Polycyclic aromatic hydrocarbons (PAHs)—organic molecules consisting of multiple aromatic rings—are ubiquitous in the interstellar medium (ISM). Based on observations of mid-infrared emission bands in the ISM, PAHs are present in abundances ~10−7 times that of hydrogen (1). PAHs are estimated to contain ≲20% of the carbon atoms in the ISM of the Milky Way (1, 2) and other galaxies (3). PAHs have been proposed as building blocks of carbon-rich dust grains, which are abundant in the ISM (4), and of higher molecular weight–insoluble organic material (IOM) that comprises most of the carbon within meteorites (5). However, it is unknown which chemical processes produce these forms of reduced carbon or where they occur (Fig. 1) (6).

Fig. 1. Potential pathways for extraterrestrial PAH formation.
The central inset shows the molecular structures of the five PAHs we investigated. The surrounding panels schematically illustrate potential formation pathways for those PAHs. In (A), (C), and (D), grayscale color bars show δ13CVPDB, δDVSMOW, and Δ2×13C values measured or predicted in extraterrestrial materials. White is isotopically depleted and black is isotopically enriched. Dots and arrows indicate values and ranges, respectively, of the source carbon and hydrogen. Δ2×13C values are estimated predictions (21) based on model results (Fig. 2), see text. (A) PAH formation in hot (≳1000 K, red) circumstellar environments by molecular mass growth reactions (6, 11). δ13C_VPDB values in AGB stars are expected to range from 0 to hundreds of per-mille depending on the stellar evolution (35, 36). δD_VSMOW values are expected to be low or zero as a result of the fusion of D in stars (35, 36). texpulsion is the time scale for PAH expulsion from stellar envelopes. (B) Shock waves and ultraviolet radiation form PAHs by breaking down carbon-rich dust but also destroy them. tbreakdown is the time scale for PAH breakdown in the ISM. (C) PAH formation in cold (10 K, blue) interstellar environments through barrierless reactions (6, 12). Reduced carbon in molecular clouds is depleted in 13C compared with interstellar CO (47), whereas interstellar hydrogen is typically D-enriched (33). (D) PAH formation or modification on a parent body at moderate temperatures (100s of K, orange). Isotopic exchange can occur with carbon reservoirs such as CO and DIC, and hydrogen reservoirs such as H2. Murchison carbonate has δ13C_VPDB values of +20 to +80‰ (48) whereas the water in parent bodies of CC meteorites has been found to be D-depleted (40, 42).

Small aromatic organics, such as PAHs containing only a few rings, can form through reactions of free radicals in the gas phase, particularly the hydrogen-abstraction-carbon-addition (HACA) reaction mechanism, which is expected to occur in hot (>1000 K) circumstellar environments around carbon-rich asymptotic giant branch (AGB) stars and on Earth by combustion (Fig. 1A) (6). Carbon-rich dust grains and IOM could potentially be formed through similar processes. However, the reaction rates of these high-temperature mechanisms are too slow to account for the amount of PAHs present within the ISM and there is no complete model of the synthesis of PAHs within the outflows of AGB stars (6, 7).

PAHs could also be formed through the breakdown of carbon-rich dust grains by shock waves, cosmic rays, or ultraviolet photolysis (8). However, these same processes destroy PAHs (Fig. 1B). This destruction occurs on time scales (9, 10) that are shorter than those expected for production of PAHs in circumstellar envelopes of AGB stars (11, 12).
A third location where PAHs could form is in cold (~10 K) molecular clouds within the ISM, through either ion-molecule reactions (13), or rapid barrierless reactions involving radicals (Fig. 1C) (7). Laboratory experiments have characterized these chemical mechanisms but it is difficult to directly observe specific PAH molecules within interstellar molecular clouds using spectroscopic methods. The only species that have been identified within molecular clouds are nitriles derived from PAHs: benzo-nitrile (14) and cyanonaphthalenes (15). Therefore, it is unlikely that circumstellar synthesis dominates the formation of extraterrestrial PAHs, but there is little evidence for interstellar formation either.

Later secondary processing reactions within a parent body—the asteroid or other Solar System object that meteorites originate from—could also synthesize PAHs or alter their composition. These reactions are often related to aqueous alteration, the modification of solid material by reactions with liquid water, which is known to have occurred on parent bodies. Potential secondary reactions include Fischer-Trospch-type (FTT) synthesis of alkanes from carbon monoxide (CO) (16) followed by aromatization; exchange with dissolved inorganic carbon (DIC) (17) or aqueous H2; or the breakdown of IOM into smaller organic molecules by catagenesis (18), which thermally cracks large organic molecules (on Earth this forms oil and gas deposits)...