Cosmic ray irradiation of comet nuclei: a possible source of cometary outbursts at large heliocentric distances

Abstract

The effect of radical accumulation in ice under high energy particle irradiation at low temperature followed by the energy release during heating has been proposed as a possible cause of cometary outbursts. Water ice samples were subjected to 15 MeV protons to simulate the cosmic ray irradiation of comet nuclei. The irradiation with 0.9 MeV electrons was performed to model a similar process on icy satellites located in radiation belts of planets. We observed the release of accumulated energy 20 J/g during the heating of the proton-irradiated ice with two pulses of fast self-heating of the sample at 84 and 110 K. Similar energy release occurred between 82 and 110 K in the electron-irradiated ice. Our calculations suggest that the recombination of radicals accumulated in the surface layer of a comet nucleus under cosmic ray bombardment can trigger the cometary outbursts far from the Sun.