A team of astronomers from Koyama Astronomical Observatory of Kyoto Sangyo University and Subaru Telescope of the National Astronomical Observatory of Japan has found that comet 21P/Giacobini-Zinner (Figure 1) is depleted in CO2 abundance with respect to water as compared to other comets by using the High Dispersion Spectrograph (HDS) on the Subaru Telescope during the 2018 apparition of the comet. This result indicates that the comet formed in a warmer region in the solar nebula compared to other observed comets. This is consistent with previous result by the Subaru Telescope, which demonstrated that the comet is rich in complex organic molecules that could be formed under warm conditions.
Figure 1: Comet 21P/Giacobini-Zinner observed in the optical on August 22, 2018. (Credit: Michael Jaeger)
About 4.6 billion years ago, comets were born in the proto-solar nebula surrounding the Sun, which is a disk-shaped cloud containing gas and dust. The comets should have formed in a low-temperature region in the solar nebula where water, the main component of the ice in cometary nuclei, can freeze (lower than -120℃ in the vacuum of space). Therefore, many comets have similar ice abundance ratios relative to water (Note 1).
From previous studies, Comet 21P/Giacobini-Zinner (Figure 1, hereafter Comet GZ) is famous as a peculiar comet with regard to its compositions of gas and dust grains. It was found that the comet includes abundant complex organics compared with other comets based on data taken by the Subaru Telescope (reported by a team of astronomers from the Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (ISAS/JAXA), Kyoto Sangyo University, National Astronomical Observatory of Japan, and Okayama University of Science). Why is Comet GZ rich in complex organics unlike other comets? Complex organic molecules are likely to be formed in relatively warm environments. So, the team was faced with the question of "was the comet formed in a warm environment in the solar nebula?"
So a team of astronomers from Kyoto Sangyo University and Subaru Telescope conducted high-resolution optical spectroscopic observations of Comet GZ using the High Dispersion Spectrograph (HDS) in three epochs: UT September 5, 9, and October 3, 2018. They focused on the abundance of carbon dioxide (CO2) with respect to water (H2O) because CO2, which is the second (or third) most abundant species in cometary ice after H2O, sublimates at much lower temperatures (-200℃ or higher in the vacuum of space) than H2O (-120℃ or higher).
Although CO2 is a major component of the Earth's atmosphere and a space telescope is indispensable to observe cometary CO2 emissions directly, no space telescopes capable of infrared spectroscopic observations were in operation. So the team focused on the special oxygen atoms (excited to higher electronic meta-stable states) that are produced by photo-dissociation reactions of H2O and CO2 by solar UV radiation. Those excited oxygen atoms emit light at certain wavelengths depending on their excitation state, which are called "forbidden oxygen lines" (Figure 2, 3). The most familiar case of such forbidden oxygen emission light is the Earth's aurora. The aurora's green light is the forbidden oxygen emissions and its red light comes from oxygen atoms in the same way and partly from telluric nitrogen molecules. The oxygen atoms produced from H2O via photodissociation tend to emit the "red" forbidden line, while oxygen atoms produced from CO2 tend to emit "green" and "red" forbidden lines almost equally. Thus, by comparing the intensity ratio of the green and red forbidden lines in comets, the abundance ratio of CO2 with respect to H2O in the cometary coma can be estimated.
Figure 2: Diagram of forbidden oxygen line emission in a cometary coma. High-energy excited oxygen atoms are produced from both water (H2O) and carbon dioxide (CO2) through photo-dissociation by solar UV radiation. The abundance ratio of CO2 with respect to H2O can be estimated from the intensity ratio of the green and red lines from these oxygen atoms. (Credit: Kyoto Snagyo University)
Figure 3: Spectra of forbidden oxygen lines of Comet GZ (vertical black tick). From left to right, the green line (557.7 nm in wavelength), and two red lines (630.0 nm and 636.4 nm) are shown. Planet Earth symbols (cross in circle) indicate the forbidden oxygen lines of the telluric atmosphere. These lines can be separated from those of the comet. (Credit: Shinnaka et al.)
Based on detailed analysis of high-quality spectra of Comet GZ, the team found that the comet has a lower CO2 abundance ratio relative to water than other comets. The derived CO2/H2O abundance ratio of Comet GZ is about 1%, while those of most comets range from a few percent to ~30%. This result is consistent with the low abundance ratio of carbon monoxide (CO) in Comet GZ because the sublimation temperature of CO is lower than that of water. From the lower CO2/H2O abundance ratio in Comet GZ (almost at the lower-end) compared to other comets and the lower sublimation temperature of CO2 relative to H2O, the formation temperature of Comet GZ in the solar nebula can be estimated to have been in the range from -200℃ to -120℃.
Where exactly was that place? The astronomers' team think that Comet GZ formed in the "circumplanetary disk" of a large planet such as Jupiter or Saturn in the solar nebula (Figure 4). When large planets formed in the solar nebula, a small disk with gas and dust surrounding these planets may have been a birthplace for satellites, just like a miniature of the Solar System. Materials (gas and dust) in this disk were warmer than those in the solar nebula at the same distance from the Sun. A comet formed in such a disk may be rich in complex organics molecules and poor in CO2 like Comet GZ.
Figure 4: A schematic view of comet formation in the circumplanetary disk of a giant planet in the solar nebula (Credit: Kyoto Sangyo University).
Dr. Akito Tajitsu of Subaru Telescope emphasizes the importance of the HDS observations, saying "Comet 21P/Giacobini-Zinner is a well-known comet that was discovered over a hundred years ago. Its return in 2018 (once per 6.6 years) was close to the earth and the best opportunity
for us to search its detailed composition. The oxygen lines we analyzed emit in the earth's atmosphere as well as in the comet. So, the highest wavelength resolving power of HDS is really important to distinguish them by their small differences in wavelength caused by the comet's moving speed (Doppler effect)." Dr. Yoshiharu Shinnaka of Kyoto Sangyo University, the lead author of the paper, comments on the future prospects, "In this research, we have revealed one mystery of Comet Giacobini-Zinner known as a peculiar comet. We would like to clear the comet formation environments in the early solar system by researching Comet GZ and other comets in more detail and by finding peculiar comets like Comet GZ."
This study was published in The Astronomical Journal on April 13, 2020 (UT) (Shinnaka, Y., Kawakita, H., and Tajitsu, A., 2020, "High-resolution optical spectroscopic observations of comet 21P/Giacobini-Zinner in its 2018 apparition".
Note 1: Many comets have abundance ratios of CO and CO2 of 10-20%, and those with more complex organics have ratios of 1-5% or less).
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