| Even before the Viking missions reached Mars in 1976, astronomers had abandoned hope of finding life on the planet. Scientists knew there were no large-scale canal systems, no surface water, almost no oxygen in the atmosphere, and no seasonal vegetation changes. The present lack of liquid water on Mars especially dims the chances for life there now. However, running water and possibly a dense atmosphere in the past may have created conditions suitable for the emergence of life long ago. In the hope that some form of microbial life might have survived to the present day, the Viking landers carried out experiments designed to detect biological activity. The accompanying pair of photographs show the robot arm of one of the landers before and after digging a shallow trench to scoop up soil samples.
All three Viking biological experiments assumed some basic similarity between hypothetical Martian bacteria and those found on Earth. A gas-exchange experiment offered a nutrient broth to any residents of a sample of Martian soil and looked for gases that would signal metabolic activity. A labeled-release experiment added compounds containing radioactive carbon to the soil, then waited for results signaling that Martian organisms had either eaten or inhaled this carbon. Finally, a pyrolitic-release experiment added radioactively tagged carbon dioxide to a sample of Martian soil and atmosphere, waited a while, then removed the gas and tested the soil (by heating it) for signs that something had absorbed the tagged gas.
Initially, all three experiments appeared to be giving positive signals! However, subsequent careful studies showed that all the results could be explained by inorganic (that is, nonliving) chemical reactions. Thus, we have no clear evidence for even microbial life on the Martian surface. The Viking robots detected peculiar reactions that mimic in some ways the basic chemistry of living organisms, but they did not detect life itself.
A criticism of the Viking experiments is that they searched only for life now living. Today, Mars seems locked in an ice age—the kind of numbing cold that would prohibit sustained life as we know it. If bacterial life did arise on an Earthlike early Mars, however, then we might be able to find its fossilized remains preserved on or near the Martian surface. Surprisingly, one place to look for life on Mars is right here on Earth. Scientists think that some meteorites found on Earth's surface come from the Moon and from Mars. These meteorites were apparently blasted off these bodies long ago during an impact of some sort, thrown into space, and eventually trapped by Earth's gravity, ultimately to fall to the ground. The most fascinating of these rocks are surely those from the Red Planet—for one of them may harbor fossil evidence for past life on Mars!
The figure at right top shows ALH84001, a blackened, 2-kg meteorite about 17 cm across, found in 1984 in Antarctica. Based on estimates of the cosmic-ray exposure it received before reaching Earth, the rock is thought to have been blasted off Mars about 16 million years ago. Looking at this specimen through a microscope, scientists can see rounded orange-brown "globules" of carbonate minerals on the rock's shiny crust. Because carbonates form only in the presence of water, the presence of these globules suggests that carbon dioxide gas and liquid water existed near ground level at some point in Mars's history, a conclusion that planetary scientists had earlier drawn from studies of Viking's orbital images of valleys apparently carved by water when the Martian climate was wetter and warmer.
In a widely viewed press conference in Washington, D.C., in the summer of 1996, a group of scientists argued, based on all the data accumulated from studies of ALH84001, that they had discovered |
fossilized evidence for life on Mars. The key pieces of evidence they presented for primitive Martian life were as follows: (1) Bacteria on Earth can produce structures similar to the globules shown in the inset. (2) The meteorite contains traces of polycyclic aromatic hydrocarbons—a tongue-twisting name for a class of complex organic molecules (usually abbreviated PAHs) that, although not directly involved in known biological cycles on Earth, occur among the decay products of plants and other organisms. (3) High-powered electron microscopes show that ALH84001 contains tiny, teardrop-shaped crystals of magnetite and iron sulfide embedded in places where the carbonate has dissolved. On Earth, bacteria are known to manufacture similar chemical crystals. (4) On very small scales, elongated and egg-shaped structures are seen within the carbonate globules. The researchers interpret these minute structures as fossils of primitive organisms.
The photomicrograph at right bottom shows this fourth, and most controversial, piece of evidence—curved, rodlike structures that resemble bacteria on Earth. Scale is crucial here, however. The structures are only about 0.5 µm across, 30 times smaller than ancient bacterial cells found fossilized on Earth. Furthermore, several key tests have not yet been done, such as cutting through the suspected fossilized tubes to search for evidence of cell walls or semipermeable membranes, or of any internal cavities where body fluids would have resided. Nor has anyone yet found in ALH84001 any amino acids, the basic building blocks of life as we know it (see Chapter 28).
These results are very controversial. Many experts do not agree that life has been found on Mars—not even fossilized life. The skeptics maintain that all the evidence could be the result of chemical reactions not requiring any kind of biology. Carbonate compounds are common in all areas of chemistry; PAHs are found in many lifeless places (glacial ice, asteroid-belt meteorites, interstellar clouds, and even the exhaust fumes of automobiles); bacteria are not needed to produce crystals; and it remains unclear whether the tiny tubular structures shown are animal, vegetable, or merely mineral. In addition, there is the huge problem of contamination—after all, ALH84001 was found on Earth and apparently sat in the Antarctic ice fields for 13,000 years before being picked up by meteorite hunters.
As things now stand, it's a matter of interpretation—at the frontiers of science, issues are not always as clear-cut as we would hope. Only additional analysis and new data will tell for sure if primitive Martian life existed long ago. Should the claim of life on Mars hold up against much skepticism in the scientific community, these findings will go down in history as one of the greatest scientific discoveries of all time. A fantastic result: we are—or at least were—not alone in the universe! Maybe.
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INTERLUDE 10-2 Life on Mars?
