The meteorite from Mars has been sliced, zapped and photographed in ultramicroscopic detail. It has undergone scores of chemical, mineralogical and magnetic tests.
And what have the scientists learned in the months of rigorous investigation since the announcement last August that a rock that fell to Earth bore possible evidence of early primitive life on Mars?
At the first large gathering of planetary scientists since the announcement, those who first reported the possibility professed to be more confident than ever that they are right.
Newly published research, they said, showed that temperatures at which certain diagnostic features in the rock formed could have been low enough, probably below the boiling point of water, to be compatible with microbial life. Also, the sizes and other characteristics of these features seemed to be similar to comparable minerals on Earth known to have biological origins.
"We have not found a show-stopper yet," said Dr. Everett Gibson, a leader of what could be called the pro-life team at the Johnson Space Center in Houston. "We feel more confident in our hypothesis now than when we published in August."
But other research groups described evidence suggesting that the rock had been exposed to temperatures much too high to be hospitable to life. They also found whiskerlike defects and chemical traces that they said were not consistent with biological processes.
The meteorite in question, AL 84001, is a 4-pound rock shaped like a potato that fell on Antarctica 13,000 years ago and was picked up by geologists in the 1980s. It crystallized in the Martian crust 4.5 billion years ago and was dislodged and flung into space by an asteroid impact about 15 million years ago. Chemical analysis has convinced scientists that the rock is definitely from Mars, but there agreement ends.
Much of the argument has centered on carbonates found deep in the meteorite's interior. They contained magnetite and iron sulfide, minerals that can be, but are not necessarily, produced by microbes.
A team of geologists led by Dr. John P. Bradley of MVA Inc. in Norcross, Ga., reported in December finding a number of structural features, including whisker-shaped markings, that they contended showed that the carbonates were formed at extreme heats that precluded a biological origin.
At a news conference, Dr. Kathie Thomas-Keprta of Lockheed-Martin Corp. reported that a study of even more of the carbonate globules failed to reveal a single one of these defects, indicating that at least some of them could be associated with life. "Maybe we are looking at different generations of carbonates," she said.
So far, the meteorite has not been put to what could be its most decisive test: a search for signs of cell walls in the features that have been tentatively identified as nanofossils of microbes. Paleobiologists say this would be the make-or-break research. Thomas-Keprta said that preparations were under way for such a test, but conceded that it was "a tough analytical problem" at the very limit of the abilities of current instruments.