Heliospheric cosmic ray irradiation of Kuiper Belt comets

Abstract

Irradiation by galactic cosmic ray particles at energies above 0.1 GeV has been shown to be a significant source of energy for chemical modification of ices in Oort Cloud comets. However, these ions have minimal efficiency for surface modification, since their energy is deposited over depths of many meters, and they must act over the lifetime of the Solar System to produce an appreciable radiation-induced mantle. Recent measurements by the Voyager and Pioneer spacecraft, now moving outward from the Sun beyond 40 – 60 A.U. into the Kuiper belt region, have found a radially increasing intensity of the “anomalous” cosmic ray component, consisting of interstellar hydrogen and heavier ions accelerated at energies up to 10 2 MeV/nucleon at the solar wind termination shock at 65 – 100 A.U. from the Sun. Significant intensities of the anomalous cosmic ray ions are expected throughout the inner Kuiper belt region beyond the termination shock and out to the heliopause. These ions would significantly affect mantle formation on Kuiper Belt comets at column depths less than 10 grams/cm 2 in the outer surface layer of material accessible to direct and remote sensing measurements.