PAH - organic elements in Space

PAH - Polycyclic Aromatic Hydrocarbon
Image from a Spitzer feature article

Another Space Theology bookmark - Understanding Polycyclic Aromatic Hydrocarbons

Robert Hurt from Spitzer Science Center wrote in 2005 clearly and interestingly about Spitzer's detection of PAHs in galactic cirrus clouds and elsewhere explaining their great significance to our understanding of the origins of life in the Universe.

...The "hydrocarbon" refers to its composition of carbon and hydrogen atoms. "Polycyclic" indicates that these molecules consist of multiple loops of carbon atoms, while "aromatic" refers to the kinds of strong chemical bonds that exist between the carbon atoms.

... Chemists classify PAHs as "organic" compounds because of their carbon atoms. By definition, any molecule containing carbon is "organic," regardless of whether or not it originated from a biological process. To an astronomer, PAHs are powerful tools for exploring the building blocks necessary for the origins of life in the stars and galaxies beyond the Earth.
R. Hurt Spitzer feature article

Astronomers deal with rather complex matters so they simplify the language where possible by the nature of the subject studied. For example, any element heavier than helium is for them some metal.

Such nomenclature would not make sense when discussing planetary compositions because rocky planets have many different elements from the periodic table. The processes in planet formation have banged together all kinds of materials from cosmic dust present in the deep clouds of star formation. But metals are less dominant in discussions about star formations, spectral analyses of light from distant galaxies and so on where usually plain hydrogen gas rules.

Similarly, anything containing carbon is by definition organic. Such a definition in planetary astronomy would be rather confusing. Curiosity rover is desperately seeking for signs of biological life on planet Mars. Presumably there is no difficulty in finding carbon there, or is there? Yet, calling carbon deposits organic on Mars would be confusing at least to the general public.


Detection technique used with the instruments on Spitzer

Like all molecules, PAHs have a characteristic manner of interacting with light that allows them to be identified remotely. When molecules are energized, or elevated to excited energy states, they settle back down by emitting light at very specific colors of light. The exact set of colors is different for every compound, providing a unique "fingerprint" that can be identified even from great distances.

When PAH molecules are energized by absorbing photons of light, they then re-emit them at lower energies. . This process is known as "fluorescence" and is the same process by which a fluorescent dye will glow under a black light. In that example, the dye absorbs the ultraviolet photons that we cannot see and re-emits them in any of a variety of visible colors that we can see.
R. Hurt Spitzer feature article

Important results from the research on PAHs

Astronomers have learned that in interstellar space, the processes that give rise to dark clouds of dust, composed largely of silicon (like earthly sand), also produce organic PAHs.

Stars are born deep within dark, dense cores of gas and dust. And while the stars are composed primarily of hydrogen and helium gas, any Earth-like planets in surrounding solar systems will be built from the solid dust and organic compounds in these clouds. PAHs provide a useful probe into such regions, and provide clues to the abundance of potentially life-bearing chemicals.
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In the distant universe, where galaxies are seen as no more than specks, spectra help astronomers trace out the earliest origins of organic chemistry. Since all of the elements heavier than hydrogen and helium, like carbon, were forged in the hearts of supernova explosions, early observations of PAH features help astronomers understand more about the origins and fates of the earliest generation of stars.
R. Hurt Spitzer feature article

This is so interesting!

I warmly recommend reading the entire article by Robert Hurt.

Better instruments for observation, better evidence for scientific analysis and so gradually approaching the greatest questions humanity is asking about the Universe - what is life!