The Rock From Mars (23 page)

Also, he added, his team had mapped the PAHs in the rock and found that they were more densely concentrated toward the rock’s core and correlated with the carbonate globules—the opposite of the pattern to be expected if the PAHs had seeped into the rock after it had arrived at Earth. Aided by the animation, Zare described his Stanford team’s immensely sensitive laser technique, which enabled them to look at targets as small as a few thousand molecules.

Zare concluded his talk by stating what some knowledgeable listeners would describe as the most indisputably important news from the rock: “By this means, we’ve been able to look and see the first organic molecules that we believe come from Mars.” Here were the long-sought signs, which the Viking landers had failed to detect, that once upon a time Mars had held crucial building blocks of life—and might still.

Next, McKay brought forth what he and others expected would be at the same time the most arresting and the most controversial portion of the evidence: the images the team thought might be fossils of ancient Martian organisms. (Only one of the fossil-like images had been reviewed by outside experts and approved for publication the following week in the journal
Science—
drawing criticism later on.)

The images on the auditorium screens zoomed in to a highly magnified view of a section of the iron-rich Oreo rim in the Grady image of a carbonate moon. The next image showed a grainy field of what looked like varieties of beads in different sizes cemented together in mounds.

McKay noted that “the biggest object in the picture is about one hundred nanometers across—that’s about one-five-hundredth the size of a human hair. . . . You can see that there’s one rod shape, there’s one with a dark line up the center of it.” Some of the shapes were probably the magnetic mineral grains Thomas-Keprta had shown earlier, “but some of these don’t look like either magnetite or iron sulfide,” he said. “They may be something else. And we’re not quite sure what that is.”

McKay then showed a structure that resembled a rough wall or reef. In the lower half of the image was a school of jelly bean–shaped objects that seemed to be “swimming” across it. “These are elongated forms, structural forms. We think that matrix they appear to be eroding out of is probably a clay mineral. We’re [in the process of] confirming that. . . . The features that you see may be any number of things. For example, they could be dried-up parts of that clay. Or they could be microfossils from Antarctica. Or microfossils from Mars. It is our interpretation, the one that we favor, . . . that these are in fact microfossil forms from Mars,” McKay said.

Once again, he added a caution. “But keep in mind that is an interpretation. We have no independent data that these are fossils. We don’t have pictures showing cell walls or internal material characteristic of cells. It is simply an interpretation at this point.”

He showed more grainy material with rodlike or wormy shapes seemingly caught in mid-squirm over the rounded surfaces and draped over the mounds, mostly in the left half of the image. “Are these strange crystals? Are they, uh, are they dried-up mud? We believe, we interpret that these are microfossils from Mars. They are extremely tiny. The longest one is about two hundred nanometers. This is very high magnification. . . . We are looking at rocks and minerals at a scale that has really not been used before.”

At this point, for comparison, McKay showed an image of possible terrestrial nanobacteria on an Earth rock—features found by a completely different group and interpreted as organisms in a 1994 paper. (The things looked something like white jelly beans and candy bits strewn on a crumbled pile of dark cake.) They were found “on calcite, calcium carbonate, the same kind of material we are looking at on Mars,” he said. At about 500 nanometers, unusually small for most known bacteria, he said, “these things are the same size and shape as many of the forms we’re seeing in the Mars sample.”

At the next image, the audience let out an audible gasp. It was the feature that would become known as “the worm,” a segmented shape resembling a stretched-out Tootsie Roll, reclining on a grainy slope. “As we move on, we see a few of these elongate forms which appear to be segmented,” McKay said. This one was about one-hundredth the diameter of a human hair, “again very tiny but now we’re getting up into the size range of common terrestrial microbes and bacteria, and whether this is a microfossil or whether it’s a dried-up mud crack we can’t really say because we have no data other than what you see. . . . But again, it is our interpretation that this and similar features have a high probability of being Martian microfossils.”

