Molecular, cellular and developmental biology Professor Norman Pace, a world-renowned biochemist and expert on life in extreme environments, said the chances of finding primitive life in thermal vents on Mars are not that promising. Perhaps the next likeliest place in the solar system to find life -- in the ice on Europa -- is significantly more of a long shot, he said.

"The basic theme here is that if you look at what is required for life, it really is a narrow window," said Pace. "Our solar system outside Earth doesn't seem too promising to sustain life, but we don't know what kind of extreme conditions conducive to life may be found elsewhere in the universe."

Pace gave a talk, "Molecular Perspectives of Extreme Life," at the 2002 American Association for the Advancement of Science meeting in Boston held Feb. 14 to Feb. 19.

Signs of life elsewhere in the galaxy or universe may be "co-occurring, non-equilibrium gases like oxygen and methane, an indication the gases are being replenished," said Pace. This most readily could be explained by the influence of life.

And should intelligent life out there be looking back, Earth could possibly be seen as a home for life by other life forms in distant galaxies working with very advanced telescopes and spectrometers like scientists on Earth are developing to locate such gaseous conditions, he said. Pace also is a member of CU-Boulder's Center for Astrobiology.

In contrast, the search for life on Mars and Europa requires a rigorous chemical analysis, a process Pace has observed first-hand both in deep geothermal vents in the sea and in geothermal vents in Yellowstone National Park. That process involves the oxidation and reduction of geothermal compounds using hydrogen and carbon dioxide to form methane, or using hydrogen sulfide and oxygen to produce sulfuric acid.

"We commonly see these processes with sediments under seawater," he said. The top 1 centimeter of some marine sediments may contain one billion microbes per cubic centimeter. However, 1,000 meters down into the sediments scientists only find about 100,000 microbes per square centimeter, "and those generally are starved."

"But if life is really going to succeed and flourish for an extended period, I think it has to take over and modify a planet on the surface, like it has on Earth," Pace said. Primitive life forms in the depths of planets or moons are not likely to contribute to changing the surface.

The key to abundant and diverse life on the surface of Earth and likely other planets is photosynthesis, which captures light energy and converts it into energetic electrons that act like tiny batteries to accomplish biochemical tasks required for life. "Life has changed the surface of Earth dramatically," he said.

In a piece for the Proceedings of the National Academy of Sciences last year, Pace wrote: "Considering the intrinsic fragility and complex organic systems coupled with the powerful force of natural selection, I venture that the physical limits of life are likely to be about the same anywhere in the universe."

The definition of life should include self-replication -- the mechanism of evolution through natural selection -- and probably carbon-based molecules since carbon is one of the most abundant of the higher elements in the universe, he said.

Given that primitive life on Earth has been found in boiling thermal vents in the oceans to microbes in ice, the temperature span for life anywhere in the universe is likely to range from roughly -58 degrees Fahrenheit to 302 F, Pace said.

"We don't know enough about Mars yet," he said. "Perhaps the soils under Olympus Mons -- a mountain nearly 90,000 feet high -- have some type of circulation method for underground water, which would enhance the chances of life."

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