CONDENSATION DYNAMICS OF COMETARY ICE ANALOGS IN VIEW OF PRIMORDIAL ACCRETION OF VOLATILES ON GRAIN SURFACES - A STUDY THROUGH INFRARED-SPECTROSCOPY

Binary and ternary gas mixtures of H2O, CO2, NH3 and CH4 were condense d at 10(-6) mbar onto a KBr cold finger at 10 K. The composition of th e few micrometre thick layers has been determined by infrared spectros copy in transmission mode. The condensation dynamics together with the predominant matrix effect studied as a function of gas composition, d eposition time and ice layer thickness reveal a lower condensation pro bability for the more volatile components CH4 and CO2 in mixtures of H 2O-NH3-CH4 and H2O-NH3-CO2, respectively, with varying composition. Th e phenomena explain representative cases of early condensation and acc retion processes of icy layers on interstellar grains.