Ioffe Physical-Technical Institute of the RAS, St. Petersburg
Simulation of energy accumulation in icy bodies during irradiation: a source
of cometary activity far from the Sun.
Many icy bodies in the solar system (e.g., comets and Kuiper Belt objects)
demonstrate activity far from the Sun. The nature of such an activity remains one
of the main enigmas of planetology. Comet nuclei undergo a constant
bombardment by galactic cosmic rays (GCR) and solar cosmic rays (SCR) during
their storage in the Oort Cloud and the Kuiper Belt. Long-term cosmic ray
irradiation produces chemical and physical changes in comet ice with the
formation of radicals. It is well-known that trapped radicals at low temperatures are
prone to fast recombination with marked energy release; recombination can be
induced by external heating or occurs spontaneously reaching the critical
concentration of radicals. A similar process could take place in the radicals-
enriched subsurface layer during the sunward voyage of a dynamically “new”
comet. Also, located in the Oort Cloud cometary nuclei can experience activity
driven by spontaneous recombination.
Several experiments on ice irradiation have been conducted to examine radicals
accumulation in proton-irradiated and electron-irradiated ice at 80 K. Based on
experimental data, we have developed the model of comet activity at large
heliocentric distances triggered by accumulated radicals. GCR influences only the
upper 10-meter layer of a comet, while much deeper layers are irradiated by decay
of radionuclides. As a results, comets in the Oort Cloud and the Kuiper Belt can
have a high concentration of radicals and be active at large distances.