NEW EVIDENCE BLOSTERS CASE FOR MARTIAN LIFE.

Byline: David L. Chandler The Boston Globe

The scientists who reported the stunning discovery of possible signs of ancient life in a Martian meteorite said last week that their case has grown appreciably stronger since last summer's initial announcement.

Several challenges raised since then have been undermined, they said, and new evidence lends support to the claim that the rock harbors evidence of past microbial life.

But other scientists remain skeptical, and the debate continued, sometimes heatedly, at the annual Lunar and Planetary Science Conference here last Wednesday.

No one yet claims that the case for Martian life has been proved decisively, nor that it has been ruled out. The debate is over levels of probability.

``We feel stronger now about the conclusions than we did when the manuscript was published,'' said Everett Gibson of the Johnson Space Center, a member of the original team examining the meteorite. But, he admitted, ``There are no showstoppers.''

In the eight months since the team's report was published in the journal Science, two main objections have been raised to their claims: First, that the apparent microfossils in the rock are much smaller than known forms of earthly life, and possibly too small to have been alive; and second, that the minerals in the rock associated with the apparent fossils must have formed at such high temperatures that life would have been impossible.

Each of these arguments has been seriously weakened by evidence presented here this week, as well as by two papers published last week in Science:

A study by researchers at the California Institute of Technology found that grains of minerals in the meteorite have magnetic fields oriented in random directions. If the rock had been subjected to high temperatures, they said, all its grains would have had magnetic fields lined up in the same direction.

A study of the chemical composition of mineral grains in the meteorite by John Valley of the University of Wisconsin found that there are sharp boundaries between different grains that could not have remained if the rock had been heated to high temperatures. The chemical evidence suggests the rock was never heated above 100 degrees Celsius, Valley said, which is the boiling point of water and the approximate upper limit for survival of known microbial life.

Arguments that the apparent fossils were too small to have been living organisms were countered by Kathie Thomas-Keprta, a member of the original team whose report on possible signs of life appeared last August in the journal Science. She showed several microscope pictures of unusual living microbes on Earth that matched the Martian microfossils both in shape and in size and showed clear signs of life in the laboratory, consuming nutrients and generating waste products.

Some of the spherical or ovoid cell-like shapes found in the meteorite were precisely matched by tiny microbes recently discovered living far underground inside solid granite in the Columbia River basin in Washington state, Thomas-Keprta showed. And some of the long, wormlike shapes found in the Mars rock matched the size and shapes of filaments that extend from some of those same microbes.

``We believe they are bacterial appendages that have become fossilized,'' she said.

Answering critics who had argued that some of these terrestrial samples were not really living organisms, Thomas-Keprta said laboratory analysis has demonstrated that they contain DNA and protein, clear evidence of their biological nature. The team now is working on finding such biological molecules inside the Martian meteorite.

Nevertheless, other researchers continued to argue that there may be other explanations for the forms found in the Mars rock.

Ralph Harvey of Case Western Reserve University said that magnetite crystals found in the meteorite, which are sometimes produced inside living microbes, contained a type of structural defect generally considered evidence that the mineral was formed by ordinary chemical processes, not by living creatures.

Harvey said these defects also suggested that the crystals formed at temperatures too high for living things to have survived.

Thomas-Keprta countered that her research has shown no sign of such defects in the meteorite.