WASHINGTON (AP) _ While scientists speculate about the possibility of life on Mars, a team of researchers has calculated that the Red Planet probably had enough energy for life to begin but not to create an Earth-like abundance.

Even over billions of years, the energy available from volcanic activity and geothermal vents would support much less life than on Earth, the scientists report in the Aug. 25 edition of the Journal of Geophysical Research.

There probably ``was sufficient energy available to support the origin of life on Mars but not sufficient energy to create a ubiquitous and lush biosphere,'' Bruce M. Jakosky of the University of Colorado, Boulder, and Everett L. Shock of Washington University in St. Louis speculate in their paper.

Two years ago, NASA scientists made the startling announcement that they had found a rock in Antarctica that had originally formed on Mars and showed evidence of tiny, bacteria-like life. Their interpretation remains subject to controversy, however.

Interest in the possibility of life on Mars has increased in recent years with the discovery that life can exist under the harshest, most extreme conditions deep inside the Earth _ conditions similar to those on Mars and Jupiter's moon Europa.

In those cases, microbes managed to extract the energy to live from chemical reactions, rather than from the sun as most life forms do on Earth.

But when the researchers studied conditions on Mars, they concluded that the amount of volcanic rock that has erupted on Mars over its lifetime is several hundred times less than that on early Earth. Therefore, the energy accessible through hydrothermal vents would probably have been proportionately less on Mars as well.

Thanks to photosynthesis, which takes energy from the sun, Earth's rate of producing life is as much as 1,000 times more than that of Mars.

Even is life did form on Mars, with its low potential density, there may not have been enough time for photosynthesis to develop there, the paper concludes.

The new research is also funded by the National Aeronautics and Space Administration, which hopes in the next decade to bring back Mars samples to study for the possibility of life.