McKay showed another image of the rock produced by a different technique, taken by team member Hojatollah Vali at his lab in Canada. This time, the scene resembled a bas-relief of snakes slithering across a frieze. “We don’t have chemistry on these,” he said, “but one possible interpretation is that they are similar kinds of Martian microfossils” to the ones seen under the microscope in Houston.

McKay concluded with a slide of “real bacteria . . . , which turn out to be about the same size and about the same shape as the things that I’ve been showing you in the Mars sample.” These organisms had been found in a sample drilled from volcanic rock more than a mile underground in the Columbia River area of Washington State. The native Martian environment from which the Allan Hills rock had been blasted might once have resembled the modern microbial habitat in the Columbia River formation: that is, a subsurface fracture in basalt, through which water flowed.

During the question period, McKay and others would elaborate further on the evolution of the Martian environment—how at some point in the planet’s history “things went bad.” The atmosphere mostly disappeared, either into space or into carbonate rocks below the surface, where it might remain locked up today. And the water dried up, some of it wafting into space, some possibly still on the planet in the form of permafrost or “even as a groundwater system.” But what had happened to any life-forms that had existed there before this sorry turn of events? They could not live on the surface, but it was possible that some of them might survive underground—like the Columbia River organisms on Earth, drawing their energy from hot springs, and hydrothermal areas, and interactions between the atmosphere and the rock.

McKay ended with another recitation of caveats. “We have a number of forms which it is very tempting for us to interpret as Martian microfossils
—but
we have no confirming evidence, and you’ll hear more about the pitfalls of identifying such things based on appearance alone. We don’t have the chemistry of these. We’ll find out if they have cell walls or not. . . . That will be part of our future work. But for now, we have to use these images and interpret them the best way we can. So I want to finish up here by simply saying that we have these lines of evidence and none of them in itself is definitive. But taken together, the simplest explanation to us is that they are the remains of Martian life.”

For David McKay, the past forty-eight hours had been a wrenching journey that had carried him light-years, at warp speed, away from the clear waters, patient cliffs, and peace of the Texas hill country. Now, despite the dislocation and fatigue, he had told his story at last.

Huntress gave the last word to Schopf, the designated skeptic.

While the other scientists were talking, it seemed to Zare, Schopf had sat there wearing the sour face of someone sniffing a carton of milk to see if it’s gone bad. A few years later, writing about the news conference in a book, Schopf would portray himself as having been sandbagged by the superior preparation and graphics of the McKay group—a version that Zare would disagree with.

Schopf began by saying that he preferred to render his comments “as part of a discussion rather than a debate. . . . I do think this is a fine piece of work. And this is not easy science.” He moved quickly to his main point by reading a quote attributed to the absent Carl Sagan, as the words flashed up on the auditorium screens: “Extraordinary claims require extraordinary evidence.”

Speaking with the assurance of the seasoned lecturer and teacher, Schopf mentioned his long history of searching for ancient life on Earth, and outlined criteria he used to test such claims on Earth. “And those criteria in my opinion must be met on Mars as well,” he said.

He showed a slide depicting his own crowning achievement: microfossils from the Apex chert (a type of quartz) in Australia. At 3.465 billion years in age, they were the oldest evidence of life on this planet, he noted. Magnified on-screen, they looked like long, sinuous filaments clearly divided into segments, again like skinny, stretched-out Tootsie Rolls. “They are demonstrably cellular, as you can see, and they are composed of organic material. Their cell walls are made of organic matter.”

On the next slide appeared another set of fossils from the same deposit: “They have conical end cells, they have rounded end cells, they have demonstrable cells, and all that. These are demonstrably fossils.” He drew attention to a minuscule strand one-half micron in thickness. “This is a bacterial strand, it’s 3.5 billion years in age, it comes from this Earth, and it is
one hundred times larger
than these microscopic objects that we have just seen from Mars. And that is one of the smallest—shown in this slide—one of the smallest fossils that has been found on Earth.”

