The Case for Mars (53 page)

The Carbonate Globules Formed at Low Temperatures

The chronology of events that brought ALH84001 into being and onto Earth is fairly well established. The events that led to the creation of the carbonate globules, however, are not as well established. McKay and colleagues hold that carbon dioxide enriched liquid water filled the fissures in the Martian stone, with the carbonatesng formed in temperatures below 80°C. Other researchers, however, have proposed that the carbonates formed when carbon dioxide rich fluids percolated into the fissures at temperatures approaching 450°C, perhaps during one of the “shock events.” Of course, 450°C is something less than a hospitable environment for life. However, it seems pretty clear to me that the NASA researchers have rebutted these arguments successfully by countering with evidence that supports low-temperature formation. This evidence includes the oxygen isotope ratio data, the presence of greigite within the carbonates, and the presence of organic molecules—all would be destroyed or degraded if heated to 450°C.

The Chemistry and Mineralogy Within the Carbonate Globules Indicate Biologic Activity

Under extremely high magnification, the carbonate globules revealed the presence individual mineral crystals that, on Earth at least, are generally produced by bacteria. (The NASA researchers selected a 50-micron diameter globule and sliced it into about fifty thin sections.) These mineral crystals were identified as magnetite, pyrrhotite, and greigite. All of these minerals can, however, sometimes be produced by nonbiologic as well as biologic processes on Earth. But the team compared the crystals found within the carbonate to known samples of biologic origin, and discovered that they closely resembled their terrestrial counterparts in size, shape, and structure. Furthermore, the minerals appeared to be extremely pure, free of contaminants
. Given this resemblance and the crystals’ purity, Gibson, McKay, and colleagues argue that the crystals they discovered were “biogenic” in origin—they were the products of biological activity.

They supported this claim with an interpretation of the overall mineralogy of the globules based, in part, on the texture of the carbonates. They point out that one region of their sample was more porous than others, and believe that the carbonate in this area was partially dissolved by acidic fluids. However, inorganic precipitation of magnetite and pyrrhotite require just the opposite conditions: high pH rather than low (acidic) pH conditions. If the two minerals had somehow formed, the researchers add, the particles should show some sign of corrosion or dissolution. In essence, the researchers hold that the combination of magnetite, pyrrhotite, and carbonate could not have formed nonbiologically.

This is a tough argument to refute.

ALH84001 Contains Organic Molecules Associated with the Carbonate Globules

Polycyclic aromatic hydrocarbons (PAHs) are organic compounds made up of carbon and hydrogen (“hydrocarbon”) and that have specific arrangements of their atoms and molecules (“aromatic” and “polycyclic”—aromatic was originally used to describe these compounds because they literally stank). If you’ve sniffed mothballs you’ve had a whiff of a polycyclic aromatic hydrocarbon (naphthalene). PAHs are ubiquitous on Earth, formed in numerous ways, from burning fossil fuels to searing meat, to the decay of once living matter.

Zare and his Stanford co-workers discovered small amounts of PAHs associated with the carbonate globules within ALH84001 via a method termed microprobe two-step laser microscopy. This process is exceptionally sensitive, capable of measuring trace levels of organic compounds, and has an extremely fine spatial resolution, down to 0.04 microns for the detection of PAHs. In the process, one laser hits a sample with a powerful but short (s pen-millionth of a second) pulse, causing organic molecules to detach from the surface. A more powerful laser then blasts this cloud of molecules with an even shorter pulse (a billionth of a second), exciting certain types of mole
cules and knocking an electron off the molecule, thereby ionizing it. All this takes place inside a very high vacuum, electrically charged chamber, so the now-ionized molecules travel out of the vapor cloud to a time-of-flight mass spectrometer. This device then determines the mass of the molecule. The second laser in the two-step system can be tuned to pick out certain families of organic molecules, in this case PAHs. It can also be set to detect amino acids—protein building blocks—as well as DNA bases.

