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The ageing effect of cosmic rays on comets

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Comets consist of relatively small aggregates of cold ice and dust formed in the outer Solar System. They remain unaltered by gravity or heating contrary to other bodies. They are thus thought to contain the best samples of pristine early Solar System materials. However, we were able to show that they age as Galactic Cosmic Rays penetrate into the outer parts of the nucleus and change the composition and structure of cometary ice, which can no longer be considered as pristine. This has strong implications on the interpretation of cometary measurements.
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Birth, life and death of comets

The Solar System formed some 4.7 billion years ago from the gravitational collapse of a Molecular Cloud (MC). A MC is a type of interstellar cloud that allows the formation of molecules.

Inside MC, molecules form and freeze at the surface of dust grains. A complex chemistry leads to the abiotic formation of complex molecules like sugar, alcohol and, amino acids. Most of the MC collapsing mass is collected in the center forming the Sun, while the rest flattens into a protoplanetary disk out of which other Solar System bodies form.

Among them, comets are small enough (typically some kilometers) and form far enough from the Sun to remain cold and preserve the protosolar nebula material. Then, they remain stored in the cold outer regions of the Solar System until a perturbation brings them in the inner Solar System where they heat up, evaporate and eventually disaggregate and disappear.

Cometary nucleus alteration by Galactic Cosmic Rays (GCRs)

During their stay in the outer Solar System, comets remain unaltered by thermal and gravitational processes. However, GCRs -a flow of high-energy protons and atomic nuclei that move at nearly the speed of light- constantly irradiates Solar System bodies.

We modeled the energy deposition of GCRs in cometary nuclei and used results from laboratory experiments to demonstrate that GCRs may significantly alter cometary ice within the first tens of meters inside the nucleus. GCRs break chemical bounds in cometary ices leading to the formation and destruction of molecules. In addition, GCRs also compact cometary ice.

Implications on the interpretation of cometary observations

Comets are seen as refrigerators preserving protosolar nebula material. This motivates the in-situ observation of comets. Comets offers a unique opportunity to sample remnant of the original material out of which the Solar System emerged, providing new insight on the Solar System formation and on the early history of planets.

BIRA-IASB is involved in Comet Interceptor, a mission in preparation, which aims at sampling material outgassed from a comet during its first approach to the inner Solar System when the outermost layers of the cometary nucleus are outgassing. It will thus be key to account for the nucleus alteration by GCRs in order to properly interpret Comet Interceptor measurements and push further our knowledge of the early Solar System.


  • Gronoff, G., Maggiolo, R., Cessateur, G., Moore, W. B., Airapetian, V. S., De Keyser, J., Dhooghe, F., Gibbons, A., Gunell, H., Mertens C. J., Rubin, M., and Hosseini, S. (2020). The Effect of Cosmic Rays on Cometary Nuclei. I. Dose Deposition, The Astrophysical Journal, 890, 89.
  • Maggiolo, R., Gronoff, G., Cessateur, G., Moore, W. B., Airapetian, V. S., De Keyser, J., Dhooghe, F., Gibbons, A., Gunell, H., Mertens C. J., Rubin, M., and Hosseini, S. (2020). The Effect of Cosmic Rays on Cometary Nuclei. II. Impact on Ice Composition and Structure, The Astrophysical Journal, 901, 136.
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Fig2: Illustration of the composition change induced by Galactic Cosmic Rays over a period of 4.5 billion years as a function of the depth inside the cometary nucleus. From Maggiolo et al. 2020.
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