WASHINGTON -- The announcement stunned the world: Scientists had found evidence of life on Mars.
Inside a meteorite from Mars, NASA researchers said, they had discovered the fossilized remains of tiny, bacteria-like animals that may have once thrived on the Red Planet.
The idea seized global attention and gave sudden popular legitimacy to the possibility of extraterrestrial life. President Clinton called for a space summit. Famed scientist Carl Sagan called it "a possible turning point in human history."
"We'll know for sure in two years or so," said NASA researcher Everett Gibson.
Now that two years have passed, excitement and applause have faded to doubt and skepticism.
Hundreds of scientists have poked, probed, crushed, dissolved and broken parts of the Mars rock known as Allen Hills 84001. So far, no one has found absolute, incontrovertible evidence that the potato-sized chunk ever contained life.
Then again, nobody has proven that the team led by Gibson and David McKay was wrong.
"Everyone was hopeful that it would just take a short period of time to prove," said Richard N. Zare of Stanford University, a key member of the McKay-Gibson team. "We've seen two years go by. I don't know of anyone who has changed their opinion."
Ralph Harvey of Case Western Reserve University, an expert on meteorites, was excited and skeptical when the McKay group made its announcement. Now he is just skeptical and believes most other scientists are, too.
"People in the field either aren't able to confirm the work or don't seem favorable to it," said Harvey.
John Bradley, a professor at Georgia Institute of Technology, is even blunter: "Early skepticism has evolved into international consensus ... that this rock does not contain Martian fossils. I do not know of a single other individual who believes it at this point."
McKay, Gibson, Zare and others on the Mars rock team, however, still believe that the presence of ancient microscopic life best explains what they found inside.
No one disputes the basic facts about the 4.2 pound rock: that it formed on Mars some 4.5 billion years ago, was catapulted into space about 16 million years ago after a comet or asteroid smashed into Mars.
The rock floated around for millions of years before it came scorching through the Earth's atmosphere about 13,000 years ago and smashed into Antarctica. It was found by meteorite hunters in 1984. Chemistry tests later proved the rock came from Mars.
What's inside has caused the controversy.
McKay's team, using powerful microscopes, found polycylic aromatic hydrocarbons, the first organic molecules ever seen inside a Martian rock. The researchers also found a crystal structure called carbonate, shaped like extremely tiny worms.
The team concluded the most likely explanation was that about 3 billion years ago, Martian microbes had crawled into the rock, thrived and then died. The wormlike structures might be their fossilized remains.
--The organic compounds can be formed in a number of ways that do not involve biology.
--The wormlike structures are crystals that formed with adjacent material when the rock cooled.
--Some minerals were formed in the rock at very high temperatures -- well above the 248 degrees Fahrenheit considered the maximum for life formation.
--The rock contains very few minerals formed in the presence of water. Since water is thought to be essential to life, it's unlikely the dry rock ever contained life.
--Organics found in the rock are of Earth origin. The molecules got into ALH84001 while it sat in Antarctic ice.
McKay, Gibson, Zare and other members of the original team have countered each argument, doing lab experiments to reinforce their findings. Gibson has debated scientists in open meetings.
The debate may never be settled, says NASA's Ed Weiler, "until we go there (to Mars) and get some samples."
No matter how or when the question is answered, notes Zare, science has been the winner.
"Prior to this study, if you talked about searching for life on another planet, you were considered a nut," he said. "It has now become a huge topic that is attracting the best scientists."