Life on a Young Planet (39 page)

The organic molecules detected by McKay’s team are just as ambiguous. Called polycyclic aromatic hydrocarbons (PAHs, for short), these molecules are well known on Earth. They have been identified in coal and oil, where they were generated by the geologic alteration of originally biological compounds. PAHs also form in industrial furnaces and automobile engines, where organic molecules in coal and gasoline crack and recombine at high temperature. As a result, these distinctive compounds can be found almost ubiquitously in small concentrations. Indeed, PAHs are widespread in the universe, not just on Earth. They have been identified in carbon-bearing meteorites formed by physical processes in the outer solar system, and occur, as well, in interplanetary dust clouds. Like carbonate minerals, therefore, PAHs are not necessarily signs of life.

Simon Clemett and his colleagues at Stanford University have convincingly demonstrated that the minute quantities of PAHs in ALH-84001 are indigenous to the meteorite and not terrestrial contaminants. These scientists do not claim that the PAHs formed from biological precursors—carbonaceous meteorites could easily have introduced the molecules to Mars. Nonetheless, by demonstrating that 4-billion-year-old organic molecules survive intact on our planetary neighbor, Clemett and colleagues have provided information of immense importance to future Mars exploration: if life did arise on Mars, it may have left a molecular calling card in rocks or sediments beneath the Martian surface. We know of no comparably old organic compounds in terrestrial rocks.

What about the tiny structures interpreted as microfossils? The objects in McKay’s images (
figure 13.2
) look like bacterial cells, but this tells us only that they are small and simple. In fact, they are
very
small, less than 100 nanometers long and as little as 20–30 nanometers wide.
¹
All bacteria are minuscule, but terrestrial bacteria, compared to the Martian microstructures, are as elephants to mice. At the molecular concentrations found in the common intestinal bacterium
E. coli
, the volume of the structures in ALH-84001 is too small to contain more than a handful of molecules. Does this preclude a biological origin? Not at all.

Figure 13.2.
Tiny structures interpreted by David McKay and colleagues as nanofossils in ALH-84001. The longest structures are only about 100 nanometers long, not much bigger than the ribosomes found in living cells. (Photo courtesy of NASA/JPL/Caltech)

In the fall of 1998, the National Research Council convened a workshop on the size limits of very small organisms, stimulated in large part by the Mars meteorite debate. Approaching the question from several different starting points, a panel of distinguished cell biologists concluded that it is hard to fit the biochemistry of free-living modern cells into a package smaller than a sphere 200–300 nanometers (0.2 to 0.3 microns) in diameter. In striking concord, a second panel of microbial ecologists reported that in nature free-living cells match but rarely fall below this predicted minimum size. (Viruses are smaller, but they depend completely on the biochemical machinery of their hosts.)

This result has been misinterpreted on both sides of the Mars meteorite debate. It does not indicate that cells smaller than 200–300 nanometers are impossible, only that free-living nanobacteria, if they exist, must have a cell biology of unfamiliar simplicity. Indeed, in the NRC workshop’s third panel discussion, molecular biologist Steve Benner proposed that a primitive organism might well fit into a 50-nanometer sphere if it had proteins
or
nucleic acids, but not both (obviating the need for relatively bulky ribosomes to link the two—think RNA world, as in
chapter 5
). Improbably tiny bacteria have, in fact, been reported
from a number of terrestrial environments, including the human bloodstream. These reports have been grasped like life buoys by proponents of Martian (and terrestrial) nanofossils, but have been greeted far more cautiously by most professional microbiologists. Conventional microbiologists (viewed by nanobacteria enthusiasts as a reactionary College of Cardinals to their own Galileo) want evidence that such reported objects are complete and free-living cells.
²
Understandably, they also want plausible molecular explanations for small size. In time, one or more claims of terrestrial nanobacteria may be substantiated, challenging us to rethink our assumptions about contemporary cell biology. But even if this happens, it will not end debate about the Mars microstructures—the observation that cells can be very small does not mean that all very small objects are cells.

Equally, the eventual rejection of nanobacterial claims won’t resolve the issue in the other direction. Perhaps the microstructures in ALH-84001 preserve life in its earliest stages—before the evolution of molecular complexity and, thus, unlike modern life on Earth. Or, they might represent conventional cells that shrank during postmortem decay. Or parts of organisms. Size, alone, is inconclusive.

This brings us to the crux of astropaleontological interpretation. We can accept the morphological or chemical patterns in rocks as biological only if they make sense in terms of known biological processes and are unlikely to be made by purely physical mechanisms. That’s the rule on Earth—dinosaurs fulfill both criteria, the filamentous microstructures in Warrawoona chert do not—and it is the rule elsewhere in the solar system.

The second criterion is particularly important in planetary exploration. Because we have no assurance that terrestrial organisms exhaust the possibilities of life, we need to think hard about how we might recognize traces of an unfamiliar biology. Extraterrestrial structures or molecules can be accepted as presumptive evidence of biology
only
if we can eliminate the alternative hypothesis that they formed by physical processes. Biology might vary from planet to planet, but physics and
chemistry should be the same, providing a consistent yardstick for biological assessment.

We don’t know the limits of nonbiological pattern formation, and we need to learn them before intelligently chosen samples are returned from Mars. What we do know, however, urges caution. With puckish delight, Spanish geochemist Juan Garcia-Ruiz has concocted chemical mixtures that spontaneously give rise to minute spheres, filaments, and corkscrews of deceptively biological appearance. Garcia-Ruiz’s creations do not (yet!) include organic structures comparable to the populations of stalked cells, spiny cysts, multicellular trichomes, and mat-forming colonies described in previous chapters, and for that I am grateful. But neither have such features been identified in ALH-84001. The problem with the Mars microstructures is not that they are too small to be biological, but that they are too simple to exclude abiological interpretations. Indeed, many observers accept a mineralogical origin for the structures. Therefore, by the purposely high standard proposed here, the evocative images published by David McKay do not qualify as evidence of biology on Mars.

