"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 dont know enough about Mars yet," he said. "Perhaps the soils under Olympus Mons -- a moun
Contact: Norman Pace
University of Colorado at Boulder