December 13, 2000
John Ira Petty
Johnson Space Center, TX
A new scientific report offers compelling evidence that primitive life existed on Mars. Tiny magnetite crystals, identical to those used by aqueous bacteria on Earth as compasses to find food and energy, have been found in the Martian meteorite ALH84001. The report on the finding is in the December issue of Geochimica et Cosmochimica Acta.
Written by a group of scientists led by Kathie Thomas-Keprta of Lockheed Martin at Johnson Space Center and funded by the NASA Astrobiology Institute, the report strongly supports the primitive life on Mars hypothesis of David McKay and coauthors in 1996.
Coauthors of the new report on a four-year investigation are Dennis Bazylinski of Iowa State University, Joseph Kirschvink of the California Institute of Technology, Simon Clemett and Susan Wentworth of Lockheed Martin at the Johnson Space Center, David McKay and Everett Gibson of NASA’s Johnson Space Center, H. Vali of McGill University in Montreal, and Christopher Romanek of the Savannah River Ecology Laboratory.
Magnetite (Fe3O4) is produced inorganically on Earth. But the magnetite crystals produced by magnetotactic bacteria are different – they are chemically pure and defect-free. Their size and shape is distinct. Magnetotactic bacteria arrange these magnetite crystals in chains within their cells.
Their characteristics make the magnetite crystals very efficient compasses, which are essential to the survival behavior of the bacteria. No one has found terrestrial inorganic magnetites, produced either naturally or in the laboratory, that mimic all the properties displayed by biogenic magnetites.
“The process of evolution has driven magnetotactic bacteria to make perfect little bar magnets, which differ strikingly from anything found outside biology,” said coauthor Kirschvink, a geobiologist. “In fact, an entire industry devoted to making small magnetic particles for magnetic tapes and computer disk drives has tried and failed for the past 50 years to find a way to make similar particles. A good fossil is something that is difficult to make inorganically, and these magnetosomes are very good fossils.”
Scientists generally agree that ALH84001 is a member of the group of 16 meteorites found on Earth that originated on Mars. The potato-sized igneous rock is the oldest of them – about 4.5 billion years. It lay in Antarctic ice for more than 13,000 years. But the biogenic-type magnetite crystals are embedded in carbonates within ALH84001. Previous work by coauthor Chris Romanek has shown that these carbonates formed on Mars. Thus the magnetite crystals must also have formed on Mars.
“These crystals are so tiny, ranging from 10 to 200 nm, that nearly a billion of them would fit on the head of a pin,” said Thomas-Keprta. Using electron microscopy, team members examined the Martian magnetites still embedded in the carbonate and also removed about 600 crystals and examined the individual particles to determine their chemical composition and crystal geometry.
The authors found that about a quarter of the Martian magnetites from ALH84001 are identical to magnetites produced on Earth by magnetotactic bacteria strain MV-1, which has been extensively studied by coauthor Bazylinski, a geobiologist and microbiologist who has developed many ways of culturing these difficult to grow microorganisms. “There is currently no known chemical means of producing these magnetite crystals with their unique morphologies,” he said.
Coauthor Clemett noted that “Mars is smaller than Earth and it developed faster. Consequently, bacteria able to produce tiny magnets could have evolved much earlier on Mars.”
When the team asserted in 1996 that Martian meteorite ALH84001 showed signs of life existing on Mars, that planet was not known to have ever had a strong magnetic field. But since then, the Mars Global Surveyor has observed magnetized stripes in the crust of Mars that show a strong magnetic field existed early in the planet’s history, about the same time as the carbonate containing the unique magnetites was formed.
"ALH 84001 has been of great heuristic value in the field of astrobiology,” said Baruch Blumberg, director of the NASA Astrobiology Institute. “Independent of its support or rejection, it has raised stimulating hypotheses that will help to focus our definition of how life, or variants of it, can be recognized."
Vic Baker at the University of Arizona and Jim Head of Brown University have inferred abundant water on early Mars from the morphology of canyons prevalent on Mars. In a recent issue of Science, Michael Malin and Ken Edgett present evidence of widespread sediment layers on Mars that they interpret as produced by numerous lakes. Adrian Brearly of the University of New Mexico has found traces of ancient water, in the form of clay minerals, in ALH84001.
Mars has long been understood to provide sources of light energy and chemical energy sufficient to support life. Early Mars, the authors note, may have had even more chemical energy produced by active volcanism and hydrothermal activity.
- end -