BOSTON — Life on Mars? After decades of speculation, a team of scientists has found "compelling" evidence of ancient life on the Red Planet. The stunning discovery moves mankind closer to solving one of the great riddles of human history: Does organic life exist elsewhere in the universe?
The proof was microscopic, and circumstantial. While examining a 16-million-year-old meteorite from Mars, researchers discovered residues of organic chemicals indicative of primitive life. They also found structures similar to fossils of ancient bacteria that may have existed billions of years ago.
The findings, reported yesterday by a team led by scientists at the National Aeronautics and Space Administration (NASA), shook astronomers, but also brought out its share of skeptics.
If confirmed by other investigators, this would be "a major first discovery" in NASA's efforts to find signs of life on other worlds, says Arden Albee, a leading Mars expert at the California Institute of Technology.
In announcing the discovery, NASA administrator Daniel Goldin acknowledged that while "the evidence is exciting, even compelling ... [it is] not conclusive." David McKay of the Johnson Space Center in Houston, one of the team leaders, explained: "There is not any one finding that leads us to believe that this is evidence of past life on Mars. Rather, it is a combination of many things...."
He added that it is the relationship of these chemical residues and microscopic forms to each other that "is the most compelling evidence." Now, he said, it is up to others to confirm or overturn his team's findings.
Those findings will be detailed in the Aug. 16 issue of Science magazine, in a paper written by nine scientists from seven NASA and university research centers. But the news was leaked this week, forcing NASA to release the information early.
The rock in question
The meteorite under study is a 4.2-pound potato-size object. It is some 4.5 billion years old, dating from the time when Mars and Earth were formed.
Scientists believe that 16 million years ago an asteroid knocked the rock out of its location beneath the Martian surface. It then traveled through interplanetary space until falling to Earth in Antarctica 13,000 years ago. There it was found in the Allan Hills ice field in 1984 - hence its name, ALH84001. It is the oldest of 12 Antarctic meteorites whose composition differs from that of other meteorites but matches the composition of Martian material analyzed by the Viking lander spacecraft.
Mars today is cold, arid, and generally inhospitable for life. But during its first billion or so years it appears to have had a more accommodating climate. At that time, primitive life could have evolved in ponds or subsurface water and in fissures in rock formations. This appears to be the habitat of the presumed meteorite fossils.
Carbon dioxide from the CO2-rich Martian atmosphere would have permeated such fissures. There, primitive life forms would transform the CO2 into carbonates. The scientific teams have found carbonate globules within the meteorite. They are associated with chemicals called polycylic aromatic hydrocarbons or PAHs for short. These compounds form when bacteria die. Also closely associated with the carbonates and PAHs are microscopic structures that look like bacterial fossils and tiny bits of magnetite similar to those found with bacteria on Earth.
While this evidence is compelling, it also offers food for skeptics. PAHs can be produced cosmically in nonbiological processes. Some of these are found in nonmartian meteorites. Then there is the question of contamination of the meteorite after it landed on Earth.
"The conclusion is at best premature and more probably wrong," says John Kerridge, a planetary scientist at the University of California, San Diego. "You should not go public with evidence that's less than 100 percent sure. This is much less than 100 percent sure."
The NASA-led team has tried to take such uncertainties into account. They note that the carbonate deposits are 3.6 billion years old and that the organic residues and "fossils" are well within the carbonate material.
The composition of the PAHs in question is of a type generally associated with life rather than nonlife chemistry. If the meteorite was contaminated with earthly PAHs, these should be most abundant on the meteorite's crust. But no PAHs show up on the crust. Their concentration increases toward the meteorite's interior, just as expected if they indeed came from Mars.
Adding it all up, members of the NASA-led team believe they have a strong case for ancient life on Mars. Nevertheless, as Dr. McKay said, the proof of their conclusions lies in the ability of other investigators to verify them.
NASA's Goldin further cautions: "I want everybody to know that we are not talking about 'little green men.' These are extremely small, single-cell structures that somewhat resemble bacteria on Earth. There is no evidence or suggestion that any higher life form ever existed on Mars."
Whatever the final outcome of this ongoing research, the current findings change the terms of debate over NASA's Mars program. That program features a series of low-cost missions over the next decade. These include both orbiters and landers with associated robots. NASA has made the search for evidence of existing fossil life and of pre-life conditions a priority.
Impact on missions to Mars
Skeptics have questioned that priority on grounds that life seemed to be a remote possibility for Mars. Now the Allan Hills meteorite shows there probably is evidence on Mars to investigate.
But this lends strength to the criticism of the program released Tuesday by the National Research Council, the operating agency of the National Academy of Sciences. The NRC points out that NASA's low-cost approach means that the Mars rover vehicles will be too simple in design to do the kind of investigation to find life signs buried inside rock formations. It says NASA needs to devote more resources to development of more-sophisticated robot-rover vehicles than now planned if it is to pursue the investigation of Martian life efficiently. The new discovery lends urgency to that recommendation.