Scientists with the Phoenix Mars Mission on Friday declared for certain there is ice on Mars, putting them an essential step closer to answering the question that has driven three decades of Mars exploration and centuries of Earth-bound speculation: Could there have been life there?

Pictures beamed 170 million miles to Earth from the Phoenix lander atop Mars' polar plain erased any doubt about the issue, they said, although in a roundabout way.

Last Sunday, several dice-sized solids were observed at the bottom of a trench that had just been dug by Phoenix's robotic arm. On Thursday, they were gone.

The only reasonable explanation, the scientists said, is that the objects were pieces of ice that evaporated into the dry Martian atmosphere through a process called "sublimation." And the presence of ice means that Mars might once have had liquid water, which is essential for life — at least as it is known on Earth.

It was too soon to know whether the entire astrophysical community will accept the disappearing objects reported Friday as proof, but the Phoenix researchers said they did not need any more convincing.

The rocket thrusters that slowed Phoenix to a soft landing revealed a white, hard substance in the ground beneath it — and tantalizingly out of reach — when the lander touched down on May 25. Similar white material was visible when the arm began to dig below the top few inches of Martian soil.

One possibility was that it was salt of some sort. But ice was always the more likely explanation.

"Salt does not behave like that," said Mark Lemmon, a scientist at Texas A&M University who is in charge of Phoenix's Stereo Surface Imager. "We found what we were looking for. This tells us we have water ice within reach of the arm."

Although Mars is much too cold now to have liquid water on its surface, scientists believe that may not always have been the case. Images from as far back as the Viking missions in the 1970s revealed channels and gullies that appear to have been carved by flowing liquid at some point in the planet's history.

The Mars Odyssey orbiter proved in 2002 that huge quantities of hydrogen existed under the Martian topsoil. Although many compounds are high in hydrogen (including petroleum), the scientists believe the only one that could be there in such quantity is water ice, which consists of two hydrogen atoms bonded to an oxygen atom.

"I don't know how you could have so much hydrogen under the surface, and something that disappears at just the temperature of ice, and have it not be ice," said Peter Smith, a physicist at the University of Arizona who is the principal investigator for the Phoenix mission.

The researchers chose Mars' northern polar plain as the landing site specifically because they believed it would give them the best chance of finding ice. They now believe their hunch was correct.

"If you were to get a big broom and sweep it off, we are on a big ice sheet," Smith said.

Water is necessary for life but is far from sufficient. Scientists will also have to find a fair amount of carbon before they are willing to say the planet might have been habitable. Carbon forms the chemical backbone of proteins and fats, and in addition to water it is the major constituent of living cells and tissue.

"The ice may always be in a frozen state," Smith said, noting that without liquid water, the formation of life is hard to imagine.

The researchers are extremely interested to know what compounds may be either frozen into the ice or, more likely, deposited in the dry, reddish soil covering it.