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Original Date: Thu, 3 Jan 2002 11:31:47 -0800
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Signs of Life: On the Lookout for
Extraterrestrial Sweet Spots
By Leonard David
Senior Space Writer
posted: 07:00 am ET
03 January 2002
BOULDER, COLORADO - Looking for life elsewhere is a tough task for
human or robot. The good news is that the scientific skill and tools
to search for, detect and inspect extraterrestrial life are advancing
A revolution in the field of microbiology is afoot, along with
extraordinary progress in understanding the "geobiological" history of
Earth. And then there's growing amazement about life on this planet
and how it can survive and thrive even in the most extreme and
bizarre of environments. For example, within the last ten years
alone, more than 1,500 new species of microorganisms have been
discovered and genetically sequenced.
In a just issued report, Signs of Life, a multidisciplinary group of
scientists grappled with techniques and technologies to detect
evidence for extraterrestrial life - either on the spot on other
worlds, or within prime pickings hauled back to Earth by robotic
Spurred largely by an April 2000 workshop held in Washington, D.C.,
report findings and conclusions were pulled together by the National
Research Council (NRC) Committee on the Origins and Evolution of Life.
"The report is based on a workshop that brought together a healthy
spectrum of senior experts and young researchers," says Jonathan
Lunine, co-chair of the committee and professor of planetary science
and physics at the University of Arizona in Tucson. Many of the
workshop attendees are developing techniques to detect life, and
modeling the environments in which such techniques might be used on
other planets, he explains.
John Baross, associate professor of oceanography at the University of
Washington in Seattle, also co-chairs the committee.
"The discussion was vigorous and exciting. This is a different world
of life detection than that in 1976, at the time of Viking," Lunine
Lunine feels the key to success in life detection in the field is to
try a range of techniques that vary in their specificity and need for
prior assumptions about the nature of life. Doing so will maximize the
chances for success in searches at the planet itself.
"With returned samples, of course, one should throw everything
possible at the effort," Lunine explains.
The Committee on the Origins and Evolution of Life, Lunine adds, is
continuing its efforts with a study on the potential nature of life
that might be very different from terrestrial...and how one would go
about detecting such life.
Since the 1976 landings of two Viking landers on Mars, the
technological ability to spot life on celestial bodies has made
impressive strides. Furthermore, understanding the nature of life and
the concomitant power of analytical tools in the biological sciences
are viewed together as "one of the most dramatic changes since
Viking," the NRC report states.
In coming to grips with the central question of what is life, the
committee assumed that if life exists on other planets or moons, it
will be carbon based and dependent on liquid water. Also, it will be
self-replicating and capable of evolving.
The quest to find life beyond Earth involves answers to several tough
questions. For instance, how does one determine if there are living
organisms in a returned sample? Secondly, can living organisms leave
tell tale traces from earlier times that can be found in a returned
sample? Lastly, how does one determine whether there are living
organisms or fossils in samples examined robotically on another solar
Lessons from the "Mars rock"
Firm answers to these questions are elusive, reports the study group.
There are great uncertainties regarding the possible range of
chemistry and morphology that could constitute life.
The committee found that "there is a disconnect between those
techniques that have been developed to an exquisite degree of
sensitivity to identify terrestrial organisms and those that could
provide the greatest probability of detecting exotic life forms from
"Given the extreme difficulty (or impossibility) of inductively
describing all possible living processes based on terrestrial
biochemistry, no single approach, or even combination of approaches,
will guarantee success on a given sample."
That view has been brought home, quite literally, by the ongoing
research of the often called "Mars rock" - the infamous ALH84001
meteorite. The claim of evidence for biological processes in that rock
of ages from the red planet remains controversial and unresolved.
ALH84001 offers an important lesson in the fundamental complexity of
identifying the faint traces of present biology or Martian life that
is long gone.
"Perhaps even more difficult, if life or its remains is detected in a
sample, will be the determination of whether it is a terrestrial
containment from Earth, and if so, whether it was delivered by the
spacecraft or in the natural process of cross-contamination via
asteroidal or cometary impact," the committee report adds.
