BioEd Online

Recent media reports that certain comet samples returned by the Stardust mission may simply be contamination have been overblown, say researchers.

In a paper to be published in the journal Energy and Fuels, astrobiologists in Spain suggest that some minerals returned by the Stardust mission might have been caused by a reaction between the rocket's fuel and the fuel tank¹. The Stardust scientists, however, say they have guarded against this possibility, and are confident that what they identified as osbornite (titanium nitride)² is the genuine article — a mineral formed near a star, and later caught up by the comet.

The comet-collecting Stardust mission, which returned its bounty to Earth early last year, was powered by a nitrogen-based fuel called hydrazine. Among the samples analysed were, surprisingly, grains of osbornite that the Stardust scientists say must have formed in a hot place, near a star. Isotopic evidence points to that star being our own Sun.

But that might not be the entire story, says Jesus Martinez-Frias at the Centre for Astrobiology, Madrid, and his colleagues. Their paper notes that another mechanism could have formed these Stardust samples: the titanium-built spacecraft and the hydrazine fuel might have reacted to form the osbornite.

Not so, says Michael Zolensky, Stardust's curator at the Johnson Space Center in Houston, Texas. "They're wrong, and for several reasons," he says of the team's suggestion. When the spacecraft was designed 10 years ago, careful attention was paid to the direction of the hydrazine jets, he says, they all pointed directly away from the collector. Extensive modelling of this set-up showed that there was no chance of hydrazine reaching the sample collector.

Silent witness

There was also a second 'witness' collector on Stardust, which was exposed to the craft but not to the comet, and so could be used as a control. This detector was remarkably free of contamination, says Zolensky. "It's amazing how clean it was."

The osbornite was also found deep inside the aerogel sample tray, and encased within other mineral grains — further bolstering the conclusion that the mineral was cometary, says Zolensky. If they were contamination products, the osbornite would be seen on the surface of the aerogel, he says.

Martinez-Frias says he is aware of these things, but still thinks contamination should be considered, with additional tests to see if the isotopic composition and physical form of the minerals shed light on their origin.

Michael A'Hearn at the University of Maryland, College Park, who led the Deep Impact mission to collect further comet material from a direct collision, says it is good to thoroughly question conclusions drawn from these data. "These questions always need to be raised," he says. A'Hearn adds that contamination issues are incredibly important, but in this case he thinks the Stardust scientists have it covered. "Most well organized science teams would have raised these questions internally," he says.

Martinez-Frias says he is not trying to open a big debate: "Our only intention was to bring attention to this problem," he says. Zolensky says there is no need: "We thought of all these things years ago."