Photochemical and thermal evolution of interstellar/precometary ice analogs

An experimental investigation of the photochemical and thermal evolution of ices deposited at 10 K which contain primarily H2O, CH3OH, NH3, and CO mixed in relative proportions consistent with the proposed composition of interstellar ices is presented. These experiments, which are relevant to both interstellar and cometary ices, are the first described in which CH3OH (methanol) is a major constituent of the ice. Ultraviolet photolysis of these ice analogs invariably produces H2CO, CO2, CO, CH4, and HCO, largely at the expense of photofragmented CH3OH. In addition, photolysis produces a mixture of more complex molecules, some of which contain nitrile or isonitrile (CN or CN) and carbonyl (CO) groups. Most of the CO and CH4 leaves the sample upon warm-up to 100 K. Most of the parent ice molecules sublime away by 200 K leaving behind a mixture of more refractory substances. Warm-up to 250 K removes a component rich in CH3 groups which may correlate with the carrier(s) of the CO and CN bonds. A residue rich in CH2 groups remains even after warm-up to 300 K. The relevance of these results to questions concerning the composition of interstellar and cometary ices, and the scale heights of photofragments in cometary comae is discussed. The results may have some bearing on the recent suggestion that polyoxymethylene is present in Comet Halley.

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