Published online 27 May 2008 | Nature | doi:10.1038/news.2008.857

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Phoenix set for three-month dig on Mars

NASA scientists hope for first taste of martian ice after soft landing for craft near Mars's north pole.

martian landscapePhoenix is now settling down to some digging in the red martian rock.NASA/JPL-Caltech/Univ. Arizona

After a dramatic deceleration through Mars's thin atmosphere that ended with a gentle landing in welcoming rock-free terrain, NASA's Phoenix mission now stands ready to go to work.

"Everything just worked like a charm," says Barry Goldstein, the mission's project manager at the Jet Propulsion Lab in Pasadena, California, which ran mission control for the landing. Within a week, Phoenix will start to satisfy the curiosity of scientists who have been waiting since 1999 to sample Mars's subsurface ice for organic molecules and signs of recent thawing.

It took less than a year for the spacecraft to travel 680 million kilometres from Earth to the northern plains of Mars. But for many mission scientists, the journey was much longer. After years of planning and development, in 1999 NASA launched Mars Polar Lander, a similar mission aimed not at Mars's chilly north but at its cool south. But something went wrong with the craft's landing, which relied on a rocket system for a gentle touchdown rather than the more rough-and-tumble airbag approach that had proved successful for the Pathfinder mission in 1997. Unwilling to risk another failure, NASA cancelled the Polar Lander's sister mission, storing its already-built parts in a warehouse.

Phoenix started as a bid by scientists and engineers — many of them from the Polar Lander team — to rescue and rehabilitate the cancelled mission within a US$420 million (£212 million) budget, revamping it in the process and retargeting it at the northern plains, where orbiting instruments had shown signs of hydrogen near the surface, presumably in the form of ice. Even with years spent scrubbing its software free of anything that looked like a possible glitch, the descent that slowed Phoenix from 5.7 kilometres per second to 2.4 metres per second within seven minutes, using heat-shield braking, a parachute and thrusters, was a tense one for the team. "I thought I was being pretty cool. But after it landed, I realized that I had been sweating like a pig,” said Aaron Zent, a scientist at NASA Ames Research Center who has had a role in five failed Mars missions.

Parachutes and polygons

phoenix parachuteThe Mars Reconnaissance Orbiter captured this snap of Phoenix's parachute descent.NASA/JPL-Caltech/Univ. Arizona

The successful descent was documented by a spectacular photo of the lander's white parachute and shell taken by the Mars Reconnaissance Orbiter, which, along with the Mars Odyssey Orbiter that provided the evidence for subsurface ice, is acting as a radio relay for the mission.

The only hint of the untoward was that Phoenix landed almost 30 kilometres east of where it was expected — at the edge of the landing ellipse that marked the teams 99% confidence expectation of where it would touch down — owing to a parachute opening seven seconds late. But it didn't matter. The site was free of awkward rocks and the lander settled down on an even keel, which will allow it to maximize solar power from its twin fan arrays. "This is a scientist's dream, right there on this landing site," says mission principal investigator Peter Smith, of the University of Arizona at Tucson.

Phoenix's first pictures showed tantalizing polygonal patterns in the terrain stretching to the horizon; the polygons, also visible from orbit, are similar to patterns associated with permafrost in polar regions on Earth. The material in the cracks, formed by the expansion and contraction of the ice underneath, could explain much about the history of water on the planet. Mars's climate may have changed over the past 100,000 years as its orbit and axial tilt have shifted.

Short active life

Mission scientists will continue to check and calibrate instruments over the next few days. Next week, they hope to begin scooping soil with a robotic arm — although early pictures indicate that there may be only a few centimetres of soil, if any. A rasp on the back of the scoop will grind samples of the dirty ice. These materials will be fed into two instruments: a 'wet lab', where water is added to surface samples in tiny beakers; and a 'hot lab' with ovens that bake the samples to determine the chemical composition of their vapours.

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In the 1970s, the Mars Viking landers, performing similar analyses, found no evidence for organic molecules. But the landers were at lower latitudes and scooped up strongly oxidized soil created by harsh ultraviolet radiation. Scientists say the polar ice could be a more hospitable environment for organics.

Phoenix scientists will have to be choosy with their samples. There are four beakers and eight ovens, each of which can be used just once. After they are used up, a meteorological station will continue to measure wind speed, temperature, pressure and cloud heights.

But mission scientists don't expect Phoenix to linger for years like the Mars rovers, which landed in 2004 and are still trucking on. As the midnight sun of martian summer sinks into the black of polar winter, Phoenix's power supply will run out. "We're going to operate until Mars freezes over," says Goldstein. 

Eric Hand is blogging from the Phoenix science headquarters in Tucson, Arizona, on Nature's phoenixlanding/">In the Field blog .

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