Phoenix fades away

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Mars lander scrapes up its last samples.

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In its final days Phoenix, the NASA lander that since May has been scraping at subsurface ice in the martian arctic, is blinking in and out of contact with Earth.

As temperatures plummeted to nearly −100 °C and dust storms and clouds obscured an enfeebled sun, the spacecraft last week missed several chances to communicate with satellites passing overhead, and plunged for the first time into a bare-bones survival mode. Although engineers may wring a few more days of erratic behaviour out of the lander, Phoenix is almost certain not to survive the coming winter. Thick slabs of frozen carbon dioxide will coat the spacecraft, and its electronics will break for good. "It's like an ageing parent in the nursing home. You know it's coming," says principal investigator Peter Smith of the University of Arizona in Tucson.

Engineers hope to initiate a final sequence of low-power experiments — mostly weather measurements — as early as 5 November, but the craft's weak condition may not even allow this. With a sudden surfeit of free time, mission scientists are returning to their notebooks to see what their data may say about the history of martian water ice and its implications for habitability. But frustrations with some of the instruments mean that the story is not complete.

The Phoenix Mars lander: a life.

For Smith, the highlight was Phoenix's flawless landing in May — a redemption for a spacecraft that was mothballed in a Lockheed Martin warehouse after a sister ship, the Mars Polar Lander, crashed near the south pole of Mars in 1999. The bold idea for Phoenix was to free the spacecraft from storage, strip it of Polar Lander's bugs and rebuild it within a $420-million budget. Operated from the University of Arizona, the lander would also be the first major mission controlled from outside of NASA's traditional centres such as the Jet Propulsion Laboratory in Pasadena, California. "There were a lot of doubters," says Smith. "It wasn't a sure thing by any means."

Summer's lease is ended: Phoenix will dig for ice no more. Credit: NASA/JPL-CALTECH/UNIV. ARIZONA/TEXAS A&M UNIV.

Yet Phoenix settled successfully on a northern plain. Originally scheduled for three months, the mission was then extended to at least 18 November, and the early part went swimmingly. A robotic arm and scoop clawed its way through a few centimetres of soil to water ice. A camera, perched atop a tall mast, caught nuggets of ice sublimating away. Panoramas showed differently sized polygonal cracks in the soil, suggesting that contractions due to freezing occurred over different temperature regimes in recent epochs, as the tilt and orbit of Mars changed. And a wet-chemistry instrument, using beakers that made soil slurries with water brought from Earth, found that the polar soil was like nothing else tested so far on Mars. The soil was basic, rather than acidic, and contained trace amounts of perchlorate, a weak oxidizer that on Earth can nourish microbes.

But Phoenix's workhorse instrument, a unit with eight ovens that baked thimbles of soil and could sniff emitted gases for organic compounds, was plagued with problems. The soil, surprisingly sticky, was hard to get past oven doors that, owing to a manufacturing error, opened only partway. A short circuit, probably caused by shaking the ovens to move the sticky soil, made NASA nervous. Worried that the instrument could fail at any time, headquarters directed the team to prioritize retrieving an ice-rich sample for analysis.

"The clock was running against us," says William Boynton of the University of Arizona, lead scientist for the instrument. Trying to nab the ice sample was particularly frustrating. "We wasted nearly half of the mission doing that," he says. In the end, Boynton got results from only five of the eight ovens. He never got to test the ice for isotopic ratios that could have said something about its age.

However, towards the end of the mission the instrument began to redeem itself. It found a strong signal for calcium carbonate, a mineral that typically forms in the presence of water. A separate, weaker signal in the soil may indicate a different type of carbonate, or even an organic molecule. Boynton hopes to settle this by comparing carbon isotopes from the two sources. With the stress of daily operations halting, he'll finally have a chance to do so.

Additional information

For an online slideshow of pictures taken by Phoenix, see http://tinyurl.com/6a3vkt See also Correspondence: Finding of unusual soil on Mars could stem from tools used

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Phoenix landing blog

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University of Arizona Phoenix website

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Hand, E. Phoenix fades away. Nature 456, 8–9 (2008) doi:10.1038/456008a

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