Aromatic moieties in meteorites: Relics of interstellar grain processes?

The carbonaceous chondrite meteorites contain a record of the formation of the solar system, part of which is present within organic matter. This organic matter is predominantly aromatic, and its sources remain controversial. The delta(13)C values for individual free and macromolecular aromatic moieties from Cold Bokkeveld and Murchison suggest that these units originate from radiation-induced "circle" reactions involving simultaneous bond synthesis and cracking. Large carbon isotope fractionations and a deuterium enrichment for these entities suggest that these reactions occurred in a dense interstellar cloud. The juxtaposition of the synthesis and cracking products implies that the reactions occurred in a restricting medium, the most likely candidate for which is the icy organic mantles of interstellar grains. In contrast, the delta(13)C record in aromatic moieties from Orgueil is mostly obscured, possibly due to the increased levels of parent body aqueous alteration experienced by this meteorite. These novel observations are consistent with the interstellar-parent body hypothesis, where the final form of meteoritic organic matter results from the transfiguration of interstellar organic precursors by aqueous reactions on the meteorite parent body.