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The Universe at Your Fingertips 2.0 DVD from the Astronomical Society of the Pacific is in our online store, filled with Earth and space science resources.
This is a drawing of the structure of the molecule naphthalene, a PAH.
Click on image for full size


PAH is the short name for a "polycyclic aromatic hydrocarbon". A PAH is a stable structure made up of multiple rings that are fused together. The rings are made of carbon and hydrogen (though other elements sometimes sneak into the structure!).

These rings are tough to break apart. They are held together by a type of hybrid bond (hybrid between a single and double bond) that is actually stronger than the single or double bond with which you might be familiar.

Many people think that all aromatic molecules contain one or more benzene rings. This is a historical misconception. Benzene rings are found in many, but not all aromatic molecules. Many of the plants that were first found to contain benzene rings, such as vanilla or sassafras, were smelly...aromatic! So, it is a common misconception that all aromatic molecules contain benzene rings...when in fact, some do not (they contain other forms of carbon and hydrogen).

PAHs form easily. PAHs are 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. PAHs are 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.

One of the chief sources of terrestrial PAHs today is automobile emissions. PAH is also relatively abundant in burnt meat. When the body tries to break down the PAHs in burnt (or barbecued meat), the intermediates can be carcinogenic.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA