Formaldehyde and organic molecule production in astrophysical ices at cryogenic temperatures.

Thermally promoted formaldehyde (H2CO) reactions in cryogenic ices have been studied to test their importance as a source of organic molecules in comets and interstellar ices. Ices containing H2CO, H2O, CH3OH, CO, and NH3 were investigated by using infrared spectroscopy. Small traces of NH3 (NH3/H2CO > or = 0.005) are sufficient to convert significant fractions (> or = 40%) of the H2CO into more complex organics. However, H2CO reactions do not proceed without NH3. Spectral evidence for reaction onset appeared between 40 and 80 kelvin, depending on the ice. Five distinct products were formed. These principally consist of polyoxymethylene and related derivatives. Polyoxymethylene itself was not made in significant amounts in cometary analogs. These products differ from those produced by ultraviolet and particle irradiation. The nature and relative amounts of the products depend on the initial composition, making these materials excellent tracers of a comet's history. About 3% of the organics in p-Halley's coma could have been produced by thermal H2CO reactions.