Most significant are discoveries that life thrives in extreme conditions here on Earth, Levin said. Just last month, the National Science Foundation reported that indigenous microorganisms were found fully metabolizing in subfreezing South Polar snow where little, if any, water is available, he noted.

Gil Levin

Levin and fellow researcher, Lawrence Kuznetz, at the University of California's department of planetary sciences in Berkeley, California, both say that "a leap of faith is no longer required" about life on Mars today.

In a series of experiments led by Kuznetz, results show that liquid water can exist under Martian environmental conditions and under atmospheric pressure as low as that on Mars.

The Central Candor Chasma in the Vallis Marineris -- are there pools of waters in its deepest recesses?

"In Valles Marineris, there could be a set of conditions where the sun angle, depth and temperature is just right, and where the atmospheric pressure is just high enough to sustain pools of water," he said.

Levin said he believes that large regions on Mars are reconstituted daily with moisture. Levin and his son, a Massachusetts Institute of Technology physicist at Lincoln Laboratory in Lexington, Massachusetts, worked together to define a way that Mars could provide moisture to the top layer of soil in amounts sufficient to support terrestrial microorganisms.

How this cycle works is straightforward, Levin said.

"We know from Viking that the surface on Mars was saturated in water vapor almost every night," Levin said.

First, the air over Martian terrain, just a few feet (1 meter) above the surface, is so cold it does not hold water vapor. Therefore, that vapor is concentrated down near the surface, Levin said.

As the temperature drops, the water vapor -- in the form of ice or frost -- is frozen into the soil. As the sun rises and heats up the surface, the frozen deposits are liquefied for a period of time -- a condition that's just right for Martian microorganisms, he said.

"I would be surprised if there weren't organisms all over the surface of Mars, just like on Earth," Levin said.

Some scientists are not as bullish as Levin about liquid water on Mars' surface and the tie to Martian life.

Michael Meyer, a NASA astrobiologist, said that Mars does have locales where the partial pressure is high enough to support liquid water. But the amount of time that water could exist is likely to be very short, he said, due to high temperature.

"The idea that there would be enough water to support a biosphere is pretty marginal," Meyer said.

Meyer said that, while Levin may have some reasonable argument, the odds that such a process is at work on Mars is not very high.

Also doubtful about claims by Kuznetz that pools of water are possible on Mars is Michael Carr, a planetary geologist at the U.S. Geological Survey in Menlo Park, California.

Carr said that two conditions -- temperature and pressure -- must be met to have pools of liquid water. Those conditions are that the temperature must be above 273 Kelvin (31 degrees Fahrenheit or minus 0.15 degrees Celsius) and the partial pressure of water vapor must be over 6.1 millibars.

Research has shown that a partial pressure on Mars, enough to sustain water, is almost impossible to build, Carr said. Also, below surface temperatures on the Red Planet are close to the daily mean of about 215 Kelvin (minus 72.7 degrees Fahrenheit or minus 58.1 degrees Celsius), where water is in a deep-freeze state.

"The bottom line is that liquid water is rendered unstable at the Martian equator by both the temperature and the pressure," Carr said.

For Levin, however, he remains convinced that he and is colleagues are on to something, and that the no-liquid-water argument is flawed.

"People are beginning to be convinced. It's a real turnaround," he said.

NASA's billion-dollar Viking mission in 1976, sent to Mars to find life, was successful, Levin said.

The answer, he added, has been staring scientists in the face for nearly a quarter of a century.