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Signs of Life: On the Lookout for Extraterrestrial Sweet Spots

Leonard David, SPACE.com, 03 January 2002

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Summary: 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 rapidly.

By Leonard David
Senior Space Writer

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 rapidly.

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 spacecraft.

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 told SPACE.com.

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.

Elusive answers

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 system body?

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 another planet."

"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.

Sterile approach

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 spacecraft components.

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 committee reports.

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 moon, Titan, by the Cassini interplanetary spacecraft after arrival in 2004.

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 says.

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.

One problem.

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.

Read more articles on this topic:

Are We Alone? Life in the Universe