Dispatching high-tech gear to scout for life -- and not drag along
hitchhiking terrestrial microorganisms in the process -- is a
difficult challenge, the committee notes. Spacecraft must be
sterilized to avoid tainting other planetary bodies with Earth biology
- a situation tagged as "forward contamination."
There remains, however, "intense debate", the NRC report observes,
over the level to which spacecraft sterilization should be achieved
for missions to particular solar system bodies.
Firstly, sterilization must be done in such a way as to avoid damaging
One procedure -- sterilization via dry heating in an oven -- was
performed on the two Viking landers that searched for life on the red
planet. However, that approach puts harsh demands on spacecraft
components and leads to a substantial increase in mission cost and,
possibly, the chances of mission failure, the report states.
Sterilization by particle irradiation of a space probe is an
alternative. Yet this technique may not reach all spacecraft
subsystems, particularly when the mission design dictates shielding
electronic components from ambient sources of radiation. That type
environment, for example, is found in the Jupiter system.
Another worry is that radiation-tolerant bacteria may dictate that
irradiation levels exceed even the extraordinary levels to be
experienced during the prime mission phase of, say, a mission to
Jupiter's moon, Europa.
Titan: cold soak
Regaining access to all parts of a spacecraft before launch to assure
that sterilization has taken place is an unsolved problem, the
Flagged in the report is the very compact Huygens probe now en route
to Saturn. The lander is to be dropped off on that planet's mysterious
Titan, by the Cassini interplanetary spacecraft after arrival
The European Space Agency-built probe was not sterilized to a high
standard on the grounds that the profoundly cold Titan environment
would sterilize the lander soon after landing. "Yet Titan is itself a
target for investigating advanced stages of organic chemistry that on
Earth might have led to life," the report notes.
In the area of spacecraft cleanliness, the committee encourages
further work to refine sterilization approaches, with an eye toward
minimizing impacts on spacecraft cost and mission success.
Hauling back the goods
Another hotly debated topic is that of back contamination, whereby
extraterrestrial samples brought back might harm biological processes
here on Earth.
At issue is whether organisms "out there" might exist that are
sufficiently different from terrestrial organisms "down here" to
escape laboratory detection, yet similar enough to pose a threat to
the health of our biosphere.
"In the debates about life detection and back contamination, this
'niche' has not been explored to the extent that it should be - in
part because of the difficulties in answering the question," the
committee report states.
The committee recommends that a focused study be done in the near
future to address the detection of microorganisms with varying degrees
of nonterrestrial biochemistry, and the possible threat that such
organisms might pose to terrestrial organisms.
Similar in view from past studies on back contamination, the committee
report states that there are practical and societal reasons for
ensuring planetary protection for all interplanetary missions.
"Although the probability that an extraterrestrial life form could be
pathogenic to humans, or even viable at all in the terrestrial
environment, is very low, it cannot be shown to be zero," the report
Back in the lab
Due to the myriad of technical woes to overcome in returning samples
back to Earth, much of the search for life elsewhere may initially be
done "in situ", that is, on the spot, by robots.
Many of the powerful and sensitive techniques for detecting life in
laboratories here on Earth are not yet "space rated". That is, they
are far too big, complex, and not ready for prime time flight. That
condition may remain so, at least in the near future.
Because of the continuing rapid improvements in technology, the
committee reports, it is not appropriate to recommend a specific set
of techniques for in situ life detection at this time. Pressing on
with the design of innovative and "miniaturizable" techniques for in
situ life detection is encouraged.
It is an almost certainly that the most interesting locales from the
point of view of the search for life will not be the easiest to get
to. Finding those comfy niches that could be just right for life
today, or were in the past, suggests the committee, is likely to mean
landing in less-than-totally-safe sites.
"It remains unclear as to which environments in our solar system
should be searched for signs of life," the committee found, beyond the
general identification of planetary targets - such as Mars, Europa,
and Titan. "In large measure, we yet do not known enough about these
bodies to target searches in particular locations."
Picking those extraterrestrial sweet spots will require a series of
missions, including orbital reconnaissance, followed by
up-close-and-personal perusals using landed vehicles.
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