An experimental study of the organic molecules produced in cometary and interstellar ice analogs by thermal formaldehyde reactions.

Thermally promoted formaldehyde (H2CO) reactions in very low temperature ices have been studied to test their importance as a source of organic molecules in astrophysical environments such as comets and interstellar ices. The infrared absorption strengths of a number of the H2CO bands were measured in 10 K ices of pure H2CO and H20:H2CO = 100:3. Infrared spectroscopy was used to monitor the formaldehyde chemistry during warm-up of ices containing H2CO and one or more of the molecules H2O, CH3OH, CO, CO2, O2, and NH3. Formaldehyde reactions do not proceed at low temperatures in the absence of NH3. However, even small traces of NH3 (NH3/H2CO > or = 0.005) are sufficient to induce conversion of a considerable fraction (> 40%) of the H2CO into organic residues. Formaldehyde reactions were observed to start at temperatures as low as 40 K for NH3:H2CO binary ices and at approximately 80 K in astrophysically relevant (i.e., H2O-dominated) ices. A total of five different organic products of these reactions can be distinguished by infrared spectroscopy. One of them is polyoxymethylene (POM), a well-known H2CO polymerization product, whereas the others are reaction products of H2CO with H20, CH3OH and NH3. These all seem to be derivatives of polyoxymethylene. The nature of the components and their relative abundances depend strongly on the initial composition of the ice mixture as well as on the ice's irradiation history. We estimate that about 1% of the organics found in the coma of Comet Halley could have been produced by thermal formaldehyde reactions taking place in the nucleus.