Ion irradiation experiments relevant to cometary physics

Abstract

During their stay in the Oort cloud (4.6 × 109 years), comets are subject to the flux of galactic cosmic rays. Experimental results on the chemical and physical changes induced by ion and electron irradiation of relevant materials have been recently reviewed with a view to their relevance for cometary astrophysics. In particular, the ability of ion irradiation of simple carbon‐containing ices to produce complex refractory organic materials has been discussed. Here we discuss further experiments carried out in our laboratory whose results are relevant to cometary physics. Experiments on targets much thicker than the penetration range of irradiating ions give support to the hypothesis that the ion‐produced cometary organic crust can “survive” gas ejection from deeper layers and be as old as the comet itself. A study of IR and Raman spectra obtained “in situ” before, during, and after irradiation of frozen benzene has clarified the processes producing, after long enough exposures, a tridimensional network of aromatic molecules linked together by linear chains. We call this material ion‐produced hydrogenated amorphous carbon. Other experimental data demonstrate the transition from crystalline to amorphous water ice induced by ion irradiation at temperatures between 10 and 100 K. Thus the observation of amorphous ice cannot be considered, as it has been up to now, a definitive proof of its formation and permanence at low T.