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R. L. Hudson (Eckerd College), M. H. Moore (NASA/GSFC), P. A. Gerakines (University of Alabama - Birmingham)
Comets contain a rich variety of organic and inorganic molecules, some tracing back to the interstellar medium while others perhaps only to a comet's formation. Since cometary ices are exposed to cosmic radiation, they are expected to change over time, as new molecules are made. Laboratory investigations on irradiated cometary ice analogues have usually emphasized production of either volatile molecules observed in cometary comae or non-volatile residues that might form a dark crust on a nucleus. Our work has covered both the radiation and photochemistries of cometary ices in hopes of not just cataloging reaction products but of understanding the underlying chemical reactions. Ice experiments have identified reactions involving cometary molecules such as H2O, CO, CO2, H2CO, CH3OH, CH4, C2H2, and C2H6 (Icarus, 1998, 135, 518; Icarus, 1999, 140, 451).
Although most of our laboratory work has focused on neutral species, we have recently investigated electron- and proton-transfer reactions in cometary ice analogues. These show that ions form in irradiated or photolyzed ices at low temperatures in sufficient abundance to observe spectroscopically. For example, ion-irradiated H2O + CO ices produce the formate anion (HCOO-), which has IR features observable in interstellar ices (Hudson & Moore, 2000, 145, 661). In other experiments, the ammonium cation (NH4+) and the cyanate anion (OCN-) have been seen by IR, which bears on the long-standing debate over the XCN spectral feature. Ions are important as they contribute to the overall elemental abundances of a comet, yet are difficult to observe. In the particular case of nitrogen ions, their formation is a case of cometary "nitrogen fixation", and is more easily understood in a mixed-molecular ice than the polymeric materials sometimes invoked to reconcile cometary and Solar Systen nitrogen balances. Ions are also more likely candidates for survival in a comet-Earth impact than are most simple organic molecules. In this presentation, a few recent experiments will be presented from our work covering these ideas.
NASA funding through NRA 344-33-01, NRA 344-02-57, and grant NAG-5-1843 is acknowledged. PAG acknowledges an NRC/GSFC Research Associateship.
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