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"We expect we will recognize life, but we have to be careful," said Kenneth Nealson, director of the Center for Life Detection at the Jet Propulsion Laboratory, during a public lecture at the American Astronomical Society conference. Life on Mars could be quite different from life on Earth, he said, so scientists must define the essential characteristics of life and how to detect them before sending experiments to the red planet. "This is the toughest challenge, by a factor of 10, that I've ever been asked to do," Nealson said.

The Viking landers of the 1970s carried two small laboratories designed to look for microscopic life in the Martian soil. One experiment added nutrients to a scoop of soil while the other looked for organic molecules. The first experiment quickly produced gases that could have been emitted by living organisms. When the second found no trace of organic molecules, though, scientists decided the gases were produced by chemical reactions in the soil.

To avoid a repeat performance on future missions, biologists must answer the question, "What it is about life that all life must contain?" Then they must turn to such fields as physics, chemistry, and information technology to help develop ways to measure those characteristics, Nealson said.

"If you're lucky, you'd see the one activity that would convince you -- you'd see something swim by," he said. If that doesn't happen, scientists should look for objects with distinctive shapes that convert energy from one form to another and create unique chemical signatures. In fact, he said, you could develop a Star Trek-like "tricorder" to detect traces of life by looking for specific chemical residues in distinct layers beneath the surface of Mars.

Although Mars is extremely cold and dry today and intense ultraviolet energy from the Sun sterilizes the planet's surface, it's possible that life evolved on Mars in the distant past, when conditions were more hospitable, and moved into more protected environments as the climate worsened.

"When conditions get very tough, life moves into the rocks," Nealson said. "Mars looks very tough. That means we'll have to study the rocks."

Communities of microscopic organisms live inside rocks in Earth's deserts -- both hot deserts, like the Mojave, and the frigid deserts of Antarctica, which biologists once thought were sterile, he noted. "It warms up to freezing about 5 or 10 days a year," Nealson said. "On those days, there's an abundant, active community inside those rocks....The rest of the time they're just asleep. If we just went there and made a measurement, it might be on the wrong day."

Nealson and his team of 20 scientists hope to design experiments that will fly to Mars in 2007. "If we run into functioning life, we won't miss it," he promised. "But if it's asleep or fossilized, that's not as sure."

And as they develop and conduct their experiments in search of Martian life, the scientists must "keep an open mind," Nealson said. "The stuff you were sure of yesterday may not be true tomorrow."

Damond Benningfield