Levin said that the likelihood is high that life, from Earth and elsewhere, exists on Mars today.

The transfer of microorganisms from one planet to another by meteoric impact is gaining increasing support, Levin said. That being the case, "it is now more difficult to propose a sterile Mars than a live one," he said.

Teamed with his son, Ron Levin of MIT Lincoln Laboratory in Lexington, Massachusetts, the former Viking scientist said their belief is that Earth has infected Mars. All links in the vital chain connecting Mars and Earth can be clearly identified.

The bottom line: Earth and Mars were hospitable over epochs that would have permitted infection of Mars by Earth microorganisms, and from other sources. "Biology offers the only fit to the Label Release data from Mars and is consistent with our new knowledge about Mars and Earth. It is time to accept the LR results," he said.

Yet even Levin admits, additional proof is required before many scientists will accept such a major change in paradigm.

Another find in the two decades-plus Viking treasure-trove of data was outlined by Joe Miller, associate professor in the Department of Cell and Neurobiology at the Keck School of Medicine of the University of Southern California in Los Angeles.

Miller has recently reviewed the Viking LR data in great detail.

"To my surprise, in their LR experiment, they seemed to have clear periodic oscillations in the release of gas from a Martian soil sample injected with a nutrient solution. The oscillations in gas release had a period of what appeared to be one Martian day. Being a circadian biologist, I became very excited," Miller told SPACE.com.

On Earth, Miller said, circadian rhythms -- oscillations with a period of nearly 24 hours -- are present in every species examined down to blue-green algae. Was it possible, he asked, that the LR experiment was recording the circadian rhythm of a Martian soil-dwelling microbe?

NASA worked with Miller, providing him the 1976 LR data sets, as well as converting the information to an electronic format. That allowed the circadian biologist to study the data using modern computer-based analytical tools.

"I found that the gas release was indeed rhythmic, with a period of precisely 24.66 hours, a Martian day," Miller said. This finding, along with other painstaking assessments about LR operations, the scientist feels that a Martian circadian rhythm in the experiment may constitute a biosignature - a sign of life.

"In conjunction with a great deal of other data from this experiment, such as the very large increase in gas immediately following nutrient injection, as well as a slow rise over the course of the entire experiment, suggest that the LR experiment was seeing biology," Miller said.

Miller said, however, that chemical interpretations of some of these data are possible, perhaps accounting for part of the LR rhythm.

"On the whole, a biological explanation seems more plausible. In all probability, Viking discovered life on Mars 25 years ago. The presence of a strong circadian rhythm in the LR experiment further suggests that circadian rhythmicity may be an excellent 'biosignature' of extraterrestrial life," Miller said.

In a sense, there's a bit of an undeclared race to prove that organisms on Mars are a fact of life.

For one, the spunky British-built Beagle 2 is set to land on Isidis Planita, a large flat region that overlies the boundary between ancient highlands and the northern plains. Beagle 2 is to be deployed from ESA's Mars Express spacecraft that is slated to begin orbiting the planet in late 2003.

"If the Beagle 2 successfully lands in 2003 and if its GCMS discovers organic molecules at the surface, then this will strengthen the case that the LR results were due to Martian biology," Warmflash of NASA told SPACE.com.

Closing in on the red planet, NASA's Mars Odyssey is on schedule to swing itself into orbit around the planet this October. Then, twin rovers are being prepped to rumble over Martian terrain in 2004. While not geared for detecting Martian life, the wheeled craft will act as robotic field geologists, geared to discern what Mars was like a long time ago.

A super-slick and highly mobile rover is on NASA's books for a 2007 liftoff. Then, no earlier than 2011, the space agency is hoping to loft the first robotic return sample mission to Mars.

Michael Meyer, astrobiology discipline scientist at NASA Headquarters here, said that answering the life on Mars question boils down to acquiring more data.

"A majority of the scientific community tends to think that Levin didn't find evidence for life on Mars. The only way to resolve the issue is to go to Mars. It's better to do take a step-wise approach, instead of trying to second-guess instruments and rework data. The answer is to go back to Mars," Meyer said.

Meyer said that simple models about life have been turned on their head, thanks to more data gained since the 1970s. "In some ways, Viking was a little too Earth-centric. It presumed life has metabolism that we can recognize. It also presumed that if you land anywhere on Mars you can measure life," he said.

John Rummel, NASA's planetary protection officer, takes a first-things-first attitude. "Looking for life on Mars is understanding the planet well enough to know where to look for it in the first place," he said.

"Either it's pathetically obvious that there is life on Mars…or it's going to be very hard to find. Most bets are that it's very hard to find. Just getting to the place where you actually would find life is going to be difficult. A lot of what we're doing in the Mars program is to really understand Mars well enough to know where to look," he said.