Comets are believed to be the most pristine objects of the solar system. The paradigm is that the composition of cometary dust is that of the original solar nebula and elemental abundances of metals therefore correspond to solar atmosphere and to the most primitive meteorites, carbonaceous chondrites of type CI. The real data of three comets visited by spacecraft as compiled by Bardyn et al. (2017) show, however, different pattern. The measured Mg/Fe ratio in comet 1P/Halley was higher than CI while in comet 67P/Churyumov-Gerasimenko it was lower. Both comets yielded Si/Fe and Na/Fe ratios higher than CI, although the uncertainties are large. Only comet 81P/Wild 2 exhibited the CI composition, with possible exception of K abundance. The question is if the measured deviations from CI abundances in cometary dust are due to measurement errors, some selection effects or if they are real.
The composition of cometary dust can be also evaluated from the spectra of bright meteors (fireballs), which contain emission lines of the major elements, in particular Mg, Fe, and Na. The archives of the Ondřejov Observatory contains high resolution fireball spectra taken on photographic films (and recently digitized). Currently, fireball spectra are photographed by digital cameras of the European Fireball Network. The network also provides trajectories, orbits and light curves of the fireballs. Cometary origin of the fireball can be revealed from its orbit and atmospheric fragmentation. In case of meteor showers, actual parent comets are often known.
In the scope of the project, the methodology of inferring chemical composition from fireballs spectra will be improved. The intensities of spectral lines depend not only on meteoroid composition but also on the process of ablation and on physical conditions in the radiating plasma. Earlier compilation of meteor spectra (Borovička 2007) indicated that Perseid and Leonid fireballs, i.e. meteoroids from comets on Halley type orbits, have composition closer to 1P/Halley dust than to CI while Taurids (related to comet 2P/Encke) have CI composition. Some doubts may, however, arise from the fact that the velocity of Perseids and Leonids is much larger than for Taurids. Since larger sample of spectra is now available, it will be possible to investigate the effect of velocity in detail. The goal of the project will be to find if there are small differences in composition of dust in various individual comets (known parent bodies of meteor showers) and in comets of different orbital types (Jupiter family comets, comet Encke, and nearly isotropic comets of Halley type). Differences in physical properties of meteoroids from various comets can be also evaluated.
References: Bardyn A. et al. (2017) MNRAS 469, S712 Borovička J. (2007) IAU Symposium 236, 107