A meteorite that made its way from Mars to Antarctica is stirring controversy among scientists. No one can say for sure yet, but the meteorite may contain evidence of past life on Mars.
by Chris Romanek, Savannah River Ecology Lab, University of Georgia, Aiken, S.C.
What is it that attracts us to Mars? Is it because Mars is one of our two nearest neighbors in the solar system, or because Mars is more like Earth than any other planet? Maybe it is the spectacular land forms that are so characteristic of Mars. Valles Marineris, a giant canyon as long as the continental United States, sprawls across the surface of the planet. Olympus Mons, one of the colossal shield volcanoes of Mars' Tharsis Region, lays claim to being the largest volcano in the entire solar system.
My link to Mars is an unassuming piece of rock about the size of a potato. This rock is a Mars meteorite that was found in Antarctica in 1984, and it is called by the name ALH84001.
It was quite by accident that I first became involved in studying ALH84001. In 1993, while working at NASA Johnson Space Center on unrelated research, a fellow colleague Duck Mittlefehldt (yes, Duck is his name), called me across the hall late one afternoon to show me his latest discovery. Duck was the proud father of a new Martian meteorite, ALH84001. He realized this particular meteorite had been misclassified when it was originally identified, and that it was actually the latest member of the exclusive clan of meteorites that hail from Mars. During that eventful first encounter, Duck pointed to small regions in the meteorite that contained very unusual grains of carbonate material. Being a low temperature geochemist, I asked if I could study the carbonates in more detail, thinking they may hold clues as to the processes responsible for their occurrence. Further study clearly showed that the carbonates had originated on Mars instead of on the Antarctic ice sheet, and that the carbonates probably formed at a temperature somewhere between 0°–100°C.
Around the time Duck showed me the meteorite, I attended a seminar given by sedimentologist Robert Folk. In his talk, Folk described a sensitive acid etching procedure he used to dissolve away thin layers of surface material from carbonate samples collected at hot springs in Italy. Entombed within the hot spring deposits were small ovoid structures that Folk determined to be the fossilized remains of ultra-small microorganisms or "nannobacteria". After returning to NASA I performed the same acid etching experiment on the carbonates in ALH84001, and similar structures became apparent. Armed with the very first evidence of potential "Martian microfossils," I brought these pictures to the attention of David McKay and Everett Gibson, and we formed the nucleus of the research team that studied these carbonates.
Our story about the controversial meteorite ALH84001 infuriates some people and thrills others but surely fascinates everyone. In the detective story below, I describe how our team of researchers came to the conclusion that features within ALH84001 can be interpreted as the product of biologic activity. It is important to stress that we have not found proof of life on another planet; rather, we observed features that are nearly identical to those we might expect had life once existed on Mars. This may be a fine distinction, but it is an important one. Whether life ever existed on Mars is beyond the scope of our research. A definitive answer to this question will have to wait for robotic or manned missions to Mars that can retrieve actual samples from the surface of the planet. Until then, scientists must rely on the clues that ALH84001 and the other Martian meteorites have to offer regarding the potential for extraterrestrial life.
The observations we consider to be evidence for extraterrestrial life come from the small pancake-shaped features Duck Mittlefehldt pointed out to me back in 1993. These features, or "carbonate globules" as we call them, are no wider than a human hair and they commonly occur along cracks and crevasses in the meteorite. The globules likely precipitated from fluids that circulated through fissures in Martian rock almost 3.5 billion years ago, when liquid water may have existed on the surface of Mars. Our observations are described below so you can decide for yourself whether the Martian meteorite ALH84001 holds potential evidence of primitive life.
Carbonate globules are visible on fracture surfaces when chips of the meteorite are viewed with an optical microscope (see figure). They are composed primarily of the mineral magnesite and are similar in structure to limestones and marbles found on Earth. Most Earth carbonates are composed of calcite—which contains mostly calcium—or dolomite, which consists of nearly equal amounts of calcium and magnesium.
(1) Several prominent carbonate globules in the Mars meteorite are shown. Their structure and chemistry suggest that they may have been formed with the assistance of primitive, bacterialike organisms. The globules are about 200 microns (0.2 mm) in diameter and contain cores of magnesium, iron, calcium, and manganese carbonate grading out to pure magnesium-carbonate near the margins. The rims are approximately 5–10 microns thick and are composed of iron-oxide and iron-sulfide bound in a carbonate matrix. Mantling the rims is a second layer of white carbonate that is nearly pure magnesium-magnesite.
In stark contrast, the Martian globules are composed primarily of magnesium with lesser amounts of iron, calcium, and manganese. They are very calcium-, iron-, and manganese-rich in their centers, their edges are almost pure magnesium-magnesite. The shapes of the globules and general trends in carbonate chemistry are similar to those found in carbonate concretions that form on Earth. Terrestrial carbonate concretions are generally subspherical in shape and range in size from microscopic features to boulders. They commonly have calcium-rich interiors and magnesium-rich outer regions, and they form in muddy sediments with the help of bacteria. The similarity between terrestrial concretions and the carbonate globules does not constitute conclusive evidence for a biologic origin because we have yet to find concretions in fractured rock on Earth that are similar to those found in the fractures of ALH84001.
The very edges of the Martian globules are surrounded by black rims that are nearly 50 times thinner than a human hair. Our research team used sophisticated scanning and transmission electron microscopes to view the rims at very high resolution. These microscopes use electrons, rather than light, to characterize the texture, fabric, and chemistry of extremely small objects. We studied the black rims in great detail to better understand the processes that produced this fine-grained material. The predominant minerals in the rims are magnetite (an iron oxide), and pyrrhotite (an iron sulfide). The minerals are so small that one billion could fit on the point of a pin. In ALH84001, these minerals have distinct shapes, chemistries, and structures that resemble biominerals that form in soils and sediments on Earth.
