Life on Mars data A University of Hawaii scientist and a colleague in London have discovered evidence shattering a NASA group's dramatic 1996 finding that a meteorite contained signs of ancient life on Mars.
is discredited
by UH scientist
Edward Scott and a colleague
say NASA's evidence is incorrectBy Helen Altonn
haltonn@starbulletin.com"We're not saying there isn't (life on Mars), just that this evidence put forward is incorrect," Edward Scott, planetary scientist in the Institute of Geophysics and Planetology, said in an interview.
The origin of iron oxide crystals or magnetites in the ALH 84001 Martian meteorite, which was found in Antarctica in 1984, has been debated since NASA scientist David McKay and colleagues argued that the 16-million-year-old rock contained evidence of microorganisms.
Scott and his associates cast doubt on that idea in several papers published since 1996 on their analysis of the famed rock.
He said conclusive evidence is presented in his latest study with David J. Barber of the University of Greenwich in London. Results of the study appear today in Proceedings of the National Academy of Sciences.
"We infer that it is unlikely that the carbonates or any minerals in them contain mineralogical evidence for ancient Martian life," they concluded.
NASA scientists have focused in the past few years on the meteorite's magnetic minerals as the most compelling evidence for Martian life, Scott said. However, he and Barber discovered the magnetite crystals formed in the rock as a result of heat from an impact and decomposition of carbonate and not inside microorganisms.
Analyzing a slice of the meteorite with an electron microscope, the researchers found atoms in the crystals "all nicely lined up" with those in surrounding carbonate material, Scott said. "That tells us they had to form inside the carbonate because they're lined up in three dimensions."
They must have removed magnesium, carbon and oxygen atoms and replaced them with iron atoms so iron oxide crystals could form, Scott said. "They were made one step at a time by changing the composition."
In additional evidence, the scientists found crystals of magnesium oxide in the carbonate lined up in the same way as iron oxides.
"There are examples in literature of carbonate minerals that have been heated up," Scott said. "Iron and magnesium oxides form inside the carbonate when carbonate is heated."
Scott and associates Sasha Krot and Akira Yamaguchi said in a research paper in 1997 that ALH 84001 likely was a "shocked rock" and carbonates formed at high temperatures in an impact.
NASA researchers accepted that, Scott said, "but still argued that there was a fraction of crystals that have very well-formed, naturally faceted crystal faces.
"They argued these particular subset of iron oxide crystals had formed in organisms because they resembled so closely the ones present in these bacteria on Earth that use magnetites as compasses to help them navigate. So, it was circumstantial evidence."
Scott expects NASA scientists who believe the meteorite had fossil-like organisms "to try and pick holes" in the new study. "They have stuck to their guns ... They've argued these magnetite crystals provide the best evidence for Martian life in the meteorite."
But the UH-Greenwich team, in continuing work on the rock, found a lot of evidence that the meteorite was heated in an impact 4.4 billion years ago that melted and even vaporized some minerals, Scott said.
"It was kind of a surprise that they heated up so much as to vaporize one of the silicate minerals." He said it seems clear that carbonate partly decomposed from the heat to form carbon dioxide gas.
ALH 84001, the oldest planetary rock found on Earth, "won't return to obscurity" because of their findings because many questions remain to be answered, the scientists said. "An ancient Martian rock is indeed a precious gift, even if it failed to bring evidence for extra-terrestrial life."
University of Hawaii