The discovery of PAHs within ALH84001 was newsworthy in that no organic compounds had previously been detected on Mars or within an SNC. The discovery of PAHs within a meteorite, though, was old news. One class of meteorites, carbonaceous chondrites, carry an abundance of PAHs, and rarely has there been a claim that these PAHs resulted from biologic activity. Further, because PAHs are so common, the NASA researchers had to convince not only other researchers but themselves as well that the PAHs they measured were not laboratory contaminants or terrestrial PAHs that contaminated ALH84001 during its residency in Antarctica. One of the longest footnotes to the
Science
paper details the extensive precautions and tests taken to prevent contamination. In the research team’s opinion, “no evidence can be found for laboratory-based contamination.” Likewise, contamination checks and control experiments, according to the NASA team, indicated that the PAHs measured were indigenous to ALH84001. They noted that PAH accumulations on the Greenland ice sheet range from ten parts per trillion for pre-industrial times to one part per billion for recent times. The PAHs detected in ALH84001 are present in the parts per million range, a thousand times higher than could be expected from terrestrial contamination alone. Further, studies of two other Antarctic meteorites showed no evidence of indigenous PAHs. The authors term these comparison samples “equivalent desorption matrix blanks”—essentially equivalent sponges. If ALH84001 had picked up PAHs in Antarctica, these samples should have too.

Perhaps the strongest evidence against terrestrial PAH contamination came from an investigation of the PAH concentrations as a function of depth within the sample. In essence, there were more PAHs inside the stone than on the outside. This is exactly the opposite of wha
t one would expect from terrestrial contamination (heavy concentrations on the outside of the sample that dropped off the deeper one went). So, the fact that the PAHs are indigenous seems well established, but what evidence does the NASA team have for their being the result of biologic activity? After all, PAHs have been measured in other meteorites, but their formation has been attributed to nonbiologic processes. The team addresses this issue by noting that though PAHs have been discovered in a wide range of interplanetary materials, each material is characterized by differing PAH distributions (remember, PAHs are a class of organic compounds; there are hundreds of them). That is to say, the PAHs found in a carbonaceous chondrite will differ to some degree from those found in interplanetary dust particles, which in turn will differ from those found in, say, a burnt cheeseburger. The PAHs in ALH84001 do indeed differ from those found in other extraterrestrial materials as well as from those found in ancient terrestrial materials. The researchers point out that the PAH distribution within the Martian meteorite is relatively simple, containing just a few of the random mix of PAHs found in other samples. Such selectivity is much more typical of life than it is of lifeless organic chemistry.

Structures That May Be Microfossils Are Associated with the Carbonate Globules

The images displayed by the NASA team are, perhaps, the most controversial evidence presented. The researchers took care to ensure that the structures imaged were not artifacts of the imaging technique. Scanning electron microscopy employs a beam of electrons to scan across the surface of a sample, which cause the sample’s surface to emit electrons. These are collected and used to create an image of the material’s surface. To capture a sharp image, it is best to make the sample’s surface electrically conducting. This is achieved by evaporating a thin film of metal onto the surface, though it is not absolutely necessary. McKay and his colleagues prepared small chips from their sample of ALH84001 with a thin, 2 nanometer gold-palladium film. To guard against artifacts, they prepared reference samples in the same manner and imaged fresh, noncoated surface samples of ALH84001. The only artifact noted in their report was a slight crazing texture t
hat showed up at very high magnifications on heavily coated samples. In their words, the textures shown in the SEM photographs “are not artifacts of the coating process but are the real texture of the sample.”

