Magnetic Chains from Mars
New research announced Thursday in the Feb. 27th Proceedings of the National Academy of Sciences (PNAS) suggest that, indeed, early microbial life appeared at about the same time on Mars as it did on our planet.
An international team of researchers studying a four billion year old meteorite from Mars found microscopic magnetic crystals inside the ancient rock. The telltale crystals were arranged in long chains, which the scientists say could have been formed only by once-living organisms.
Right: Backscattered scanning electron microscope images of (above) a modern magnetotactic bacteria with a chain of magnetite crystals and (below) magnetite crystals and chains of magnetite crystals in the Martian meteorite ALH84001. One conspicuous chain is indicated by arrows. The diameter of a single crystal is approximately one-millionth of an inch.
"The chains we discovered (in the martian meteorite ALH84001) are of biological origin," asserted Dr. Imre Friedmann, an NRC senior research fellow at NASA's Ames Research Center and leader of the research team. "Such a chain of magnets outside an organism would immediately collapse into a clump due to magnetic forces."
Both the chain-like arrangement of the martian crystals and the traits of the crystals themselves bear a striking resemblance to similar crystals produced by bacteria on Earth.
The bacteria, which are mostly from the Magnetospirillum genus, grow the "magnetite" crystals (Fe3O4) atom by atom in small internal pouches, then line up several of these crystals to collectively act as a bar magnet. Following this internal "compass needle" allows the bacteria to search the body of water that it lives in for suitable oxygen concentrations in a more efficient, straight-line path.
Above: One example of a "magnetotactic" (magnetite-producing) bacterium from Earth. Note the line of slightly-elongated magnetite crystals down the bacterium's center. These crystals act as a compass, aligning the bacterium with the Earth's magnetic field. Image courtesy of Dr. Dennis Bazylinski of Iowa State University.
Friedmann's team says the magnetite chains in the meteorite probably were flushed into microscopic cracks inside the martian rock after it was shattered by an asteroid impact on Mars' surface approximately 3.9 billion years ago. This cataclysmic event also may have killed the bacteria. The same, or a later, asteroid impact ejected the rock, now a meteorite, into space.
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"These magnetites (from the meteorite) are basically indistinguishable from certain biogenic (i.e., biologically-produced) magnetites on Earth. And furthermore, we know of no other mechanism to make them, either on Earth or Mars," said Dr. Everett Gibson, an astrobiologist at JSC, who also participated in the Thomas-Keprta study.
Crystals made by magnetite-producing bacteria are chemically pure and free from defects in the crystalline structure. They are slightly elongated along a particular crystalline axis, and they range in size from 35 to 120 nanometers (a nanometer is one-billionth of a meter). They also show a particular pattern of faceting -- like a cut diamond. These properties are so unusual that they have only been seen in magnetite crystals produced by biological processes.
Below: This side-by-side comparison shows the marked similarity between the magnetite crystals found in the martian meteorite, on the left, and those made by bacteria on Earth, on the right. Click on the image for a more detailed
Friedmann's team discovered the crystal chains using a technique that enabled them to 'see' the tiny chains inside the meteorite without destroying them. Besides the chain-like formation, the team discovered that the individual crystals in a given chain are all of similar size and shape, do not touch each other, and that the chains themselves are flexible, further evidence of biological origin.
The possibility that a small (about 4-pound) meteorite from Mars contained large numbers of bacteria suggests that long ago such bacteria were widespread on the Red Planet, the researchers say.
The magnetite crystals probably formed about 3.9 billion years ago, when the rock was ejected from Mars. For comparison, the earliest well-documented life on Earth dates back 3.6 to 3.7 billion years. Both planets formed about 4.5 billion years ago.
The next step is to find the remains of the martian bacteria themselves, Friedmann said.
"Finding evidence of life on Mars is one of the central problems in astrobiology research today," said Dr. Michael Meyer, head of NASA's astrobiology program, which funded the research.
The martian meteorite ALH84001, pictured right, was found in the Allen Hills region of Antarctica in 1984 by researchers supported by the National Science Foundation's Antarctic Search for Meteorites Program, a joint effort by the NSF, the Smithsonian Institution and NASA. Case Western Reserve University in Cleveland manages the program. [more information]
Ames Research Center is NASA's lead center for astrobiology, the study of the origin, evolution, dissemination and future of life in the universe. NASA Ames is the location of the central offices of the NASA Astrobiology Institute, an international research consortium.
Chains of magnetite crystals in the meteorite ALH84001 -- abstract of the research paper by Friedmann et al. that explains the finding and why it strongly suggests microbial life once existed on Mars, with link to full text.
NASA's Astrobiology Institute -- Home page
Mars Global Surveyor -- Home page
Fossil Life in ALH84001? -- Summary of the 1996 Science article by Dr. David McKay et al. that first announced possible evidence of Martian life in the Allan Hills meteorite
What is ALH84001? -- Information on the Allan Hills meteorite, including how scientists know it came from Mars and how it got to Earth
Animation -- A 980 kb mpeg movie simulating a meteor impact that could have launched the Allan Hills meteorite from the face of Mars
Science@NASA Stories about Mars:
Martian Micro-Magnets -- The Allan Hills meteorite from Mars is peppered with tiny magnetic crystals that on our planet are made only by bacteria.
Layers of Mars -- Last year Mars Global Surveyor spotted terrains on Mars that look like sedimentary rock deposits. If the mysterious layers formed underwater, as some scientists suspect, they may be a good place to hunt for Martian fossils.
Sedimentary Mars -- New Mars Global Surveyor images reveal sedimentary rock layers on the Red Planet that may have formed underwater in the distant martian past.
Making a Splash on Mars -- On a planet that's colder than Antarctica and where water boils at ten degrees above freezing, how could liquid water ever exist? Scientists say a dash of salt might help.
Unearthing Clues to Martian Fossils -- The hunt for signs of ancient life on Mars is leading scientists to an otherworldly lake on Earth.
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