NASA TEAM DEFENDS MARTIAN LIFE FINDINGS : NASA CHALLENGES MARS-LIFE SKEPTICS.

Byline: John Noble Wilford The New York Times

Scientists and space agency officials Wednesday reaffirmed their claim of finding strong evidence for past life on Mars and asked skeptics among the world's scientists to join them in conducting even more rigorous tests needed to confirm or disprove it.

Daniel S. Goldin, the NASA administrator, said the possibility of Martian life, however primitive, would prompt a thorough review of the nation's program of exploring the planet. Some of the planned 10 robotic missions to Mars might be re-evaluated, he said.

Asked if the findings might bring more money for NASA, Goldin said science should dictate the scope and design of future projects. ``I think exploration is necessary,'' he said, but he cautioned that NASA should not simply say, ``Give us money. Let's have a big mission.''

The White House announced that Vice President Al Gore would convene a ``bipartisan space summit'' by the end of the year to consider the future of the nation's space program, in light of the Mars discovery.

As he was leaving for a flight to California, President Clinton told reporters, ``A significant purpose of this summit will be to discuss how America should pursue answers to the scientific questions raised by this finding.''

Clinton pledged that ``the American space program will put its full intellectual power and technological prowess behind the search for further evidence of life on Mars.'' For if the discovery can be confirmed by others, he added, ``it will surely be one of the most stunning insights into the universe that science has ever uncovered.''

At a news conference Wednesday at the National Aeronautics and Space Administration, the research team explained and vigorously defended their reasons for concluding that a meteorite that fell to Earth from Mars bore strong chemical and possibly fossil evidence of primitive life on Mars several billion years ago, when the planet was warmer and wetter. They conceded that their evidence was not conclusive but it certainly pointed to past microbial life processes on Mars as the most reasonable and simplest explanation.

The team's analysis of the 4.2-pound, potato-size meteorite, which was found in Antarctica, was described in added detail after unofficial word of the discovery became widely known Tuesday. A full report on the findings is being published in the Aug. 16 issue of the journal Science.

Dr. David McKay, a geochemist at the Johnson Space Center in Houston, who directed the research, pointed to four lines of evidence supporting the team's conclusion. These included the detection of an apparently unusual pattern of organic molecules, the carbon compounds that are the basis of life, and also several unusual mineral samples that are known products of primitive microscopic organisms on Earth.

Even more startling, and potentially controversial, was McKay's interpretation of microscopic but distinct black and white circles embedded in the dark rock as being fossils of the Martian microbes themselves. Highly magnified pictures of the tiny circles were shown at the news conference.

``This is the most controversial part of our conclusions,'' McKay said. ``But the explanation for these structures that we favor is that these are in fact microfossils from Mars. It's simply an interpretation at this point.''

As more scientists learned of the discovery, many advised caution and some were frankly skeptical that the researchers indeed had sufficient evidence to back up their conclusions about life on Mars. One somewhat skeptical scientist, Dr. William Schopf, a paleobiologist at the University of California, Los Angeles, was invited to speak at the news conference.

Calling the findings ``exciting and very interesting,'' Schopf said: ``I personally regard this as a preliminary report. I think a certain amount of additional work needs to be done before we can be sure that this report is really about life on Mars.''

Schopf recalled that Dr. Carl Sagan, the Cornell University astronomer and writer, once said, ``Extraordinary claims require extraordinary evidence.'' Accordingly, he contended that several points of evidence made by the research team fell short of being extraordinary. The organic molecules and certain mineral compounds, he noted, could have been of nonbiological origin.

When asked what it might take to persuade him that the evidence indeed suggested Martian life, Schopf said he wanted to see a more detailed examination of the purported microfossils to determine if they show the presence of cell walls. He also wanted to see evidence for a ``population of organisms,'' not just a few, and traces suggesting cell division, showing the life cycle of such organisms.

McKay said future studies with electron microscopes should be able to identify the presence of membranes or other signs of cells walls, if they existed. Other investigations will examine the rock for signs of amino acids, the building blocks of proteins and of life. The researchers said they had already met other objections by skeptics, who argued that the meteorite has been exposed to high heat, by saying that the presence of iron sulfides and certain organic molecules was proof that the meteorite had not been exposed to extremely high temperatures.

In their report for publication in Science, the research team said the rock they studied was one of 12 meteorites that have fallen to Earth and been identified as Martian, based primarily on an examination of pockets of trapped gas in the rock. The researchers determined that the rock formed about 4.5 billion years ago, when the Martian crust itself was newly formed. The rock presumably came from underneath the Martian surface. And from 3.6 billion to 4 billion years ago, when the Martian climate was much less cold and arid than it is now, the putative organisms would have left their marks in the rock much like the formation of fossils in limestone on Earth.

Then 16 million years ago, a comet or asteroid struck Mars, ejecting the piece of rock and sending it off into space, where it eventually fell to Earth 13,000 years ago. The rock was found in Antarctica in 1984 and named Allan Hills 84001. Current research that led to the supposed discovery of Martian fossil life began two years ago, mainly at the Johnson Space Center.

Goldin, the head of NASA, set the tone for the news conference by cautioning that the researchers were not claiming ``ultimate proof'' of life beyond Earth, but circumstantial evidence that is highly suggestive of past life on Mars.

``All of us are skeptical, but thrilled and humbled by this prospect,'' Goldin said.

CAPTION(S):

3 Photos

PHOTO (1 -- color) Geochemist Dr. David McKay discussesthe Mars meteorite tests Wednesday at a news conference in Washington, D.C.

(2) The meteorite from Mars was discovered in Antarctica in 1984.

(3) An image of a particle from the rock shows carbonate globules.

Associated Press