The Dirt on Mars Phoenix Lander

訪客

Could the Mars Phoenix lander have been contaminated by bacteria from Earth?

The possibility was raised by rumor-multipliers feasting on an Aviation Week report that the White House had been briefed on "major new Phoenix lander discoveries concerning the 'potential for life' on Mars."

The buzz has since quieted, but it raised the prospect, if only wildly, that the Phoenix found Martian soil so habitable because transplanted microbes flourished there. But that, say researchers, is highly unlikely.

Mars explorers have a profound self-interest in ensuring that bacterial hitchhikers don't confound their results: imagine asking for NASA funding after claiming a plucky strain of underarm bacteria as extraterrestrial life. And if Earthly bacteria survives a trip and then flourishes, it could upset an alien ecosystem -- the equivalent of finding something rare and priceless by stepping on it.

"Not every mission is something we worry about. The concern is when we go to a place where life could exist. The Phoenix lander is in Category Four, the highest level of concern," said Margaret Race, a planetary protection expert at the SETI Institute.

The lander was assembled in a chip fabrication-style clean room, said Race, and thoroughly cleaned afterwards. A few microbial spores inevitably make it through, or are picked up during launch -- "but what's probability that it will stay through launch, go through re-entry, and stay on the spacecraft?" she said. "And then, if it gets jostled off, could it survive? And grow and reproduce? At each point there's a lower and lower probability."

Of course, bacteria are fabled for their ability to colonize extreme environments: strains have evolved to survive everything from Antarctic ice to nuclear reactors to deep-sea volcanoes.

"But of all the organisms on Earth that might survive in unusual places, what are the chances you'll find those in a clean room at the Kennedy Space Center?" said Race.

The Phoenix lander does contain water taken from Earth for mixing with Martian soil. (Since water is ubiquitous in the chemistry of life on Earth, adding it to just about any Earthly organism gets some sort of rise; the same, we hope, holds for Mars.)

But Jim Bell, a Cornell University geologist who served on a National Academies of Science panel on Mars mission contamination, isn't worried. "There are rigorous controls on the purity of that water," he said. It's distilled, purified, and rigorously checked for the presence of anything other than hydrogen and oxygen.

The last remaining contamination possibility resides in sabotage: perhaps, tempted by the God-like act of introducing life to another planet, an engineer secretly tucks away microbes modified to withstand solar radiation, deep-space cold and the heat of atmospheric entry.

There's no guarantee against that. Though spacecraft assembly is tightly monitored, and contamination is measured at every step of the process, final responsibilities lie with a small group of people -- and people can be tricked or corrupted.

But between conscientious, competent engineers and the rigors of interplanetary travel, the odds against contamination are astronomical.

Of course, there's nothing a good conspiracy theory likes more than astronomical odds, but that could be a positive thing. Contemplating the possibility of contamination forces people to confront questions of environmentalism in space: what's our responsibility to non-Earthly life?

More on that question soon. In the meantime, read "What to Do About Little Green Goo."

Note: The Aviation Week story that kicked off the life-on-Mars rumor-storm has since been follwed by a report that Martian soil is unexpectedly bad for life. The latter story, at least, was sourced and seems plausible: NASA's scheduled a press conference tomorrow to review the soil results. But to engage in a little rumor-mongering myself: my own sources suggest that upcoming findings will ease any disappointment you might feel tomorrow.

Update: I originally posted that the Aviation Week story was "retracted." This was not the case; my apologies to the journalist, Craig Covault, for my careless reporting.

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