RADIATION CHEMISTRY OF CARBON AND NITROGEN COMPOUNDS IN H₂O AND NH₃ ICES ; RELEVANCE FOR CHEMICAL EVOLUTION IN SPACE

Abstract
Radiolysis and photolysis of ice and ice-mixtures lead to the formation of new chemical compounds. Preexisting chemical compounds can be changed by the interaction with radicals formed in the frozen matrices and by direct radiolysis. The actual product distribution in an icy substrate is a function of radiation dose (fluence) and dose rate (flux). The reactivity of irradiated H₂O-ice depends on the equilibrium concentration, the mobility and reaction velocity of H*, HO* HO₂* radicals, electrons, etc. The nuclear recoil process (1,2) offers an elegant method to introduce carrier-free probes without disturbing the macroscopic composition of the matrix. The kind of products formed by the radioactive atom is indicative for the redox properties of the ice. The accompanying radiation, e.g. of protons to induce the ¹⁴N(p,α)¹¹C process in NH₃ or the ¹⁶O(p,αpn)¹¹C in H₂O, allows to modify the radiation load. Fig. 1 shows the typical behaviour of ¹¹C products in H₂O-ice at 77 K as a function of the deposited proton dose (2). A neutral phase at low doses is characterized by relatively constant yields of the major products CH₂O, CH₃OH and CO₂. It is followed by a reducing phase at medium dose (CH₃OH increase) and an oxidizing phase at higher dose (CO₂ becomes the only product).