Published online 6 June 2008 | Nature | doi:10.1038/news.2008.878

News

'Dandruff' could contaminate Phoenix landing site

Dead microbes and skin flakes from Earthlings may scupper the search for organic molecules.

martian dustThe most detailed photo ever of martian dust - but Phoenix may have less success in finding organic molecules.NASA/JPL-Caltech/Univ. Arizona

Could Phoenix’s search for organic molecules on Mars be foiled by dandruff from Earth? After a successful landing last month on the planet’s northern plains, the NASA spacecraft is busily scraping through the martian dirt. Next week, the mission team plans to use one of its premier instruments, the Thermal and Evolved Gas Analyzer (TEGA), to test its first baked soil sample for molecules containing carbon.

The search for the organic building blocks of life has been a major selling point for Phoenix; many press accounts have eagerly, yet mistakenly, foreshortened the mission’s raison d’etre to ‘the search for life’. Yet some mission scientists say that it is the science goal least likely to succeed, partly because TEGA is so sensitive that it may end up sensing only contamination from Earth.

“We will see organics, for sure, because we’re bringing them,” says Aaron Zent, a mission scientist from NASA’s Ames Research Center in California. Likely contaminants include skin flakes, dead microbes and volatile lubricants. “The problem with an instrument so sensitive is all you detect is your own schmutz,” says Zent.

From the ashes

In some ways, Phoenix is better suited to finding organic molecules than the 1970s-era Viking landers, which detected none in the caustic soil. Ultraviolet radiation creates strong oxidizers, which seem to have destroyed even the organic content expected from comet and meteor impacts.

“We will see organics, for sure, because we’re bringing them.”

Aaron Zent
NASA Ames Research Center

Phoenix has two possible advantages. First, its ovens will bake soil and ice samples to 1,000°C – 500ºC hotter than the Viking ovens – and this might vaporize stubborn molecules that the Vikings missed. Second, Phoenix sits on soil nearer the martian north pole (which receives less ultraviolet radiation), atop ice that may neutralize the oxidizers.

But the advantages end there. NASA spent about US$800 million, in 1975 dollars, on the Vikings, which were encased entirely in a biological shield prior to launch. And NASA is spending almost US$2 billion for a 2009 launch of the Mars Science Laboratory, which will be able to 'bake out' its ovens – ridding them of contamination upon arrival at Mars – before ingesting samples.

The $420 million Phoenix mission, by comparison, is low-budget, built from parts recycled from a cancelled mission — the Mars Surveyor Lander — that had been kept in a warehouse – and how much dust those parts gathered is a worry. “We’re doing a quick and dirty organic analysis,” says TEGA lead scientist William Boynton, of the University of Arizona in Tucson. “We’re kind of doing it on a shoestring.”

It's all about mass

The mass spectrometer in TEGA – the tool that sniffs the gas coming out of the ovens – was added to the spacecraft as an afterthought; the rest of the instrument wasn’t built for sensitive organic detection. And so, for instance, TEGA contains motors with wax components that cannot be baked out to high temperatures. “It’s being pressed into service for something it was not designed for,” says Zent.

The biological barrier that encased the robotic arm during space flight has received much attention. But it was motivated mostly by planetary protection, NASA policies (some mandated by international treaties) that aim to prevent the forward contamination of Mars with living spores or microbes from Earth.

ADVERTISEMENT

While built in clean rooms, many parts of the spacecraft were not wrapped in prophylactics. Organic films could creep into TEGA. Organic particles can move during descent, or with the wind. And just because Phoenix’s surfaces and instruments were treated with heat or alcohol doesn’t mean the problem disappears. Dead organisms contain organic molecules just like living ones.

The Phoenix team is making the best of the situation. Engineers added a protective film that covered the eight TEGA oven doors; it was retracted one week after landing. The team has catalogued the chemical signature of 20 possible contaminants such as Braycote, a widely used lubricant, so that it can discount them if they appear in TEGA. And all the practice digging and dumping that Phoenix has performed could dilute residual contamination in the scoop.

But the real key in the quest to find organic molecules, Boynton says, resides in a small white block of glass-ceramic that sat inside the biological barrier, next to the robotic arm. This 'organic-free blank', engineered to contain no carbon, is the experimental control. Using the rasp at the end of the robotic arm, the TEGA team will feed powder from the blank into one of TEGA’s eight precious single-use ovens. If the gas measured at the back end contains no organics, then scientists will assume that background contamination levels for all eight ovens are low. But if the blank produces a high level of background contamination, then the Phoenix team has little hope of detecting the small signal of Martian organic molecules. In that case, Boynton bluntly acknowledges, “we’re screwed.” 

Commenting is now closed.