While the team did take care, there still is some concern that they may have not ruled out all possible artifacts in the imaging process. But, more to the point, the simple fact that something might look like a bacterium does not, per se, make it a bacterium. The elongate structures imaged by the NASA researchers may simply be odd bits of microscopic geology, not biology. The alleged microfossils are also exceptionally small—0.4 microns—about ten times smaller than the smallest universally accepted microfossil found on Earth. (Controversial maybe-fossils of “nanobacteria” have been found on Earth that are about the same size as McKay’s critters.) Unfortunately, they are so tiny that there is no way, at present, to chemically analyze them, so there is no way to determine just what they might be made of. As it stands, while the photographic evidence might be the most intriguing presented, it may well also be the weakest evidence presented. At a minimum, the NASA researchers will have to try to image one of the structures in cross-section to determine what, if anything, is inside these minute forms. Imaging a cell wall would certainly bolster their microfossil interpretation, as would photographs of colonies of cells, or a clear image of one of these puzzling objects dividing or budding.

In opening the August 7 press conference, Dan Goldin stated that, “We must investigate, evaluate, validate this discovery.” To this end, he announced that NASA would make available samples of the stone to “meritorious proposals.” Undoubtedly, there will be quite a clamor for those samples by investigators eager to lend additional evidence to the microfossil theory or to pummel it. McKay, Gibson, and colleagues are already at work trying to bolster their arguments and evidence by searching for amino acids within the meteorite and by continued imaging in hopes of photographing a cell wall or colony of cells. The investigation of ALH84001 for evidence of past life has really just started.

Already, however, by November 1996 additional evidence has been turned up by British investigators who have found strong evidence of biogenic gases with another SNC meteorite—EETA79001—this one less than two hundred million years old. The British team, includ
ing Ian Wright and Colin Pillinger of the Open University in Milton Keynes, and Monica Grady of the Open University and Natural History Museum in London, found organic compounds within their rock that are usually produced from microbe-generated methane. Moreover, they were able to measure the ratio of carbon isotopes found within these compounds, and found that they matched those which are typical of life and anomalous for nonlife. This is strong evidence, and if it holds up, the odds are high for life on Mars not just in the distant past, but in the present.

A CHANGE IN THE AGENDA

While not constituting final proof of Martian life, the discoveries in ALH84001 and EETA79001 are powerful, so powerful in fact that they will necessarily change the course of NASA’s and the world’s Mars investigation programs. While the question of life on Mars has (rightly) always been the central scientific question regarding the Red Planet as far as the public was concerned, among the post-
Viking
professional priesthood of planetary scientists, exobiology has been frowned upon. Indeed, at gatherings of NASA’s Mars Science Working Group (MarsSWG) held over the past decade, the few exobiologists present were generally regarded as grad students who never grew up. Instead, according to the dominant factions in the MarsSWG, the real, serious questions of scientific investigation concerning Mars had to do with seismology, meteorology, and geochemistry.

The Mars rock discoveries have changed all that. Now exobiology and paleontology have moved to the fore. And while seismology and meteorology can be done well, and geochemistry done badly, with robotic probes, exobiology and paleontology cannot be done competently without human explorers on the surface. Fossil hunts require the ability to hike miles across unimproved terrain, to climb up boulder fields, to do both heavy work and delicate work, and to exercise subtle forms of perception and intuition, all of which are far beyond the capabilities of robotic rovers. To find the fossil beds that will reveal the ancient Martian biosphere in its true glory will take human explorers, real live rock hounds, on the scene. To drill dee
p into the ground to bring up subsurface water in which Martian life may yet exist will take human prospectors and drill-rig teams working out of a permanent Mars base.

President Clinton has called for putting “the full intellectual resources and technological prowess of the United States behind the search for life on Mars.” If this commitment is to be honored, then the United States will have to send human explorers to Mars. Administrator Goldin is leery, at least publicly, of using the NASA team’s findings as a reason to launch an immediate, aggressive campaign of Mars missions, especially human missions. Having weathered several years of fairly vicious budget cuts and sensing a distinct hostility toward new, possibly big programs on the Hill, Goldin has been cautious. “Our missions should be driven by scientific potential,” he notes, “and the potential for economic opportunity.” If we go to Mars, via robotic missions or human missions, we should know “why we’re there.” Goldin wants mission planning to be “science driven,” not the result of an “emotional” response.