Planetary Science Research Discoveries

October, 1996

by G. Jeffrey Taylor

Polycyclic Aromatic Hydrocarbons

Benzene Organisms are made of complicated hydrocarbons (compounds made mostly of hydrogen and carbon), so their presence should be marked by high concentrations of hydrocarbons produced when the organisms decayed. One group of hydrocarbons produced by decomposition of ancient organisms on Earth are called polycyclic aromatic hydrocarbons. These are certainly aromatic: they stink! The simplest one is benzene, depicted here. The corners of the hexagonal structure are occupied by carbon atoms, and a hydrogen atom is bonded to each carbon. The structure of benzene is usually drawn as a hexagon with a circle in the center. The circle represents six electrons in the molecule that are not associated with specific carbon atoms, but are spread out above and below the plane containing the carbon atoms. (Graphic by Brooks Bays, PSR Discoveries graphic artist.)


PAH More complicated aromatic hydrocarbons consist of benzene molecules linked together, such as phenanthrene, shown here. When two or more benzenes are joined the compounds are called "polycyclic aromatic hydrocarbons," or PAHs for short. A number of PAHs were detected in ALH 84001. Researchers at Stanford University, working with colleagues at the Johnson Space Center, have shown that the PAHs in ALH 84001 are not contaminants from the laboratory or Antarctica. PAHs are produced by decay of organic materials; for example, PAHs are abundant in coal deposits. Their presence in ALH 84001 suggest to the Stanford-NASA team that organisms were present. The researchers acknowledge that PAHs are also present in carbon-rich meteorites and in interplanetary and interstellar dust, in which PAHs formed by nonbiological chemical processes, but show that the PAHs in ALH 84001 are different from those in other meteorites, except for a type called "CM carbonaceous chondrites." CM chondrites contain clay-like minerals, organic compounds, magnetite, and iron sulfides. Astronomical observations of asteroids suggest that many asteroids may be like CM carbonaceous chondrites. (Graphic by Brooks Bays, PSR Discoveries graphic artist.)



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Planetary Science Research Discoveries
October, 1996

by G. Jeffrey Taylor

Magnetite and Iron Sufide Grains

Elongated FeS McKay and co-workers have identified very small grains of magnetite (iron oxide) and two types of iron sulfide. These have similar sizes and shapes as magnetite and iron sulfide grains formed by bacteria on Earth. This photo shows an iron sulfide grain from the Martian meteorite (left) and a similar grain in a terrestrial bacteria living in the cell of a plant root. (Photo adapted from Science.)






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Magnetite-Sulfide Whether the shapes can be produced by non-biological processes or not, McKay and colleagues argue that the types of minerals present and evidence for some of the carbonate dissolving suggests that biological activity was involved. This photograph shows the distribution of small magnetite (left) and sulfide grains in a carbonate matrix. (Photo adapted from Science.)


Carbonate Dissolution This photograph shows a light band cutting across a carbonate grain. McKay and co-workers suggest that this band was formed by partial dissolution of carbonate. It is in these areas that the magnetite and iron sulfides shown above are found. According to the research team, dissolution of the carbonate required that the water be acidic, but formation of magnetite and iron sulfide from water would have required alkaline (far from the acidity needed to dissolve carbonate), unless bacteria or other microorganisms were involved. The lack of a simple non-biological way to produce the minerals existing together leads them to conclude that magnetite and sulfide formed as the result of biological processes. (Photo adapted from Science.)


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