ION IRRADIATION EXPERIMENTS RELEVANT TO THE PHYSICS OF COMETS

We present the results of new experiments on some physical-chemical effects induced by fast ion colliding with solids of relevance for the physics of comets. In particular we have studied the transition from crystalline to amorphous water ice induced by keV ion irradiation at temperatures between 10 and 100 K. After ion-induced amorphization occurred, at low T, the samples have been heated and their crystallization at around 150 K has been studied. Our results are compared with those recently reported in the literature (Hudson and Moore, J. Phys. Chem. 96, 6500, 1999). The production of molecular solids, polymer-like materials and amorphous carbon by irradiation of simple carbon-containing ices is discussed as well as some preliminary results on the changes in the sublimation rates of irradiated CO-ices. We also report on a set of experimental results obtained irradiating methanol and water-methanol mixtures. As a consequence of bombardment, different species form. Among these, the formation of CO, CH4 and CO2 is evident. The production of formaldehyde questionable is; upper limits are given. The ratios among different molecules have been evaluated as a function of the deposited energy. The results are compared with those obtained by UV irradiation of the same mixtures (Allamandola et al., Icarus 76, 225, 1988). The experimental results are finally discussed in the light of their relevance to cometary physics.