  Comets are small bodies thought to contain the most pristine material in the
solar system. However, since their formation 4.5 Gy ago, they have been altered
by different processes. While not exposed to much electromagnetic radiation,
they experience intense particle radiation. Galactic cosmic rays and solar
energetic particles have a broad spectrum of energies and interact with the
cometary surface and subsurface; they are the main source of space weathering
for a comet in the Kuiper Belt or in the Oort cloud; and also affect the ice
prior to the comet agglomeration. While low energy particles interact only with
the cometary surface, the most energetic ones deposit a significant amount of
energy down to tens of meters. This interaction can modify the isotopic ratios
in cometary ices and create secondary compounds through radiolysis, such as O2
and H2O2 (paper II: Maggiolo et al., 2020). In this paper, we model the energy
deposition of energetic particles as a function of depth using a Geant4
application modified to account for the isotope creation process. We quantify
the energy deposited in cometary nucleus by galactic cosmic rays and solar
energetic particles. The consequences of the energy deposition on the isotopic
and chemical composition of cometary ices and their implication on the
interpretation of cometary observations, notably of 67P/Churyumov Gerasimenko
by the ESA/Rosetta spacecraft, will be discussed in Paper II.
