Volatiles in comets as probes to the early solar system

Comets are considered as the remnants of the planetesimals (building blocks of the planets) formed in the proto-planetary disk of the Sun. They have retained the information about the formation and evolutional history of the early solar system. To investigate the chemical and physical conditions of the proto-planetary disk, comets have been studied as probes to the solar system formation. In the last two decades, thanks to advances in technology, near-infrared (NIR) observations have been carried out to detect the various kinds of molecules (with and without permanent electric dipole moments) released directly from the nucleus. As the physical temperature could control chemical reactions, we expect to find chemical diversity among comets that have different dynamical origins. To investigate chemical diversity in the proto-planetary disk, we have observed several comets with NIR high-dispersion spectrometry. Although the number of samples is still small relative to the number of samples obtained by optical studies, the HCN, $C2H2$⁠, $CH4$⁠, $C2H6$⁠, $CH3OH$⁠, $H2CO$⁠, and CO content in more than 10 comets have been measured. We compared our samples with other samples obtained by NIR observations and found no clear differences in the chemical compositions of the comets, even though the comets originated in different dynamical reservoirs (i.e., the Oort Cloud and the trans-Neptunian regions). Although there was a small variation in the mixing ratios among the OC comets, all the samples were consistent within error limits. This variation (if it exists) may be supporting evidence for the Nice model. The sublimation temperature of $H2O$ is relatively higher than that of other hyper volatiles, such as CO, $CO2$⁠, and $CH4$⁠. Thus, in the proto-planetary disk, there was a region where $H2O$ could exist as ice and be incorporated into the planetesimals, while other hyper volatiles were in gas phase and could not be incorporated into the planetesimals. Alternatively, the differences in the chemical composition of ices between comets and interstellar environments might be caused by some kind of nebula processing after interstellar ices fell into the solar nebula. To provide more rigid constraints on the conditions in the proto-planetary disk, we need more observational samples of comets and realistic models of chemical and dynamical evolution in the proto-planetary disk.