Probe ends historic Mars mission

By Jonathan Amos Science reporter, BBC News

The probe had surpassed its expected lifetime by more than two months

Nasa says its Phoenix lander on the surface of Mars has gone silent and is almost certainly dead.

Engineers have not heard from the craft since Sunday 2 November when it made a brief communication with Earth.

Phoenix, which landed on the planet's northern plains in May, had been struggling in the increasing cold and dark of an advancing winter.

The US space agency says it will continue to try to contact the craft but does not expect to hear from it.

"We are actually ceasing operations, declaring an end to operations at this point," Phoenix mission project manager Barry Goldstein said at Nasa's Jet Propulsion Laboratory in Pasadena, California.

"However, since we've been surprised by the robustness of this vehicle, we're going to keep listening. As the orbiters fly overhead every two hours, we'll constantly turn on the radio and try to hail Phoenix to see if it is alive."

Fiery plunge

Launched from Earth in August 2007, the robot arrived on Mars on 25 May, landing further north than any previous mission to the Martian surface.

To make it down, the probe had to survive a fiery plunge through the Red Planet's thin atmosphere, releasing a parachute and using thrusters to control its descent.

The mission was scheduled to last just three months on the surface, but continued to work for more than five months.

During its ground operations, the robot dug, scooped, baked, sniffed and tasted the Martian soil to test whether it has ever been capable of supporting life.

Phoenix's major achievement was in becoming the first mission to Mars to "touch water" in the form of the water-ice it found just centimetres below the topsoil. Chunks of ice were seen to vaporise before the lander's cameras.

"This was quite a thrill for everybody and it has been the study of that ice that has kept us busy for the last five months," said Phoenix principal investigator Peter Smith of the University of Arizona in Tucson.

"We've excavated that ice, we know its depth, we know how it changes over the surface; we've seen different types of ice."

Phoenix landed further north than previous missions

The spacecraft found the Martian soil to be mildly alkaline, quite different from the acidic soils seen by previous missions to other parts of the planet.

Other key results included the identification in the soil of calcium carbonate, which on Earth is a chief component of limestone rock.

Phoenix also detected sheet-like particles, which were probably clays of some kind.

The significance of both minerals is that they form only in the presence of liquid water - which could have supported life.

The lander also detected perchlorate (an ion containing chlorine and oxygen) which is an oxidising chemical and, on Earth, can sustain some microbes.

Perhaps the key achievement has been to touch the water-ice in the soil

Phoenix even recorded snowfall; and took more than 25,000 pictures, from the panoramas of its Arctic landing site to the atomic scale images of dust grains delivered to its microscope.

"Right now at this epoch in Martian history it is certainly too cold for organisms to be alive, certainly in the sense of Earth organisms," said Peter Smith.

"But we do think that over time as the Mars climate changes that it can get warm enough that, perhaps, we are getting at least films of liquid water or dampness in the soil; and that could create an environment where life could exist. That would be in the last few million years; very recent in Mars history."

The Phoenix mission scientists have a mass of data that will keep them busy for decades. They have not yet given up hope of seeing a signature in the data for organics, the carbon-rich molecules that can be considered the "feedstock" of biochemistry.

One disappointment, however, from the final days of the mission was the failure to get a microphone on the lander to work. This would have returned the first sounds of Mars.

Overall, though, the US space agency is delighted with the achievements of the mission.

"This is an Irish wake rather than a funeral," commented Doug McCuistion, director of the Mars exploration programme at Nasa Headquarters in Washington DC.

"We should celebrate what Phoenix and the Phoenix team has done and where it is going to take us in the future. [There were] a lot of lessons learned in this mission for us that will feed forward to future missions. We learned a lot about handling of soils, soil consistency, and how difficult it can be."

Cracking up

Phoenix was never expected to be a long mission. At its high latitude (68 degrees North), it was always destined to be starved of light as the Arctic winter deepened.

In the end, though, the demise of Phoenix was hastened by a dust storm which obscured the Sun's precious rays still further.

With so little energy getting into its solar panels, the batteries on Phoenix were regularly going flat, preventing the robot from heating its systems in temperatures that were heading down to minus 100C.

Nevertheless, Nasa says its Mars Reconnaissance and Odyssey satellites will continue to listen for Phoenix for a further three weeks, until Solar Conjunction, when Mars moves behind the Sun as viewed from Earth.

The probe has sent back more than 25,000 pictures

As winter progresses, Phoenix will be covered in a thick layer of carbon dioxide frost. As the ice builds up on the solar arrays, they are likely to crack and fall off. The electronics will also break up in the cold that could see temperatures go down to minus 180C.

Phoenix had risen from the ashes of two previous failures.

In September 1999, the Mars Climate Orbiter spacecraft crashed into the Red Planet following a navigation error caused when technicians mixed up "English" (imperial) and metric units.

A few months later, another Nasa spacecraft, the Mars Polar Lander (MPL), was lost near the planet's South Pole.

Phoenix used hardware from an identical twin of MPL, the Mars Surveyor 2001 Lander, which was cancelled following the two consecutive failures.

Nasa's robot rovers, Opportunity and Spirit, continue to work at their equatorial landing sites five years after arriving at the planet.

The next mission to the surface of Mars is due to leave Earth next year. The Mars Science Laboratory is a "smart" rover that will be dropped on to the surface of the Red Planet by a rocket-powered "skycrane".

At almost three metres in length and weighing 850kg, MSL is considerably bigger than the current rovers.

Phoenix was sent to Mars with seven science instruments

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