In a report titled ?Signs of Life,? a multidisciplinary group of scientists talk about techniques and technologies that can be used to detect evidence for extraterrestrial life on other worlds.

?The report is based on a workshop that brought together a healthy spectrum of senior experts and young researchers,? says Jonathan Lunine, professor of planetary science and physics at the University of Arizona in Tucson. ?The discussion was vigorous and exciting. This is a different world of life detection than that in 1976, at the time of Viking.?

The Committee on the Origins and Evolution of Life is studying the potential nature of extraterrestrial life that might be very different from life on Earth, and how to detect it.Since the 1976 landings of two Viking craft on Mars, we have learned a lot about how to spot life on other planets.

The committee wants to answer the central question of what is life. We?ve always assumed that if life exists on other planets or moons, it will be like life on Earth: carbon-based and dependent on liquid water. Also, it will be self-replicating and capable of evolving. But now there are uncertainties regarding the possible range of chemistry and morphology that could constitute life.

This controversy is illustrated by the ALH84001 meteorite known as the ?Mars rock.? The claim that it contains evidence that biological processes once existed on Mars remains controversial and unresolved.

We need to figure out how to tell if there are living organisms in a returned sample from a manned or robot craft. Can organisms that are no longer alive leave telltale traces from earlier times that can be found in a returned sample? ?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 says.

Choosing the planets and moons on which to launch a search for extraterrestrial life will require a series of reconnaissance missions, to be followed by landings. ?It remains unclear as to which environments in our solar system should be searched for signs of life,? the committee says. ?In large measure, we yet do not know enough about these [planets] to target searches in particular locations.? When we do find a location that?s likely to harbor extraterrestrial life, it may to dangerous for humans to travel to, so we may need to send robots to collect samples instead.

Dispatching high-tech robots to other planet to search for life, while not bringing Earth microorganisms along with them, is a difficult challenge. Spacecraft must be sterilized to avoid tainting other planetary bodies with Earth biology, which is called ?forward contamination.? Sterilization must be done in such a way as to avoid damaging spacecraft components.One procedure ? sterilization by dry heating in an oven ? was performed on the two Viking landers that searched for life on Mars. 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 of environment, for example, is found in the Jupiter system.

Another problem is that radiation-tolerant bacteria may mean that irradiation levels need to exceed even the extraordinary levels that will be experienced by the craft during a mission to Jupiter?s moon, Europa.

Gaining access to all parts of a spacecraft before launch to assure that sterilization has taken place is an unsolved problem, the committee reports. They are concerned about the compact Huygens probe which is now en route to Saturn. It will be dropped off on Saturn?s moon Titan by the Cassini interplanetary spacecraft 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.

Another concern is back contamination, whereby extraterrestrial samples brought back might harm biological processes here on Earth. Scientist wonder whether extraterrestrial organisms might exist that are sufficiently different from ours to escape laboratory detection, yet similar enough to pose a threat to the health of our biosphere.

?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.

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