Simulations Of Ice Chemistry In Cometary Nuclei

©NASA

Comet NEAT

The first computational model of solid-phase chemistry in cometary nuclear ices is presented. An astrochemical kinetics model, MAGICKAL, is adapted to trace the chemical evolution in multiple layers of cometary ice, over a representative period of 5 Gyr.

Physical conditions are chosen appropriate for ``cold storage'' of the cometary nucleus in the outer Solar System, prior to any active phase. The chemistry is simulated at a selection of static temperatures in the range 5 - 60 K, while the ice is exposed to the interstellar radiation field, inducing a photochemistry in the outer ice layers that produces significant formation of complex organic molecules. A treatment for the chemistry resulting from cosmic-ray bombardment of the ices is also introduced into the model, along with a new formulation for low-temperature photochemistry. Production of simple and complex molecules to depth on the order of 10~m or more is achieved, with local fractional abundances comparable to observed values in many cases.

The production of substantial amounts of O2 (and H2O2) is found, suggesting that long-term processing by high-energy cosmic rays of cometary ices {\em in situ}, over a period on the order of 1 Gyr, may be sufficient to explain the large observed abundances of O2, if the overall loss of material from the comet is limited to a depth on the order of 10 m. Entry into the inner solar system could produce a further enhancement in the molecular content of the nuclear ices that may be quantifiable using this modeling approach.

Robin T. Garrod
(Submitted on 8 Sep 2019)

Comments: 62 pages, 18 figures, accepted for publication in the Astrophysical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA); High Energy Astrophysical Phenomena (astro-ph.HE)
Cite as: arXiv:1909.03366 [astro-ph.EP] (or arXiv:1909.03366v1 [astro-ph.EP] for this version)
Submission history
From: Robin Garrod
[v1] Sun, 8 Sep 2019 01:53:09 UTC (1,157 KB)
https://arxiv.org/abs/1909.03366
Astrobiology, Astrochemistry

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