For as long as people have gazed at the night sky, they have wondered if neighboring planets could be populated by living things. In fact, recent explorations of our solar system have relayed several enticing hints that the life-supporting conditions on Earth may not be so unique.
Evidence for water and organic compounds on Mars and Europa has astrobiologists seriously pursuing the possibility that primitive life once existed on other planets and moons. As they gear up for the real acid test - collecting samples from these distant bodies to examine them directly for evidence of life - they are tackling nothing less profound than the origins of life in the universe.
But this pursuit is nagged by an uncertainty: We have never seen our extraterrestrial cousins before. How will we recognize them if we meet face to face? Peter Buseck and Martha McCartney, new members of ASU's arm of the NASA Astrobiology Institute, are among many scientists who predict the best clues are to be found in lowly bacteria.
Buseck, Regents Professor of geological sciences and professor of chemistry and biochemistry at ASU, and McCartney, a research scientist at ASU's Center for Solid State Science, were recently funded by NASA to help develop reliable criteria for identifying traces of life, or "biomarkers," for use during future astrobiology missions.
Study of organisms from Earth, Buseck and McCartney argue, is the most promising way to start. After all, Earthly life is the only life we know, making it our one reference point in judging whether extraterrestrial life exists. Therefore, Buseck reasons, "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.
Because astrobiologists expect extraterrestrial life, if it exists, to be simple, terrestrial bacteria are getting top billing as model Martians. Bacteria are single-celled organisms, among the most primitive life forms on Earth.
But the hunt for ancient bacteria presents some special challenges. Bacteria, all soft parts and no bones, do not usually leave any traces in the rock record, making their presence hard to prove. To unequivocally demonstrate that bacteria were ever present, Buseck stresses that "you need some sort of biomarker, some sort of remainder." Preferably, that biomarker should be a durable material, such as a mineral, that can survive for billions of years.
Just such a long-lasting biomarker may have already been found - in a NASA scientist team's 1996 claim of fossil bacteria in a 4.5 billion-year-old Martian meteorite, perhaps the most stunning evidence to date of extraterrestrial life. Not surprisingly, the claim continues to spark heated controversy. Buseck and McCartney aim to moderate the debate by putting the Martian life hypothesis to a very thorough test.
The group of scientists originally studying the now-renowned meteorite - known as ALH84001 - presented a slew of findings, including organic chemicals and "bacterium-shaped objects," that collectively cried "life." Since then, intense scrutiny by other researchers has shown 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 is still thought to be a unique indicator of 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, no inorganic process could have produced the meteoritic crystals. Only so-called "magnetotactic" bacteria, which form the magnetite grains through a controlled process, can generate these particular shapes.
Magnetotactic bacteria, common in aquatic and marine habitats, produce and carry the magnetic crystals in a chain. The chain, which looks like a faux backbone under a microscope, acts like a compass as the bacterium swims along Earth's magnetic field lines.
These crystals are at the center of Buseck and McCartney's planned work. If bacterial synthesis is the single possible explanation for the magnetite grains found in ALH84001, they could be the one clear indication that life ever existed outside Earth. But, Buseck worries, if no major holes have yet been punched in this argument, that may be because it has not been examined closely enough.
And when Buseck says "closely," he means it quite literally. "These crystals are at the limit of what one can see, even with powerful electron microscopes," he says.
At 40 to 100 billionths of a meter wide, magnetite nanocrystals have evaded clear three-dimensional imaging. That's a problem for the hypothesis of life on Mars, which now hinges on precise matching of the complex shapes of the magnetite crystals from ALH84001 and from magnetotactic bacteria.
"There are questions about how well we know the shapes of these tiny crystals and how secure the identity is between those in the meteorites and those in the bacteria," says Buseck.
To be able to match the crystals from the two sources with confidence, Buseck says astrobiologists must first fulfill four clear objectives. "What we need to do is 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."
Of these four steps, Buseck and McCartney intend to test the first three. They are studying the shapes, chemical composition, and magnetic properties of both the meteoritic and bacterial magnetite grains in unprecedented detail. New developments in transmission electron microscopy, a technique in which samples are viewed with a beam of electrons rather than a beam of light, have only recently made such precise study of crystal shapes possible.
Using the recently improved techniques, the team will generate dozens of two-dimensional images taken from different angles as well as three-dimensional holograms of each magnetite grain. The resolution of their images will be in the range of hundreds of trillionths of a meter.
In these efforts, Buseck and McCartney plan to continue ongoing collaborations with fellow scientists Dennis Bazylinski (of Iowa State University), Richard Frankel (of the California Polytechnic State University), Rafal Dunin-Borkowski, (of Cambridge University, England), and Mihály Pósfai (of the University of Veszprém, Hungary).
Their work will provide improved data and criteria for use in evaluating whether other magnetite grains, from meteorites or from samples collected in outer space, have a biological origin. Of course, ALH84001 will be the first Martian rock subjected to Buseck and McCartney's uncompromising analysis.
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