PAHs

Astrobiological Motivations


Potential DIB Carriers

Not one atom or molecule has been unambiguously paired with any of the hundreds of diffuse interstellar bands. Despite this fact, there are clues from laboratory studies, observations, and models that lend evidence that at least some DIBs may be the absorption profiles of organic molecules in the gas phase of the ISM (Herbig 1995; Snow 2001a). Snow (2001a) and Salama et al. (1996) argue that many of the DIBs are possibly due to polycyclic aromatic hydrocarbons (PAHs). PAHs are organics comprised of benzene rings (C6 - see image/menu on main research page). Along with PAHs, other possible candidates include simple carbon chains and fullerenes (Ehrenfreund & Charnley 2000), polycyclic aromatic nitrogen heterocycles (PANHs - Hudgins et al. 2005), and possibly carbonaceous material such as quenched carbonaceous composites (QCC-Wada et al. 2009). It must be stated that the DIB-PAH hypothesis is highly debated in the literature. Some of the current interest in DIBs being the electronic absorption profiles of neutral or ionized PAHs comes from eliminating or lessening other possibilities including the following list from Snow (2001a) and references within:

Many of the PAHs analyzed in the laboratory have absorption features in the optical, near the wavelengths where DIB absorption occurs (e.g. Salama et al. 1996). However, identification of specific DIB carriers is problematic due to the difficulties in recreating diffuse ISM conditions in the laboratory. For example, some of the current research in the laboratory consists of isolating relatively small PAHs within a solid matrix, which has the consequences of changing the line profile shapes and central wavelengths of the observed spectra (see Snow 2001a, Salama et al. 1996). Ionized PAHs are consistent with all known astronomical constraints, but the larger cations, which are probably more abundant in the diffuse ISM, are currently infeasible to analyze in the lab (Snow 2001a). Figure 6 below, from Salama et al. (1996), shows some typical PAHs that are abundant on Earth. The benzene rings that comprise the PAHs can be more centrally concentrated (pericondensed), or can be more chain-like (catacondensed). The pericondensed structures are more centrally condensed allowing them to withstand higher ultraviolet fluxes. The catacondensed species of PAHs are more linear in structure and are more easily photo-dissociated.

Figure 6

Figure 6: Some typical structures of PAHs. Figure from Salama et al. (1996).


Astrobiological Significance

The DIBs and the infrared emission bands of PAHs are both observed in the solar system and nearly all ISM conditions including planetary atmospheres, atmospheres of cool giant stars, the diffuse interstellar medium, photo-dissociation regions, emission nebulae, and planetary nebulae (Messenger et al. 1998; Botta 2005; Sagan et al. 1993; Ehrenfreund & Charnley 2000; Bada & Lazcano 2002; Herbig 1995; Tielens 2005). Among organic molecules in the gas phase of the ISM, PAHs are considered the most abundant (Ehrenfreund & Charnley 2000; Puget & Leger 1989; Tielens 2005). The likely abundance of PAHs in the ISM of the Galaxy disk may not be surprising given the relatively high metallicity of the Galaxy and the strength of the carbon bonds; PAHs require significant energies to be dissociated (see Snow 2001a; Salama et al. 1996).

Besides their abundance in the ISM, PAHs are considered an important and abundant early constituent of organics on the early Earth, much of which may have arrived on Earth after formation (Bada & Lazcano 2002). Shown in Figure 7 is an illustration of how organic molecules could have come to Earth, courtesy of Andy Christie of Slimfilms.com (for July '99 Scientific American). As the pre-solar nebula condenses to form the solar system, the PAHs accrete directly onto the forming planets as well as ariving through asteroid and comet impacts much in the same way as the volatiles are thought to have been seeded on our planet.

Figure 7

Figure 7: Illustration of organics seeding the early Earth. Image courtesy of Andy Christie of Slimfilms.com (for July '99 Scientific American).

Shown in Figure 8, from Bada & Lazcano (2002) and courtesy of the Leiden astrobiology website, is a cartoon portraying likely pathways to the formation of life on the early Earth. For the discussion and astrobiological motivation of my work, note the potential role "organics from space" may play in the chain of events that leads to the "protein/DNA world". There was likely a presolar abundance of PAHs given the wealth of PAHs found in both carbonaceous and ordinary chondrite meteorites (Botta 2005; Ehrenfreund & Charnley 2000; Messenger et al. 1998). Those organics may have contributed to the prebiotic soup and prebiotic polymers that were the precursors of biological molecules.

Figure 8

Figure 8: DIBs may be the absorption profiles of organics in the ISM, such as PAHs, that may have played a crucial role in the later formation of complex biological molecules on the early Earth. Figure is adapted from Bada & Lazcano (2002), and courtesy of the Leiden astrobiology website.

The connection and pathway from simple organics to more complex biological molecules, such as the nucleic acids that make up RNA and DNA, remains uncertain. Illustrated in Figure 8 is the possible importance of abiotic synthetic reactions in creating the organic molecules within the prebiotic soup. It is unclear which process, organics from space or organics created on early Earth, has the greatest role in creating the constituents contained in the prebiotic soup. PAHs are a potential candidate for playing a role in the formation of complex biological molecules. If DIBs are the absorption profiles of organics, they may shed light on the abundances of these organics in a variety of astrophysical environments. Those environments that are more conducive for organic creation/survival will be of greater astrobiological interest. Through studying DIBs, we may begin to understand those places and times in the Universe where life is more probable.


Sources of Further Information

The following websites provide more detail on the current state of astrochemistry and astrobiology where it relates to DIBs, PAHs, and the Universe.


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