Scientists with the Phoenix Mars mission yesterday declared for certain that there is ice on the Red Planet, 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's northern polar plain erased any doubt about the presence of ice, they said.

But the evidence came in a roundabout way. Last Sunday, several dice-size solids were observed at the bottom of a trench that had 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 is too soon to know whether the entire astrophysical community will accept the disappearing objects reported yesterday as proof, but the Phoenix researchers said they do not need any more convincing...