The Effect of Cosmic Rays on Cometary Nuclei. II. Impact on Ice Composition and Structure

Abstract

Since their formation in the protosolar nebula some ∼4.5 billion years ago, comets are in storage in cold distant regions of the solar system, the Kuiper Belt/scattered disk or Oort Cloud. Therefore, they have been considered as mostly unaltered samples of the protosolar nebula. However, a significant dose of energy is deposited by galactic cosmic rays (GCRs) into the outermost tens of meters of cometary nuclei during their stay in the Oort Cloud or Kuiper Belt. We investigate the impact of energy deposition by GCRs on cometary nuclei. We use experimental results from laboratory experiments and the energy deposition by GCRs estimated by Gronoff et al. (2020), to discuss the depth down to which the cometary nucleus is altered by GCRs. We show that GCRs 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.

A Survey of CO, CO2, and H2O in Comets and Centaurs

“…These results are inconsistent with models that predict that the least-processed comets should have more CO than CO 2 and raises questions about comet formation and evolution models. A low CO/CO 2 production rate ratio for dynamically new comets may instead be explained by a model that includes galactic cosmic-ray processing that alters the ice composition down to meters below the surface, resulting in CO depletion with respect to CO 2 (Gronoff et al 2020;Maggiolo et al 2020). This highly processed layer is then eroded during the first perihelion passage, revealing relatively fresher surface layers with pockets of CO ice that are liberated during subsequent passages.…”

Unusual polarimetric properties for interstellar comet 2I/Borisov

“…By contrast, at the time of our observations, comet 2I/Borisov was polarimetrically homogeneous, showing no sign of active areas contributing to the coma formation. Prior to its recent perihelion passage, comet Hale-Bopp probably was near the Sun at least once, and possibly only once, ~2250 BC 65 ; at the time of that first approach, the original material was removed from the surface and active areas were open 66 , hence Hale-Bopp could manifest activity during its recent perihelion passage. Comet 2I/Borisov instead, most likely never passed close to the Sun or any other star, and may represent the first truly pristine comet that has ever been observed.…”

The Effects of Early Collisional Evolution on Amorphous Water Ice Bodies

“…Impacts are not the only known process that can affect AWI content; cosmic-ray exposure can affect the abundance of AWI in these icy bodies (Dartois et al 2015;Maggiolo et al 2020). Galactic cosmic rays are highly energetic particles (mostly protons, alpha particles, and electrons/positrons) with typical energies on the order of ∼GeV (∼10 −10 J), but cosmic rays with energies of up to 3.2 × 10 11 GeV (∼51 J) have been detected (Bird et al 1995).…”

“…These energies are sufficient to knock water molecules out of position in the crystal lattice, creating AWI (Leto & Baratta 2003). Moreover, cosmic rays are prevalent enough to fully amorphize crystalline water ice near the surface of icy bodies over timescales of only ∼60 Myr (Maggiolo et al 2020). However, cosmic rays are adsorbed as they pass through the material of the icy body, and therefore they only affect the outer few tens of meters of material (Maggiolo et al 2020), leaving the interior of the icy body unaffected.…”

“…Moreover, cosmic rays are prevalent enough to fully amorphize crystalline water ice near the surface of icy bodies over timescales of only ∼60 Myr (Maggiolo et al 2020). However, cosmic rays are adsorbed as they pass through the material of the icy body, and therefore they only affect the outer few tens of meters of material (Maggiolo et al 2020), leaving the interior of the icy body unaffected. It is currently unclear whether the small impact flux into present EKB and Oort Cloud objects is sufficient in the present solar system to overcome cosmic-ray amorphization and crystallize the near surfaces of these objects; such an investigation is beyond the scope of this work and left for future studies.…”