Schopf’s final slide showed a confidence-rating chart that used seven criteria to compare the evidence of life on Earth to the evidence of life on Mars as just presented. Schopf punched his words. “I want to emphasize this is
subjective.
It says
subjective.
It is italicized
subjective.
It is my
o-pinion.
”

Noting that the Olympics had just ended, he said he was using a similar scoring technique—a scale of one to ten. He gave the probability that the meteorite was from Mars a nine. The age of the carbonate globules, too, seemed “pretty well established” at around 3.6 billion years. “I give that a confidence rating eight.”

Evidence of the rock’s environment and history was the subject of debate, he pointed out. Another team of scientists had recently argued in a published paper that the carbonates had been formed at 450 degrees Celsius (842 degrees Fahrenheit), much too hot for biological activity. They had also suggested that the rock had been fractured during the impact that threw it off the Martian surface, rather than well before, as the McKay team maintained. “I’m not taking sides in the matter. I’m simply saying that this is not a resolved issue as yet in the minds of some people,” Schopf said, adding with a smile, “. . . although I think the guys here at NASA make pretty good arguments.”

Schopf found it likely that the organic matter and the fossil-like objects, like the rock that contained them, were also from Mars. “I think it’s very likely that even though they occur in fractures where groundwater [on Earth] can introduce things, I think the data are good,” he said. “I give it an eight or nine rating that in fact those things are as old as the fractures in that rock” and therefore are from Mars.

However, addressing the possibility that the PAHs might have come from a
biological
source, Schopf turned feisty: “I take a rather different view. With regard to the polycyclic aromatic hydrocarbons, I
note
that such compounds are found in interstellar dust grains. I
note
that PAHs are found in interplanetary carbon grains. I
note
that PAHs are found in other sorts of meteorites, like carbonaceous chondrites. In none of those cases have they ever been interpreted as being biological. This is, after all, a meteorite.” Therefore, assuming they’re not contamination from industrial pollution on Earth, he said, “I’d say the first guess would be that they are probably nonbiological, just like PAHs that occur in other meteorites. The burden of proof is on those who claim that they’re biological.”

As for the fossil-like objects, he said, “I note that they are one hundred times smaller than such fossils that have been found on this planet. I note that there is no evidence of their composition. The best guess at this point would be that they are made of mineralic material. At least, there are no data—and it’s because they are so small there are no techniques at present to analyze their chemical composition. But there’s no evidence that they’re made out of carbonaceous [i.e., organic] material. We don’t know that yet. Thirdly, there’s no evidence that there is a cavity within them, a compartment, a cell. Why do you need that? Well, that is where the juices of a living organism reside. That’s where the chemistry that makes things live works. You’ve gotta look inside these things, see if they have cell walls, see if they’re compartmentalized, see if they’re cellular, see if they’re composed of organic material. There is no good evidence as yet of life cycles, or of cell division—the tests that we also apply to the fossil record [on Earth].”

In his opinion, he said, the biological interpretation was probably unlikely. “I finally come back to Carl Sagan’s quotation, which I think is applicable. . . . Personally, I think that this is exciting. I think it is very interesting. I think they are pointing in the right direction. But I think a lot more, or a certain amount of, additional work needs to be done before we can have firm confidence that this report is of life on Mars.”

In the months and years to come, McKay would hear the Sagan quote repeated constantly by Schopf and by others. And he would confess now and then that it annoyed the hell out of him, given that Sagan had helped referee the very
Science
paper that was at issue.

There followed an hour-long barrage of questions from the assembled journalists as well as from reporters at other NASA facilities across the country, linked in by satellite. These exchanges focused heavily on what the findings might mean for public policy in space- and earth-based research. But there were a few queries about specific problems with the data, triggering a brief point-counterpoint between Schopf and Gibson. Someone asked what it would take to convince a doubter like Schopf. What was the “smoking gun,” and was it obtainable?