Polyoxymethylene as parent molecule for the formaldehyde extended source in comet Halley

Among unsolved questions raised by observations of comets is the origin of extended sources, i.e., the distribution of molecules in the coma which cannot be explained by a direct sublimation from the nucleus. Polyoxymethylene [formaldehyde polymer: (-CH(2)-O-)(n), also called POM] is sometimes invoked as a potential parent compound, the degradation of which could produce the required amount of H(2)CO across the coma, but no quantitative study has ever been undertaken with relevant parameters. From new experimental data, we are now able to consider multiphase chemistry: POM in the solid state on cometary grains slowly degrades by solar photons and heat and produces H(2)CO in the gaseous phase. This is a new approach to cometary organic chemistry. We show, by considering simple assumptions about the cometary environment, that the hypothesis of POM on grains leads to a very good agreement with Giotto observations if we assume that the cometary grains are similar to7% POM by mass at a temperature of 330 K.