You might expect some excitement at a press conference to announce the discovery of life on Mars, but the scene at NASA headquarters in Washington, D.C., last August was over the top. In the front of the room, a pack of reporters and cameramen were jostling for position around the specimen, throwing elbows for a better look. A panel of scientists waited on the podium, their jet-lagged faces pasty in the glare of the tv lights. Suddenly a piercing, inhuman shriek filled the auditorium. NASA officials barked frantically from the lectern: Does anybody have a molt box? Can we hook up a molt box? Those of us on the fringe of the crush could only wonder: Was the Martian actually shedding its skin, right here on E Street? And if NASA didn’t have the equipment to handle the situation, for God’s sake, who would?
All this, before the press conference had even begun. A glitch in the sound system had left the scientists without microphones, delaying the proceedings and letting loose that screech of feedback. After the problem was solved by the connection of a mult box (a piece of electronic gear unfamiliar to this correspondent), the meeting was eventually called to order. In the meantime, the center of attraction--a meteorite which might, just might, hold within it the discovery of the century--lay before the scientists on a cushion of blue velvet, calm and composed as only a rock can be.
The news, of course, was worth the wait. Today we are on the threshold of establishing whether life is unique to Earth, announced NASA chief Dan Goldin, before yielding the microphone to the team of scientists who had made the discovery. With the help of an animated video, Everett Gibson of the NASA Johnson Space Center in Houston summarized the life story of the putatively life-bearing rock: how it crystallized from magma below the surface of Mars 4.5 billion years ago; how half a billion years later the impact of meteorites on Mars fractured the surface, allowing water to penetrate and carbonate minerals to form in its fractures; how some 16 million years ago a comet or asteroid struck the planet, sending pieces of its crust hurtling into space; and finally, how a mere 13,000 years ago, or roughly around the time when man was first learning to plant crops, this particular morsel of Mars entered our atmosphere and fell onto the ice of Antarctica, to be plucked up and pocketed by a research team in 1984.
The dramatic turning point in the rock’s story was the formation of those carbonate minerals. As the scientists who followed Gibson made clear, four lines of evidence hint that the process may have been aided and abetted by tiny Martian bacteria. First was the presence of the carbonate globules themselves, which were likened by the team to carbonates formed by bacteria on Earth. Associated with the carbonates, furthermore, were fine- grained iron sulfides and magnetite minerals whose morphology and chemistry also resembled bacterial products. Even more compelling, perhaps, was a concentration of polycyclic aromatic hydrocarbons (pahs) in the vicinity of the carbonates; pahs are a common chemical result of organic decay. Finally--and most controversially--a high-powered scanning electron microscope revealed egg-shaped and segmented, Tootsie-roll-like structures that might be fossils of the Martian bacteria themselves.
The scientists emphasized repeatedly that none of these observed phenomena demonstrate that life once existed on Mars, since each could be derived from inorganic processes as well. But the fact that they were found clustered together in the meteorite, the researchers argued, meant the most sensational possible origin was also the most likely--a reasonable interpretation of the evidence, in the words of David McKay of the jsc.
To finesse the skeptics, NASA included on the panel a highly credentialed independent investigator ready to say why everyone else up there was probably wrong. ucla paleobiologist J. William Schopf, discoverer of the earliest form of life on Earth--3.5-billion-year-old microfossils from northwestern Australia--questioned the NASA conclusion on several fronts. Noting that pah molecules can be formed inorganically--they’re in car exhaust, for instance--he pointed out that they have been found before on detritus from space without anyone’s claiming they were evidence of extraterrestrial life. Other investigators who had studied the same Martian meteorite, meanwhile, had recently suggested that the carbonates had been formed inorganically at temperatures far too hot for life. As for the alleged microfossils, in Schopf’s expert opinion they were simply too micro to be fossils: they were a hundredth the size of the smallest terrestrial bacteria. To show that they were once alive, the NASA researchers would have to get inside and find a cell wall or membrane, and, if possible, evidence of cell replication. Without that smoking gun, said Schopf, the biological interpretation is unlikely.
