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Project Progress

We have made substantial progress in all three of the focus areas pertinent to our task.

(1) In the area of spectroscopy of PAHs under astrophysical conditions we have published a number of papers in main journals. Four are all about the IR spectra of PAHs in water ice at low temperature as they are thought to exist in pre-solar ices (in dense molecular clouds) the environments in which new solar systems are conceived. These will enable searches for PAHs in dense molecular clouds and deepen understanding of the connection between interstellar material and new planetary systems. We have also published the first catalog of near-IR spectra of PAH ions, resolving a major puzzle regarding the PAH model. It was traditionally thought that UV radiation was required to pump the ubiquitous cosmic PAH emission features, but NASA’s Spitzer space telescope has detected these features in UV poor regions. We followed this up with publication of a detailed model that includes these new results and that is now used to account for NASA’s Spitzer observations of the emission from these complex organic species. In a major publication in the Astrophysical Journal we have shown that cosmic PAHs contain nitrogen, a very important biogenic element. The distribution of nitrogen throughout the interstellar medium has long been a puzzle. This is a step toward solving that mystery and provides a new nitrogen tracer. Lastly we have also continued our studies of PAH ions in cosmic ices relevant to both icy planets and the interstellar medium. We are beginning to understand the important role of concentration and PAH size. Remarkably many PAH ions remain trapped and stable in ices even at temperatures well above 50 K and the largest ion we have studied to date (C₄₀H₂₀⁺) remains unmoved in water ice at temperatures up to 120 K. As this is on the small end of the extraterrestrial PAH size distribution, these observations suggest that ions in icy bodies could play an important, but previously unrecognized, role in the chemistry, spectroscopy, and physics of icy bodies both in the Solar System and the interstellar medium (ISM).

2) We also continue our studies of the photochemistry of aromatics in solid H₂O at low temperatures, extending our previous work on naphthalene to the structurally equivalent nitrogen heterocycle, quinoline. This nitrogen-containing aromatic is closely related structurally to current bio-molecules. The paper on this subject by our NAI post-doc Jamie Elsila has just come out recently: Jamie E. Elsila, Matthew R. Hammond, Max P. Bernstein, Scott A. Sandford, And Richard N. Zare, UV photolysis of quinoline in interstellar ice analogs Meteoritics & Planetary Science 41, 785-796 (2006). Jamie Elsila is also studying the production of amino acids in ice irradiation experiments and these samples are being analyzed at Goddard as part of our cross team collaborative efforts.

3) Our third area of interest is the interaction of PAH and functionalized PAH with various vesicular model membranes and lipid monolayers. Langmuir compression isotherms (Charles L. Apel, Ames and SETI NAI team member) and NMR measurements (Andre Simpson, Univ. of Toronto) are revealing details about the orientation of the PAH within each system. Preliminary dynamic light scattering measurements (Pierre Monnard, LANL) suggest that the inclusion of PAH into vesicle membranes doubles the average diameter of these structures. The data from these studies are now being organized for publication. Experiments to measure the ability of PAH to shield genetic informational molecules from the harmful effects of UV exposure have also been initiated with collaborator Wenonah Vercoutere (Ames Life Sciences Division and NAI team member).

Scott Sandford, a key member of this team, is CoI on the very successful Stardust mission. He was among those who retrieved the capsule, and has been intimately involved with sample extraction, distribution, and analysis. Scott also continues to spearhead the proposal effort for two missions dedicated to Astrobiology, ABE the Astrobiology Explorer and ASPIRE, the Astrobiology Space Infrared Explorer, which is now a mission concept study.