Interstellar Ice Photochemistry and the Deuterium Enrichment and Chemical Properties of Meteoritic Polycyclic Aromatic Hydrocarbons S
Total Page：16
File Type：pdf, Size：1020Kb
63rd Annual Meteoritical Society Meeting 5003.pdf INTERSTELLAR ICE PHOTOCHEMISTRY AND THE DEUTERIUM ENRICHMENT AND CHEMICAL PROPERTIES OF METEORITIC POLYCYCLIC AROMATIC HYDROCARBONS S. A. Sandford1, M. P. Bernstein1,2, L. J. Allamandola1, J. S. Gillette3, and R. N. Zare3, 1NASA-Ames Research Center, MS 245-6, Moffett Field CA 94035 ([email protected]), 2SETI Institute, 2035 Landings Drive, Mountain View CA 94043, USA, 3Department of Chemistry, Stanford University, Stanford CA 94305, USA. Introduction: Polycyclic aromatic hydrocarbons surroundings over time. However, exchanged D atoms (PAHs) are known to be abundant and ubiquitous in at the other sites are labile only under UV irradiation; the interstellar medium (ISM) [1]. At the temperatures once irradiation ceases these atoms will not exchange typical of dense interstellar molecular clouds (T = 10–- with the environment. 50 K), the birth sites of stars and planetary systems, Because the aromatic fractions of meteoritic PAHs will be frozen out of the gas phase into icy grain organics are known to be enriched in deuterium [6], mantles. These mantles contain a number of molecular this raises the possibility that interstellar species, the most abundant being H2O, CH3OH, CO, photoprocessing of PAHs in D-enriched interstellar CO2, NH3, CH 4, and possibly N2 and O2 [2]. Many of ices has occurred [7]. The measured D/H ratios of these molecules are expected to be enriched in interstellar ices [8] and the observed exchange rates for deuterium via a number of interstellar chemical coronene (C 24H12)-D2O and d12-coronene (C24D12)-H2O processes [2]. While frozen in these D-enriched ices, isotopic substitution experiments show that PAHs in the PAHs will be exposed to ultraviolet photons from interstellar ices could easily attain the D/H levels nearby stars, the attenuated diffuse ISM radiation field, observed in meteorites [7]. Since this process and cosmic ray interactions with the dust. We report produces deuteration in characteristic locations on the the results of laboratory simulations that suggest links PAHs, it may be possible to constrain the extent to between interstellar photochemistry in dense clouds which this process is responsible for meteoritic D and the chemical and isotopic properties of meteoritic enrichments. aromatic compounds. Photochemical Alteration of PAHs: When H2O- References: [1] Allamandola L. et al. (1999) ApJ, rich ices containing PAHs are exposed to UV 511, L115. [2] cf. Sandford S. (1996) MAPS, 31, 449. radiation, and the resulting photoproducts are analyzed [3] Bernstein M. et al. (1999) Science, 283, 1135. [4] using infrared spectroscopy and mass spectrometry, we Bernstein M. et al. (1996) ApJ, 472, L127. [5] Hahn J. find that peripheral carbon atoms on the PAHs can be et al. (1988) Science, 239, 1523; Cronin J. and Chang oxidized to yield aromatic alcohols, ketones, and ethers S. (1993) in The Chemistry of Life's Origins (J. [3]. In some cases peripheral atoms accumulate excess Greenberg et al., eds.), p. 209, Kluwer, Netherlands. H atoms, thereby converting edge rings from aromatic [6] Kerridge J. et al. (1987) GCA, 51, 2527. [7] to aliphatic structures. The resulting class of species Sandford S. et al. (2000) ApJ, in press. [8] Teixeira T. (Hn-PAHs) may account for the enigmatic 3.4 µm et al. (1999) A&A, 347, L19. interstellar emission feature [4]. Representatives of these various classes of compounds are found in meteorites [5], raising the possibility that many of these molecules in meteorites may have an interstellar origin. Some of these molecular structures may have important implications for the role of extraterrestrial organics in biogenesis. For example, the oxidized PAHs with ketone (C=O) edge groups are known collectively as quinones. Quinones are used in living systems for electron transport. Deuterium Enrichment of PAHs: The PAH coronene (C24H12) rapidly accumulates deuterium when it is frozen in a D 2O ice at interstellar temperatures and pressures and then exposed to UV radiation. Exchange occurs via three different chemical processes - D atom addition, D atom exchange at oxidized edge sites, and D atom exchange at aromatic edge sites. The D atoms associated with exchange at oxidized edge sites are labile and would be expected to equilibrate with their.