IF you're looking for signs of life on Mars, look for signs of water.
That's the advice planetary scientists are giving each other as a new phase of Mars exploration gets under way.
America's Mars Observer, which was launched on Sept. 25, two Russian/European missions to be launched in 1994 and 1996, and American and European follow-on missions now being planned should give scientists an intimate knowledge of the Red Planet within a decade. That prospect prods those involved in the exploration to sharpen their concepts of how to look for evidence of past or present Martian life.
Such evidence - even if it is only a fossil microbe billions of years old - is the "Holy Grail" of Martian explorers. Uncertain as the likelihood of finding such evidence is, the quest for it gives the efforts of those involved an overarching goal. As Daniel S. Goldin, administrator of the National Aeronautics and Space Administration (NASA) notes, scientists realize that "finding some lower form of life, or fossilized life, on Mars would alter our perception of the universe far beyond anything Copernicu s or Columbus ever dreamed."
When it comes to assessing the possibility of life, "the key for Mars is liquid water," says Rocco L. Mancinelli of the NASA Ames Research Center at Moffett Field, Calif. A thin atmosphere, fiendishly cold temperatures, and sterilizing solar ultraviolet radiation would make things tough for Martian life forms. But they aren't showstoppers.
Dr. Mancinelli explains that organisms could adapt to an atmosphere where surface pressure is less than 1 percent of that on Earth. They could live on a world where surface temperatures vary between -140 degrees C (-220 F.) and 17 degrees C (62 F.). But without water, organic life as we know it wouldn't have a chance.
That means "Mars's temperature and pressure regimes are important to life only because they determine the state of water," Mancinelli says. He adds that, therefore, "the probability for life to have arisen and evolved on Mars is almost solely related to the probability of liquid water existing on the planet for hundreds of millions or billions of years."
This basic point was a major theme that ran through several sessions the recent World Space Congress devoted to the prospect for Martian life. Assessing that prospect depends on understanding the geological nature and history of Mars.
Mars does have water. What appear to be flow channels and other water-formed features cover much of its surface. These suggest that water may have been abundant in the distant past. Bruce M. Jakosky of Cornell University in Ithaca, N.Y., estimates that enough water has come from the interior to form a layer 200 meters deep if it were spread evenly all over the planet. Something like 50 to 100 meters of that has been lost to space. Some is still accessible on the surface, mainly as ice at the polar caps a nd within a meter of the surface over about half of the planet, according to Dr. Jakosky.
He also notes that Martian climate undergoes long-term changes as the planet's orbit around the sun goes through regular cycles. The atmosphere's water content varies with these changes from about a tenth of its current value to a thousand times that value.
For Martian water to be useful for life, it has to be available in liquid form. United States Geological Survey geologist Michael H. Carr points out that Mars is too cold to have much liquid water today. He says "you only have to stick your finger into the ground" and you're down to well below freezing temperatures. Therefore, he adds, "I think the chance for liquid water at anything other than an active volcanic site is very poor." He advises biologists interested in present day Martian life to look for
volcanically active regions where they expect also to find a water-rich outer crust on the planet. There may be such sites because, he says, "I believe there is good evidence that Mars is volcanically active today."
Many scientists are skeptical about present Martian life. NASA's two Viking landers found only sterile soil in 1977. University of Massachusetts biologist Lynn Margulis says "looking for life on Mars is like looking for Father Christmas." She points out that, as yet, there is no evidence for organic material on the planet. There also is no sign of the kind of atmospheric composition biologists would associate with biological processes. Instead, the makeup of Martian air is consistent with an absence of l ife.
Even if there is no life on Mars now, there may have been in the past. Mars, like Earth, is about 4.5 billion years old. Life may have arisen on Mars during its first billion years as it did on Earth. In fact, Wanda L. Davis of the NASA Ames Research Center says she believes that "Mars may hold the best record of events that led to the origin of life." Earth has lost much of that record.
Amos Banin of Hebrew University at Rehovot, Israel, says his analysis of available information shows that Mars has all the essential chemical elements for organic life as we know it. He concludes that "a biosphere having a similar chemical code to life on Earth could have evolved on Mars."
Thus the question of the possibility of ancient Martian life comes down to a question of the availability of water. What was the early water inventory? Where was it located? How long did it stay there? What was its state? These are the questions scientists need to answer to assess the past history and origin of life on Mars, Mancinelli explains.
These are also the questions that the upcoming Mars missions are designed to help answer. They are essentially geological questions. But the fact that they are relevant to the search for Martian life gives them added bite.
For many of the scientists involved, this search is more than a quest for intellectual understanding of our neighboring planet. It is a passion. NASA administrator Goldin put it this way: "This search for life - this urge to explore to the very limits of our technology - is not idle curiosity. It's a biological imperative.... It's what defines us as human beings.... Exploration is what we live for. It's how we grow as intelligent beings."