Unidentified Infrared Emission Features in Mid-infrared Spectrum of Comet 21P/Giacobini-Zinner


Best-fit thermal emission model for comet 21P/G-Z for fitting with all wavelength regions (8–18.8 µm). The data at 9.4–9.8 µm affected by the strong absorption of telluric ozone are not used for the fitting. The best-fit spectrum cannot reproduce the observed spectrum of 21P (especially, the emission peaks at 8.2, 8.5, and 9.2 µm and the photometric point at 18.8 µm).

Comet 21P/Giacobini-Zinner (hereafter, comet 21P/G-Z) is a Jupiter-family comet and a parent comet of the October Draconids meteor shower.

If meteoroids originating from a Jupiter-family comet contain complex organic molecules, such as amino acids, they are essential pieces of the puzzle regarding the origin of life on Earth.

We observed comet 21P/G-Z in the mid-infrared wavelength region using the Cooled Mid-infrared Camera and Spectrometer (COMICS) on the 8.2 m Subaru Telescope on UT 2005 July 5. Here, we report the unidentified infrared (UIR) emission features of comet 21P/G-Z, which are likely due to complex organic molecules (both aliphatic and aromatic hydrocarbons), and the thermal emission from amorphous/crystalline silicates and amorphous carbon grains in its mid-infrared low-resolution spectrum. The UIR features at ~8.2 micron, ~8.5 micron, and ~11.2 micron found in the spectrum of comet 21P/G-Z could be attributed to polycyclic aromatic hydrocarbons (or hydrogenated amorphous carbons) contaminated by N- or O-atoms, although part of the feature at ~11.2 micron comes from crystalline olivine. The other feature at ~9.2 micron might originate from aliphatic hydrocarbons. Comet 21P/G-Z is enriched in complex organic molecules.

Considering that the derived mass fraction of crystalline silicates in comet 21P/G-Z is typical of comets, we propose that the comet originated from a circumplanetary disk of giant planets (similar to Jupiter and Saturn) where was warmer than the typical comet-forming region (5-30 au from the Sun) and was suitable for the formation of complex organic molecules. Comets from circumplanetary disks might be enriched in complex organic molecules, such as comet 21P/G-Z, and may have provided pre-biotic molecules to ancient Earth by direct impact or meteor showers.

Takafumi Ootsubo, Hideyo Kawakita, Yoshiharu Shinnaka, Jun-ichi Watanabe, Mitsuhiko Honda
(Submitted on 8 Oct 2019)
Comments: 26 pages, 6 figures, 3 tables, accepted for publication in Icarus
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1910.03485 [astro-ph.EP] (or arXiv:1910.03485v1 [astro-ph.EP] for this version)
Submission history
From: Takafumi Ootsubo
[v1] Tue, 8 Oct 2019 15:49:24 UTC (922 KB)
https://arxiv.org/abs/1910.03485
Astrobiology, Astrochemistry

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