NASA's Phoenix lander, already struggling to keep its batteries charged in the increasingly cold, dim martian winter, was buffeted by an unexpected dust storm Oct. 27 that sharply reduced the amount of power its solar panels could provide to run critical heaters. Waking up later and later to phone home via satellite, Phoenix failed to wake up at all on Nov. 2, prompting NASA managers to reluctantly cease science operations.

"At this time, we're pretty convinced the vehicle is no longer available for us to use," said Barry Goldstein, Phoenix project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "So we are actually ceasing operations, declaring an end to mission operations at this point."

He said NASA's orbiting Mars Odyssey and Mars Reconnaissance Orbiter spacecraft will continue to listen for calls from Phoenix just in case it somehow manages to store enough energy to wake up and check in, but engineers are not optimistic. In all likelihood, the $475 million mission is over.

Unlike the space agency's hardy Spirit and Opportunity rovers, still operating after five years in the more benign equatorial regions of Mars, mission managers said from the beginning that Phoenix would be lucky to survive to the end of the year at its extreme northern latitude. The spacecraft landed May 25 and operated for two full months beyond its 90-day mission design lifetime.

"We always knew with Phoenix that five years was not in the cards," Goldstein said. "It's rather tough living up north above the Arctic Circle. So we always knew the end would be coming for us."

Spirit and Opportunity, located on opposite sides of Mars, have been studying the local geology to help scientists understand the role of water in the red planet's past. The rovers have confirmed that water existed on Mars earlier in its history. Pictures from Mars orbiters clearly show huge channels and other features on the surface that almost certainly were carved by flowing water and there is evidence that Mars once featured a vast ocean in the northern hemisphere.

Mars today is cold and dry, the presumed victim of an environmental catastrophe of some sort in the remote past. But the Mars Odyssey orbiter, equipped with an instrument that can detect evidence of water up to three feet below the surface, discovered vast amounts of ice locked in the soil of the planet's polar regions, ice that could be the remnant of an ancient sea. Phoenix, equipped with a robot arm and a suite of ovens and other instruments to assess soil chemistry, landed on that frigid surface May 25.

Peter Smith, the Phoenix principal investigator at the University of Arizona at Tucson, told reporters today the mission was a complete success.

"What have we done? The first thing was to discover ice," he said. "When we landed, we looked around, we saw a field of dirt and rock that was spread out to the horizon and we didn't see ice right away. It wasn't until we looked under the spacecraft that we found out we were standing on it. This was quite a thrill for everybody.

"Studying that ice has kept us busy for the past five months. We've excavated to the ice, we know its depth, how it changes over the surface, we've seen different types of ice. Really, the mission was all about water and it's going to keep us busy for some time now as we try to understand what we've got."

Cameras aboard Phoenix snapped some 25,000 pictures, ranging from high-resolution panoramas to high-power microscope views of collected soil samples. Its sophisticated instruments analyzed soil and ice samples to help scientists characterize the past and present habitability of the landing site.

"We noticed that the soil was alkaline and its filled with carbonates and clays," Smith said. "On Earth, we would conclude immediately there was liquid water in this soil. For Mars, we have to be a little more careful and we're going to develop this story as we interpret our data. But definitely liquid water has been a part of this soil.

"We also see salts and the perchlorate molecule, which is totally unexpected. It has profound implications for Mars. Perchlorate, of course, is an energy source for microbes on the Earth, its easily soluable, it's something you have to worry about in your drinking water, it has implications for human health. So we've seen nutrients and energy sources. So that leads to the question, is this a habitable zone? Have we found such a thing on Mars?"

The jury is still out. Smith said scientists have not yet completed their analysis of data from Phoenix and he would not rule out the possibility that organic compounds may yet be found.

Phoenix already was struggling to stay alive in the harsh northern environment and mission managers were planning their final "high power" experiment runs when a dust storm cropped up Oct. 27. The storm cut the lander's life short by about three weeks, Goldstein said.

"We were executing what we thought would be the last of what we call our high-power science days," he said. "Those are days on Mars that we dedicate a significant amount of energy to do science. We were planning at that point to turn off the heaters on some of the electronics and then move into a more dormant state where we were going to do some weather monitoring, maybe the occasional image. Unfortunately, Murphy had a little problem for us ... and threw a dust storm at us. It just came out of the blue for us."

"So immediately the team ... went to work really hard to try to shed load, to turn unnecessary power off the vehicle to keep it from going into a power starvation mode. Unfortunately, the end of the mission was coming. We knew this would happen eventually, but we were unsuccessful in keeping the batteries from browning out. What I mean by browning out is, we always go to sleep every night on Mars and ever since the sun's been going down, it's become more imperative. At night, we rely on our batteries to keep the vehicle warm. When the batteries reach zero, kind of like your laptop computer running out of juice when you're on an airplane, the vehicle would just shut down."

For a few days, Phoenix struggled to stay alive.

"The vehicle would brown out, it would die, the sun would come up the next morning and then as the sun came up, the power would come out of the solar arrays, the vehicle would wake up and for a couple of (days) there actually communicated with us, told us what it was doing," Goldstein said.

But the dust lingered "and it became increasingly harder and harder for the vehicle to wake up, and therefore increasingly colder and colder each morning," Goldstein said. "So much so that on Nov. 2, last Sunday, was actually the last time we heard from Phoenix."

Next April, the sun will drop below the horizon at the Phoenix landing site for about three months. The spacecraft will be covered in carbon dioxide ice and "overnight" temperatures will plunge below minus 300 degrees Fahrenheit. Circuit board coatings and even the solar panels themselves might crack or even break off. By next October, the sun will be high enough once again to provide the energy needed, in theory at least, to wake Phoenix up. NASA will listen, but Goldstein said there is little chance the lander will revive.

"This has been a great mission," he said. "I think we have had a huge success."