But this work is doable, he concluded encouragingly, and I’ll bet, as soon as these guys can get on a plane, they’re going to shoot back down to Houston and get another paper in Science. I hope they can nail this thing shut.
And so that’s what they have been trying to do, at least when they aren’t fending off the media or testifying before Congress. Slicing open a structure a millionth the size of a pinhead to look for a cell wall isn’t simple, but McKay and his colleagues are developing a procedure to do just that. Convincing their colleagues that the Martian structures really are fossils would also be easier if equally tiny bacteria could be found on Earth. Several years ago, Robert Folk of the University of Texas first reported the discovery of such bantamweight microbes living in travertine and limestone. In size and shape, McKay says, Folk’s electron microscopic images match the Martian pictures tit for tat. So far the biological community has been slow to accept Folk’s structures as bacterial. According to Folk, that’s mainly because they are so small they pass through the standard laboratory filter used to sieve life out of a solution. McKay’s group would like to confirm the existence of Folk’s nannobacteria. We’re pushing ahead into new areas, says McKay. We don’t want to rest on our laurels.
One of those areas involves the laser mass spectrometer that discovered the pahs in the meteorite. Richard Zare of Stanford, another member of the nasa-led team, is now refocusing it to look for amino acids. The presence of these building blocks of proteins in the meteorite would not be conclusive proof of biological activity either--they too have been found in other meteorites--but if Zare finds them clustered with the other alleged life traces, rather than spread out uniformly, it would constitute powerful new circumstantial evidence.
It will take more than that to persuade the skeptics, however, if only because the stakes are so high--and because everyone is mindful of the lessons of 1961, when another group of investigators claimed (erroneously) to have found fossils in a meteorite that had fallen in France. If their interpretation of the evidence is true, this would be the most important scientific find ever, says Ralph Harvey of Case Western Reserve University, who led the team that recovered the Martian meteorite in the Allan Hills region of Antarctica 12 years ago. So the standard of proof has to be extraordinary.
Just a month before NASA dropped its bombshell in August, Harvey and colleague Harry McSween of the University of Tennessee had published their own analysis of the meteorite. Noting the absence of water-bearing minerals, they hypothesized that the carbonates in the fractures may have been formed by the sudden infusion of carbon dioxide into the Martian crust by an asteroid impact. Their study was not aimed at finding signs of life on Mars, but since the swift chemical reaction that they think produced the carbonates would have occurred at 1,200 degrees Fahrenheit, the issue would be moot. The NASA team believes the carbonates were formed over a much longer period at temperatures no higher than 180 degrees. Harvey remains insufficiently impressed. If they crack open one of these rocks and find something clearly like terrestrial life, well, maybe, he says. But it’s going to take a lot to convince me.
More, probably, than just an independent confirmation of the NASA findings--which is what a British team reported at the end of October. The British researchers looked at a second meteorite as well as the Allan Hills one, and they found different evidence--a large quantity of organic carbon, a biotic-looking ratio of carbon isotopes. But it was still circumstantial evidence, for which life was still just a reasonable interpretation.
I’m open to someone coming up with another interpretation, says Zare of the NASA team. But suppose we are able to establish that there was life long ago on Mars. Then some really interesting questions come up.
Really interesting, as in Could something up there still be alive? The surface of Mars today is waterless and cold, and its atmosphere is too thin and dry to support a biota. But there is clear evidence that Mars was warmer and wetter early in its history, and some of that water may still be present beneath the surface. In 1995, Todd Stevens and James McKinley of the Pacific Northwest National Laboratory in Richland, Washington, reported the discovery of bacteria living 4,900 feet underground in basalt formations near the Columbia River, apparently thriving on nothing more than water and hydrogen derived from the basalt. Could similar organisms exist under Martian ground? We think that’s a good possibility, says Stevens. There is no reason it couldn’t work that way on Mars, or any other planet with liquid water and basalt.
Which raises another, perhaps ultimately interesting question. If there is life on both Mars and Earth, where did it all start? Did some chip off the old block of Earth fall onto Mars billions of years ago, bringing microbes with it, or was it the other way around? Or did life emerge independently on the two neighboring planets, and who knows where else?
These questions have answers, says Zare. But we may not be able to find them without going to Mars.
