ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
tokens: −1 , with an average value of 0.2 eV molecule −1 . For crystalline ice the energy needed to convert it into compact amorphous ice is higher. For instance, Leto Baratta ( 2003 ) estimated that this energy ranges between 0.59 and 1.42 eV molecule −1 for pure crystalline water ice and for various projectiles. Famá et al. ( 2010 ) obtained an energy of eV molecule −1 for a pure crystalline water ice impacted by H + ions with an energy of 225 keV. Note that Raut et al. ( 2008 ) suggested the existence of a stopping power threshold of 4 eV Å −1 to trigger the destruction of the open porosity of the ice. However, the presence of such a threshold in compaction at low stopping power was not confirmed for the high-energy cosmic rays encountered in interstellar space (Dartois et al. 2013 ). These results are not surprising since every molecule is coordinated with the neighbors in the lattice. The photodissociation of a single water molecule, breaking and reformation of bonds, and formation of radicals or other species such as H 2 O 2 affect the unit cell of the crystalline structure, and hence a larger number of molecules are affected (Leto Baratta 2003 ). Figure 8 shows the impact of the energy needed to compact ice on the depth down to which cometary ice inside the nucleus is fully converted into compact amorphous ice. For an energy varying between 10 −2 and 10 eV molecule −1 , this depth varies between ∼28 and ∼8 m. The depth down to which the ice structure is fully changed by irradiation is not strongly dependent on the energy needed to compact ice but is rather controlled by the sharp drop of the deposited energy by GCRs inside the nucleus. Figure 8. Maximal depth until which the cometary ice is completely converted into compact amorphous ice as a function of the energy required to convert the ice into amorphous ice. One important implication of this result is that, regardless of the water ice structure inside the cometary nucleus, its outer layer (the first ∼10–20 m) consists of compact amorphous ice and is not necessarily representative of the whole nucleus. This has strong implications for dynamically new comets (i.e., a comet that is approaching the Sun and outgasses for the first time). For instance, recent observations by Meech et al. ( 2009 ) showed that the activity at large heliocentric distances of comets passing through the solar system for the first time may be caused by the annealing of amorphous water ice. According to our results, such observations of the altered outer layers of a cometary nucleus cannot be taken as a proof that comets as a whole are composed of amorphous ice and cannot be used to infer the temperature and region where comets have formed in the protosolar nebula. 5. Discussion and Conclusion In this paper we analyzed the effects of GCRs on the isotopic composition, chemical composition, and ice structure inside a cometary nucleus. We did not consider surface effects such as sputtering, which leads to erosion of the