Researchers who are searching for evidence of water and organic compounds on Mars need to know how to recognize Martian life forms if Mars rocks are eventually brought back to Earth. Peter Buseck and Martha McCartney, of the NASA Astrobiology Institute, decided that clues can be found in bacteria here on Earth.
They were hired by NASA to develop reliable criteria for identifying traces of life, or ?biomarkers,? for use during future astrobiology missions. They decided that the study of organisms from Earth is the best way to start. Buseck says, ?If you find something in extraterrestrial samples that resembles life on Earth then it?s reasonable to think that you have found traces of life? on other planets.
Astrobiologists expect extraterrestrial life, if it exists, to be simple, single-celled organisms. They are searching for more simple, ancient bacteria here on Earth to use as a comparison, but it?s hard to do because bacteria do not leave traces in the fossil record, since they have no bones. To demonstrate that bacteria were ever present, Buseck says ?you need some sort of biomarker, some sort of remainder,? preferably of a durable material, such as a mineral, that can survive for billions of years.
They thought that such a long-lasting biomarker had been found in a NASA scientist team?s 1996 claim of fossil bacteria in a 4.5 billion-year-old Martian meteorite. The group of scientists originally studying the meteorite found ?bacterium-shaped objects? that seemed to indicate life. Other researchers showed that most of that evidence could have resulted from non-biological processes or artifacts introduced during study of the meteorite.
Only one of the original findings was still thought to indicate life: Crystals of an iron-based mineral called magnetite. The crystals found in the meteorite are striking because magnetite grains with similar size, purity, and structural perfection previously have been seen only in bacteria found on Earth. According to the NASA group?s report, ?magnetotactic? bacteria, which form the magnetite grains through a controlled process, can generate these particular shapes.
Magnetotactic bacteria, which are common in marine habitats, produce and carry magnetic crystals in a chain. The chain acts like a compass as the bacterium swims along Earth?s magnetic field lines. If bacterial synthesis is the only possible explanation for the magnetite grains found in the Mars meteorite, this could be a clear indication that life once existed outside Earth.
To be able to match the crystals from the two sources with confidence, Buseck says, ?What we need[ed] to do [was] determine the shapes in the meteorites with high accuracy, determine the shapes of the crystals in bacteria with comparable accuracy, demonstrate their identity, and then somehow determine that there are no other ways of forming such crystals. Then we?d have a tight case.? But at 40 to 100 billionths of a meter wide, magnetite nanocrystals from Earth were too small for three-dimensional imaging, so there was no way to match them to the shapes of the crystals on the meteorite.
New developments in transmission electron microscopy (Tem), a technique in which samples are viewed with a beam of electrons rather than a beam of light, made such precise study of crystal shapes possible. Using the recently improved techniques, the team generated dozens of two-dimensional images taken from different angles as well as three-dimensional holograms of each magnetite grain. The resolution of their images was in the range of hundreds of trillionths of a meter.
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Now Peter Buseck and his team say that the small magnetic grains found in the Martian meteorite are not similar enough to those formed by terrestrial bacteria to be evidence of life. ?Although the similarities are intriguing, we believe that they do not provide evidence that the crystals are ?Martian magnetofossils? or that they constitute evidence of the oldest life yet found,? says the research team.
But advocates of the life on Mars theory say that it?s not necessarily the case that simple life forms on Mars would have produced magnetite grains similar to those produced by microbes on Earth. Martian bacteria could have come in types unknown on Earth today.
One researcher says, ?Perhaps the only way to sort it out would be to bring rocks directly back from Mars.?
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