Biominerals can be formed by both plants and animals, and they comprise the teeth, bones, shells, and other skeletal materials that are produced by living organisms. Biominerals serve three purposes: they trap or remove the potentially toxic form of elements that poison bacteria, they take part in biochemical reactions that bacteria use to store energy for growth, and they can even be used as tiny compasses for bacteria to find their way as they move. When bacteria die, they leave behind these resistant little minerals as fossils, much like the dinosaur bones that paleontologists study.
The tiny magnetite and sulfide grains in ALH84001 can be viewed as evidence of fossilized bacterial remnants from Mars. Alternatively, they could have been produced by inorganic reactions. Grains of inorganic origin occur on Earth; we know they are inorganic because they formed at temperatures considered too high to sustain life.
We also know that inorganic magnetites and pyrrhotites occur in meteorites, including the carbonaceous chondrite Orgueil, which never experienced high temperatures. While low and high temperature forms of inorganic iron oxide and sulfide are known to exist, they are significantly different in texture, structure, and chemistry from those found in ALH84001, still leaving us with the possibility of an organic origin for these features.
A major breakthrough in the search for a biologic "fingerprint" occurred when members of our research team discovered the first hydrocarbon compounds in ALH84001. Life, as we know it, is largely built from carbon and the other biochemical elements such as hydrogen, oxygen, sulfur, nitrogen, and phosphorus. These building blocks bond together to form amino acids, proteins, carbohydrates, and a myriad of other organic compounds including DNA. Together, they constitute the organic structures we know as living organisms. A very sophisticated one-of-a-kind mass spectrometer was used to search for a specific type of carbon-bearing compound in the carbonate globules called polycyclic aromatic hydrocarbons (PAHs). These compounds can be thought of as fused carbon rings. Analyses from the spectrometer provided the first direct evidence for hydrocarbons from Mars, a feat that even the Viking Landers did not achieve.
Not all scientists agree that the PAHs on Mars suggest a history of organic processes or that the PAHs are even from Mars at all. Some think the PAHs are terrestrial contaminants or pollutants that were introduced to the meteorite on the Antarctic ice. PAH molecules are produced by humans in many ways. They are found in car exhaust, cigarette smoke, and even barbecue foods. PAHs can also form naturally through the burning of organics or the breakdown of dead organisms as they decay. Common examples of buried organics on Earth that have undergone this type of processing are oil and coal deposits. It is possible that the PAHs in ALH84001 were created by organic processes on Mars, but we do not know for sure. Inorganic PAHs have been detected not only on Earth, but also in space and on meteorites. Clearly, extraterrestrial processes are responsible for these inorganic PAHs. What makes the PAHs found within the carbonate globules of ALH84001 so compelling is that they are unlike any of the inorganic PAH samples from Earth or space in both molecular distribution and concentration. Though we cannot yet be sure, there is a possibility that these most unusual carbon compounds formed by organic means on Mars.
The final and most controversial piece of evidence found within the carbonate globules are tiny oval and tubular shapes that are associated with the thin black rims (see figure. These features are about one thousandth the width of a human hair (100 to 200 nanometers) and must be viewed with a powerful scanning electron microscope. Although these features are ten to one hundred times smaller than the smallest known terrestrial bacteria, microbiologists have successfully cultured ultra-small bacteria or "nannobacteria" in the lab. They found that when normal-sized bacteria are exposed to stressful conditions they expel most of their internal fluids and can shrink to one thousandth of their normal size. This is well within the range of our larger purported microfossils.
(2) High-resolution scanning electron image of ovoid and tubular structures associated with carbonate near black rims. Tubular features in the center and lower right central region of the image are approximately 200 nm (0.2 microns) in length. These features are similar in shape and size to nannobacteria entombed in terrestrial carbonate rocks.
The mineralized shapes found in ALH84001 do not constitute the smoking gun for fossilized life, since inorganic processes are also capable of producing structures of similar size and shape. When synthetic minerals are grown in the lab and viewed with the SEM, crystal faces often display very delicate growth structures on their surfaces. Some of these inorganic structures look similar to the features we observe in ALH84001 and the fossilized remnants of nannobacteria on Earth.
As you can see, all the evidence presented here can be explained by both organic and inorganic processes. This is where the problem lies and the controversy begins. Each line of evidence, in itself, is not a powerful statement regarding a biologic origin for the globules. When taken together, though, these closely linked observations make a strong argument for a biogenic origin of the features. In fact, our research team feels the evidence is strong enough to suggest that, for the first time, we may be seeing fossilized remains of an ancient Martian biota. But even we are not so sure. Are you?
I am currently an assistant ecologist at the University of Georgia's Savannah River Ecology Lab working on environmental problems at the Department of Energy's Savannah River site. I study the natural chemical reactions that occur on or near the Earth's surface to understand how low-temperature processes affect the fate and transport of pollutants in our environment. I spent the early years of my life in New Jersey before moving to Greenville, South Carolina, in 1972. My attraction to geology stems back to my grade school days. When asked, I remember telling my third-grade teacher, Mrs. Young, "I want to be a paleontologist when I grow up." Her response still rings clear after all these years: "You had better learn how to pronounce the word correctly first." Well, I wonder what Mrs. Young would think of me now.
After receiving a B.S. from Furman University in 1982 and a M.S. from the University of Florida in 1985, I attended Texas A&M University, where I completed my Ph.D in geology in 1991. My very early work on the carbonate globules in ALH84001 established the initial link between carbonate formation on Mars and the Martian atmosphere. In addition, my work suggested that the carbonates formed at a temperature low enough to sustain life. I am now working on more Earthly concerns, including the characterization and remediation of environmental contaminants at the Savannah River site.
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