As a postscript, Andrew Steele of the Carnegie Geophysical Laboratory
has
found undeniable microorganisms in ALH-84001, but they are terrestrial bacteria. During the long interval when ALH-84001 sat in Antarctica, this dark rock absorbed sunlight to form a small oasis of warmth in the polar desert. Bacteria took refuge in cracks once washed by Martian water—undeniable testimony to the ubiquity and hardiness of life on Earth, but yet another roadblock in the search for
Martian
biosignals.

Of the four lines of evidence originally proposed by the McKay team, only one has survived into the new millennium, the unusual magnetite crystals in ALH-84001’s carbonate globules. The magnetic properties of magnetite are well known, but its biological associations are not. In fact, magnetite crystals can provide evidence of biology
because
of their magnetism—some bacteria sense direction using a chain of elongated magnetite crystals synthesized within their cells. The tiny grains have a crystal form and chemical purity not found in magnetites from igneous or metamorphic rocks. After cells die, bacterial magnetite can be deposited in sediments, and magnetofossils have been found in rocks as
old as 2.0 billion years. On Earth, then, if you find a really pure magnetite in nature, it’s a sign of life.

Magnetite grains in ALH-84001 come in various shapes and sizes. Some have structural defects in their crystals that preclude a biological origin. A subset, however, display mineralogical features associated on Earth with biology. Joe Kirschvink, introduced earlier as father of the Snowball Earth hypothesis, believes that similar crystals require similar explanations. To Joe and the McKay team, the magnetite grains in ALH-84001 provide a smoking gun for Martian biology. They argue that the industrial importance (think magnetic tapes) of tiny, chemically pure magnetite crystals is such that if physical synthesis were possible, some enterprising chemist would already have discovered how to do it. Possibly so, but this argument can be stood on its head. Perhaps the person who penetrates the secrets of Martian magnetite will land a patent of immense commercial value.

And so it may. Earlier, I noted that while the carbonates in ALH-84001 may have formed at mild temperatures, they subsequently experienced transiently high temperature and pressure when bombarded by meteorites. With this in mind, a team of mineralogists from the Johnson Space Center and nearby engineering firms devised a most revealing laboratory experiment. Mixing sodium bicarbonate and iron, calcium, and magnesium salts in CO
₂
-charged water (approximating the chemical composition of the precipitated globules in ALH-84001), the team induced mineral precipitation at relatively low (150ºC) temperatures. Then they subjected the deposited minerals to a simulated meteorite impact, sharply increasing the pressure and temperature (to 470ºC) of the reaction vessel for a short time. When the mixture cooled, the team found chemically pure, defect-free magnetite crystals much like those in ALH-84001. Moreover, these newly synthesized magnetites display the distinctive crystal form associated on Earth with biology.

The great twentieth-century philosopher Karl Popper famously maintained that scientific hypotheses can never be proved, only disproved—one thousand black swans can’t prove the hypothesis that all swans are black, but a single white swan can show it to be wrong. This seems straightforward enough, but as the
New Yorker
’s Adam Gopnik has observed, real scientific debates are seldom so tidy. Confronted by a white swan, advocates of the black swan hypothesis are likely to question the
evidence—“You call that a swan?” in Gopnik’s droll telling. So it goes with Mars magnetite. Those who see biology in Martian magnetite vigorously dispute the claim that the loyal opposition has produced anything like the crystals found in ALH-84001.

Regardless of how this disagreement turns out, one more line of evidence challenges the notion that life played a hand in forming Martian magnetite crystals. Based on careful electron microscopic and X-ray microstructural studies, British scientists David Barber and Edward Scott have confirmed earlier observations that the crystal structures and orientations of magnetite grains in the Allan Hills meteorite reflect the crystallographic properties of the carbonates that surround them—something we might predict if the magnetites formed during meteor impact, but unlikely if biology grew the crystals.

Debate about martian magnetite continues, but support for a biological origin is waning, and with it, the last hope that questions of extraterrestrial life might be solved easily. Thinking back to the Proterozoic rocks of Spitsbergen, Gunflint, and the Great Wall, we have to recognize that
none
of the compelling biosignatures found in those rocks have been identified in ALH-84001. People argue about the crystal structure of Martian magnetite because there are no unambiguous microfossils, no steranes, and no microbial mat structures in ALH-84001 or any other meteorite from Mars.

Where does all this leave us? The cynic might answer that where fossils are absent, it’s hard to detect ancient life. A more forward-looking conclusion is that if we want to know whether Mars has ever been a biological planet, we’ll have to go there. And before we go, we’ve got some homework to do.

If ALH-84001 nurtured the discipline of astrobiology in its infancy, what will become of this field in maturity? One dividend of astrobiological thinking, already apparent, is the growth of a planetary perspective on terrestrial biology. Microbial ecologists, biogeochemists, paleontologists—all of us now routinely consider the ramifications of new discoveries for our understanding of the Earth system as a whole. But an astrobiology that truly meets the challenge of its name cannot remain Earthbound. It requires exploration, and the entire universe beckons.

Whatever one might conclude from the meteorite debate, Mars
remains our prime target for astrobiological investigation, and not just because of its proximity. The present-day Martian landscape is bleak beyond imagination, hardly a place to look for life. But channeled terrains and other surface features preserved over 4 billion years tell us that early in its history, Mars was much more like Earth (
figure 13.3
). Both planets had relatively thick atmospheres, active volcanism, and, at least intermittently, liquid water. On Earth, these conditions incubated life, and it is reasonable to hypothesize that they could also have done so on Mars.