This is a drawing of the structure of the molecule naphthalene, a PAH.
Click on image for full size
JPL/NASA
PAH
PAH is the short name for "Polycyclic aromatic hydrocarbons". Thank goodness this molecule has a short name! These are long strings of carbon atoms which form a ring. (That accounts for the "cyclic" and "hydrocarbon" part of its name). An aromatic molecule is one which forms a benzene ring. Polycyclic means the molecule forms many rings. The only PAH which non-scientists may be familiar with is napthalene, the chemical in mothballs.
PAH's form easily.
PAH is very abundant in the solar system as well as in space. The red color in many interstellar clouds, such as this one comes from radiation in red wavelengths of PAH chemicals in the clouds. PAH is found in comets and in meteorites, even ones which come from Mars. This was especially important when scientists thought there might be fossils of ancient life in a Martian rock. Trees and other sophisticated plant life emit PAH as part of their photosynthesis/respiratory cycle. But the chief source of terrestrial PAH today is automobile emissions. PAH is also relatively abundant in burnt meat and are suspected to be hightly carcinogenic.
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