The effects of cosmic rays on cometary nuclei

Comets are small bodies considered as mostly unaltered samples of the protosolar nebula. Since their formation some 4.5 billion years ago, they are in storage in cold distant regions of the solar system, the Kuiper Belt/scattered disk or Oort Cloud, where they experience intense constant particle radiation.

Galactic cosmic rays and solar energetic particles are the main sources of space weathering for a comet in the Kuiper Belt or in the Oort Cloud. While low-energy particles interact only with the cometary surface (down to a few centimeters deep), the most energetic ones deposit a significant amount of energy down to tens of meters.

We model the energy deposition of energetic particles as a function of depth using a Geant4 application and quantify the energy deposited in the cometary nucleus by galactic cosmic rays and solar energetic particles. We then consider the implication of the interaction between energetic particles and cometary material. Modifications of the isotopic composition are estimated with Geant4 while the alterations of the ice chemical composition and of the ice structure are estimated using experimental results from laboratory experiments.

We show that energetic particles, mostly from Galactic Cosmic Rays, do not significantly change the isotopic composition of cometary material but modify the chemical composition and the ice structure in the outer layers of the nucleus which cannot be considered as pristine solar nebula material.

We discuss the effect of the collisional history of comets on the distribution of processed material inside the nucleus and its implication on the observation of comets.