Two different evolutionary types of comets proved by polarimetric and infrared properties of their dust

Two different evolutionary types of comets proved by polarimetric and infrared properties of their dust ABSTRACT Aims. We consider polarimetric and thermal-emission properties of comet dust and show how and why they can be used for classification of comets. Methods. We provide a statistical analysis of comet polarimetric, thermal, and orbital characteristics. We perform computer simulations of polarization and infrared spectra considering comet particles as ballistic particle-cluster and cluster-cluster aggregates (BPCA and BCCA) consisting of submicron spherical grains. Results. Comets can be divided into two groups: Type I, characterized by high gas/dust ratio, low polarization, and a weak or absent 10 µm silicate feature, and Type II, for which a low gas/dust ratio, high polarization, and strong silicate feature are typical. We show that the low polarization is the apparent result of depolarization by gas contamination at low dust concentration, which, in turn, results from the dust in Type I comets being concentrated near the nucleus. The simulations of thermal emission show that for more porous particles (BCCA), the silicate feature is more pronounced than more compact ones (BPCA), for which it even vanishes as the particles become larger. We also show that in both types of comets the main contribution to light scattering and emission comes from particles larger than 10 micron. Conclusions. The strength of the silicate feature in the cometary infrared spectra suggests that the dust in Type II comets consists of high-porosity aggregates, whereas the dust of Type I comets contains low-porosity ones. This is consistent with the polarimetric features of these comets, which indicate that the dust in Type I comets tends to concentrate near the nucleus. This may result from the predominance of highly processed particles in Type I comets, whereas in Type II comets we see pristine or slightly-processed dust. This conclusion is in accordance with the orbital characteristics of the comets. We have found that the strength of the silicate feature correlates with the semi-major axis of periodic comets and, for short-period comets, with the perihelion distance. Thus, the silicate feature weakens due to compaction of aggregate particles if a comet spends more time in the vicinity of the Sun, which allows the comet to accumulate a mantle on the surface of its nucleus. Key words. comets – dust – polarization – infrared spectra – silicate feature – semi-major axis – perihelion