Comets
Résumé
Comets are low-density, kilometer-sized bodies of ices (mostly H2O, CO2, and CO) and dust (silicates, carbon, and organics), which orbit the Sun along elongated orbits. Cometary nuclei have not undergone significant warm-up processes, keeping the compositions resembling that of the protoplanetary cloud. Approaching the Sun, comets develop tenuous atmospheres (comae) consisting of sublimated ices and dust particles lifted by gas flow. The spectra of comets indicate emission of gases, absorption of solid species, and a continuum produced by the scattered solar light. Although the scattering can be produced by free electrons (Thomson mechanism) and gas molecules (Rayleigh mechanism), the study by Dobrovolsky (1966) of the relative roles of different types of scattering showed that the continuum spectra of comets are due to scattering by dust particles. Scattering on the cometary nucleus can be observed, but only in those rare cases when the coma is negligible and that usually occurs at large heliocentric distances. Extensive reviews of the physical properties of cometary nuclei, and gas and dust comae may be found in Festou et al. (2004).
In this chapter we focus on polarimetry of cometary dust, as such studies have accumulated the largest amount of data and produced the most significant results in the characterization of comet materials and their variations within a comet and between comets.
The sunlight scattered by cometary dust becomes elliptically polarized making polarimetry an essential tool for studying the optical and physical properties of cometary dust: size distribution, complex refractive index of its material, and the shape/structure of the particles. However, in the introduction to the book Planets, Stars, and Nebulae Studied with Photopolarimetry, Tom Gehrels (1974) wrote that: “In organizing this book we had not invited a review article on polarization of comets. The reason for this is not a lack of observations but a lack of conclusive interpretations.”