- 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
- 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
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.
Editor's Note: The original news release can be found at http://www.asu.edu/asunews/current_releases/extraterrestrial111501.htm
Note: This story has been adapted from a news release issued by Arizona
State University for journalists and other members of the public. If
you wish to quote from any part of this story, please credit Arizona State
University as the original source. You may also wish to include the following
link in any citation: http://www.sciencedaily.com/releases/2001/11/011120042817.htm