Scientists studying a meteorite that fell to Earth from Mars have identified organic compounds and certain minerals that they conclude "are evidence for primitive life on early Mars."
The discovery of the first organic molecules ever seen in a Martian rock is being hailed as startling and compelling evidence that at least microbial life existed on Mars long ago, when the planet was warmer and wetter. The molecules found in the rock, which left Mars some 15 million years ago, are being described as the fossil trace of past biological activity.
In a statement issued Tuesday, as unofficial word of the discovery spread, NASA Administrator Daniel Goldin confirmed that scientists had "made a startling discovery that points to the possibility that a primitive form of microscopic life may have existed on Mars more than three billion years ago."
A detailed description of the research, conducted at the Johnson Space Center in Houston, is to be given at a NASA news conference at 1 p.m. Wednesday in Washington. The journal Science is publishing a full report on the work in its Aug. 16 issue.
A draft of the Science paper, written by a team led by Dr. David S. McKay of the Johnson Space Center, identified the telltale compounds as polycyclic aromatic hydrocarbons, called PAHs, which often have biological origins and are associated with coal and diesel exhaust, and the minerals magnetite and iron sulfide, both of which are related to bacterial action on Earth. In addition, they found some carbonate globules that are similar in texture and size to materials produced by bacteria on Earth.
"The PAHs, the carbonate globules and their associated secondary mineral phases and textures could thus be fossil remains of a past Martian biota," the researchers concluded.
Although the authors of the paper were under orders from NASA not to talk before the news conference, scientists familiar with the research spoke in unusually enthusiastic terms about the work.
One scientist called the results "unequivocal." Another said it was one of the most stunning discoveries in the solar system in recent decades. But they cautioned that it would take more research to assess the full implications of the discovery for the longtime quest for extraterrestrial life.
Dr. David C. Black, director of the Lunar and Planetary Institute in Houston, praised the research as being "very, very cautious" work that would not have been possible two or three years ago, before new analytical instruments for electron microscopy were available.
"The signs are all consistent with biological activity," Black said. "But what makes the case much more interesting is that the compounds are found all together in proximity within the meteorite. That is really suggestive."
Reservations about the interpretation of the discovery were expressed by Dr. Jack D. Farmer, a geologist and paleobiologist at the Ames Research Center in Mountain View, Calif. He said the meteorite, as one of the oldest rocks ever identified in the solar system, contained evidence that Mars had water early in its history and that this environment, with liquid water and organic molecules, was one where life could have existed.
"We didn't know this about Mars until now," Farmer said. "It gives us a lot of compelling reasons to go back to Mars. But it doesn't tell us anything definite about life. It's interesting, but not convincing."
Because of its possible scientific and philosophical ramifications, Goldin briefed President Clinton and Vice President Al Gore last week about the discovery. They were reported to be "excited" about the research.
After the space agency makes its formal announcement, White House officials said, Clinton is likely to call for more study of the question of fossil life on Mars and may propose adding resources to ongoing NASA projects to place unmanned spacecraft and roving vehicles on the planet.
The last major thrust of Mars exploration by the United States was the Viking project, which landed two robotic craft on the cold, arid surface and conducted weeks of tests, sampling the soil for any trace of biological activity.
The results appeared to rule out any lingering hope of Martian life in some microbial form, though other investigations showed that water must have flooded the Martian surface in the distant past, raising the possibility that life existed there soon after the planet's formation 4.5 billion years ago and then disappeared.
The scientific plans for future missions call for focusing on searches for fossils in regions where water once stood and might still be present in a kind of subsurface permafrost.
The meteorite bearing the organic molecules is one of only 12 identified as having come from Mars, based on analyses of their chemistry, compared with that of Martian surface materials examined by the Viking instruments. It is the oldest, having crystallized from molten rock about 4.5 billion years ago, in the planet's infancy. The carbonates, chemicals closely associated with the PAH molecules, dated from 3.6 billion years ago.
About 15 million years ago, scientists estimate, a huge asteroid collided with Mars, gouging many pieces of rock from beneath the surface and scattering them into space. One of them eventually fell on the icy Allan Hills in Antarctica about 13,000 years ago.
Scientists found the meteorite in 1984 and named it Allan Hills 84001, but identified it as Martian only recently. Experts believe that many other fragments of Mars probably lie unrecognized in meteorite collections.
As soon as the meteorite was found to be Martian, McKay and his colleagues began investigating its pitted and fractured dark surfaces, making thin slices of the rock where they detected the organic molecules. The work involved scientists from the Johnson center, Stanford University, the University of Georgia and McGill University in Montreal.
The scientists described the PAH molecules, which contain multiple connected rings of carbon atoms, were clear and distinct. They studied the samples further to make sure the organic molecules had originated on Mars, not as the result of terrestrial contamination.
One telling piece of evidence was the fact that the concentration of the molecules increased as the analysis moved inward; if those molecules had been contamination, they would have been concentrated on the surface.
These PAH molecules are common on Earth, and many of them have biological origins related to bacteria. They are abundant, for example, in ancient sedimentary rocks, coal and petroleum, the residue of the chemical changes that occurred to the dead marine plankton and plant life that were the source of these rocks and so-called fossil fuels. The PAHs also occur during partial combustion, as when a candle burns and when a steak is grilled on charcoal.
The scientists began to sense they might be approaching a big discovery when they noted other characteristics of the rock's chemistry and mineralogy. The highest concentrations of PAHs were associated with carbonates. The carbonates, found in the rock fissures, were younger than the rock itself. The magnetite and iron-sulfide particles inside the carbonate globules are in many ways similar to particles produced by bacteria on Earth.
In their planned journal report, the scientists wrote: "None of these observations is in itself conclusive for the existence of past life. Although there are alternative explanations for each of these phenomena taken individually, when considered collectively, particularly in view of their spatial association, we conclude that they are evidence for primitive life on early Mars."
The authors of the report, other than McKay, were Dr. Everett K. Gibson Jr., of the Johnson Space Center; Dr. Kathie L. Thomas-Keprta, of Lockheed Martin in Houston; Dr. Hojatollah Vali, of McGill University; Dr. Christopher S. Romanek, of the University of Georgia's Savannah River Ecology Laboratory in Aiken, S.C.; and Dr. Simon J. Clemett, Dr. Xavier D.F. Chillier, Dr. Claude R. Maechling and Dr. Richard N. Zare, of Stanford University in California.
Dr. Ray Arvidson, a planetary geologist at Washington University in St. Louis, said that McKay had a reputation as an extremely careful scientist and that his findings could be trusted. "If this holds up," Arvidson remarked, "this is pretty darned exciting."
Goldin said NASA "is ready to assist the process of rigorous scientific investigation and lively scientific debate that will follow this discovery."
With the recent detections of planets around other stars and the planned return of robotic spacecraft to Mars, with launchings beginning late this fall, the space agency has stepped up efforts to explore the possibilities of life elsewhere in the universe.
In Goldin's statement about the Martian meteorite, he emphasized that the discovery had nothing to do with the "little green men" of past Martian lore.
"These are extremely small, single-cell structures that somewhat resemble bacteria on Earth," he said. "There is no evidence or suggestion that any higher life form ever existed on Mars."
One member of the discovery team, Zare, a professor of chemistry at Stanford University, said this could be a first step in answering the ages-old question of whether people are alone in the universe and to what extent life is unique to Earth. The meteorite and the recent discoveries of planets around other stars, he said, are changing thinking "from life being special to life being ubiquitous."
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