Further Analysis of ALH84001
Polycyclic Aromatic Hydrocarbons
by G. Jeffrey Taylor
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.)
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.)
Tiny Grains of Magnetite and Iron
by G. Jeffrey Taylor
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.)
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.)
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.)