Scientists are generally in accord on two overall points. First, they agree that ALH84001 was hurled from Mars by an asteroid impact about sixteen million years ago. Second, they agree that Mars was warmer and wetter three to four billion years ago, and that some forms of bacterial life could have evolved there. Here, however, is where agreement stops and debate begins.
Although excited by the prospects of discovering fossilized prebiotic life in a martian meteorite, the scientific community has not exactly embraced the ALH84001 findings as interpreted by the NASA team. After all, the “fossils” don’t definitively resemble terrestrial bacterial forms, especially in size, and even the NASA scientists admit the structures might just as well have been formed by nonbiologic processes.
The evidence stands for all to see in the August 16, 1996, issue of Science. In the paper, cumbersomely titled, “Search for Past Life on Mars: Possible Relic Biogenic Activity in martian Meteorite ALH84001,” the nine-member science team claim their examination turned up five compelling pieces of evidence that, taken together, point toward signs of primitive life on Mars.
1) An igneous rock – one formed from a molten state – originating from the planet Mars was penetrated by water along fractures and pore spaces. Mars
2) Scattered throughout these fissures in the rock are oval-shaped structures called “carbonate globules.” The globules are dated at 3.6 billion years, a time when water is thought to have been still flowing on the surface of Mars.
3) Close-up images of the carbonate globules taken with a scanning electron microscope reveal tiny rod-like features, no longer than 200 nanometers, that resemble microfossils seen on Earth.
4) Analysis of magnetite and iron sulfide particles embedded in the rims of the carbonate globules show they could have been formed by oxidation from microbes. Moreover, many of these particles have similar shapes and compositions as those produced by microorganisms on Earth.
5) Most intriguing of all, in the vicinity of the carbonate globules, the scientists found organic molecules called polycyclic aromatic hydrocarbons, or PAHs. On Earth, PAHs are produced when microorganisms decay. By themselves, the presence of PAHs, which are abundant in ancient sedimentary rocks, coal, and petroleum, and have been observed in meteorites and interstellar dust, would not normally be considered a definitive biologic marker. In ALH84001, however, it was where the PAHs were located that was distinctive. The highest concentration of PAHs were found in regions rich in carbonates and well away from the meteorite’s fusion crust. In fact, their concentration increased with increasing depth inside the rock. Had these PAHs been the result of Earth contamination, the NASA team asserts, just the opposite would have been found.
“None of these observations is in itself conclusive for the existence of past life,” the scientists state in the conclusion to their August 16 paper in Science. “Although there are alternative explanations for each of these phenomena taken individually, when they are considered collectively, particularly in view of their spatial association, we conclude that they are evidence for primitive life on early Mars.”
Other scientists, however, are not so sure. In the August 17 issue of New Scientist, Robert Clayton, a geochemist from the University of Chicago, takes the NASA team to task. “PAHs are very widespread compounds in asteroids and not diagnostic of life,” he said, adding that the distribution of PAHs in ALH84001 is a thousand times less diverse than that found in fossils and biologic material on Earth.
What about the increased PAH concentration inside the meteorite? The answer depends largely on how polluted a place Antarctica is. In the same New Scientist article, Robert Gregory, a geologist at Southern Methodist University in Dallas said, “Antarctica isn’t as sterile a place as people think it is.” Being dark in color, he explained, the meteorite would have absorbed more solar heat. Melted snow containing contaminants could have then leeched deep within the rock’s many fissures. Ultraviolet radiation, meanwhile, would have broken down any PAHs on the surface of the meteorite.
Other geologists, however, argue that Antarctica is still an unpolluted, unsullied region. Hence, though it may be true that PAHs break down under ultraviolet radiation, terrestrial contamination cannot be the source for the carbonate globs.
Scientists in other quarters also have offered alternative explanations for the formation of the meteorite’s carbonate globules. They suggest that the impact of the asteroid on Mars created a hot fluid rich in carbon dioxide that deposited carbonate on the rock as it was ejected into space. Christopher Romanek, the geochemist at the University of Georgia who initially determined the isotope ratios in ALH84001, has challenged this hypothesis. Romanek said his research shows the isotopic signatures support the idea that the globules were deposited by life forms in a cooler environment. “Comparing the isotopic composition of the carbonate ‘globs’ to the surrounding matrix material clearly points to a low-temperature origin,” he said.
Still another objection is raised concerning the age of the meteorite’s carbonates. The NASA team calculates the meteorite was formed on Mars 4.6 billion years ago and that microorganisms permeated the rock between 3.6 billion and 4 billion years ago. Scientists from the University of Chicago and the Scripps Institute of Oceanography, however, argue that the meteorite’s age may be far younger. Based on its abundance of the soft metallic elements strontium and rubidium, they derive an age of only 1.4 billion years. If this is true, the fossils may have arisen from inorganic rather than biologic processes, especially when one considers that water has not been abundant on Mars for at least the past three billion years.
The rod-shaped structures found in ALH84001, too, have their limitations. The martian rods measure between 20 and 100 nanometers long. Earthly bacteria are typically 25 to 200 times larger, between 0.5 and 20 microns. The martian rods, therefore, are very near the minimum size limit of genetic material in living systems. Claims of evidence of terrestrial bacteria in the nanometer size range are controversial at best, though, again, it depends on which scientist you talk to. Obviously, until electron microscopy reveals a cell compartment or some other internal structure that can be definitely linked to an organic process, the origins of these curious features will likely remain uncertain.
Paleobiologist Stephen Jay Gould summed up the problems with the evidence in a New York Times editorial, when he wrote, “The evidence [found in ALH84001] is chemical and inferential, not ‘solid’ like an unambiguous bone or shell. The authors found organic chemicals and mineral precipitates associated with globules that may have been formed, in part, by organic activity. The globules themselves are not fossils.”