomb shape, suggesting ice below the surface was cracking in the winter.

Phoenix landed on the edge of one of these polygons. The Dodo-Goldilocks dig site was located in the trough around it. The latest dig site, called Wonderland, lies near the middle of it. NASA reported yesterday that Phoenix had encountered a hard layer at the same depth as the earlier white clumps while digging a trench, "Snow White 2," at this site. Images sent back today showed bright material resembling those seen in Dodo-Goldilocks.

That proximity to ice is what Phoenix is all about, said its Surface Stereo Imager co-investigator Mark Lemmon of Texas A&M University. "That's the thing that really excites me, that we can actually reach out and touch the ice on Mars now."

Smith said the plan now is to prepare to quickly gather some of the ice quickly using the robotic arm's rasp, a pinky-sized scraping tool. The team will have Phoenix deliver the material to one of the ovens in its Thermal and Evolved Gas Analyzer (TEGA) so that its composition can be understood. From there the scientists can begin to figure out when the water was last liquid and whether any parts of Mars are or were once habitable.

"We're following the water, but it's liquid water we're really interested in," he said. "It's going to take a few weeks for us to get down to answer those big questions."