Comet physical properties

Water is a unique substance that plays a major role in geochemistry and cosmochemistry and is a critical component of life. The physical properties of water control the environment on the Earth s surface and have played significant roles in the history of other planets, comets, and asteroids. Most plants and animals are about 60% water by volume, and most biological reactions involve water. It is no exaggeration to say that water is the key to our existence. 

The analysis of cometary observations suggests the existence of very fluffy dust aggregates. Differences are observed in the light-scattering properties, e.g. stracture of the comae, polarization phase curves maxima and minima, polarization wavelength dependence. They could be a clue to the temporal evolution of the physical properties of the dust particles, with collisional processes as well as evaporation of icy mantles and organic compoimds. Table 1 presents some polarization properties of dust particles in comets, asteroids, in the interplanetary dust cloud, and on Mars, as retrieved by remote sensing. 

Veiy few significant results on the physical properties of cosmic dnst, as revealed by in-situ experiments, may be expected in the coming years. The samples collected by the Stardust (interplanetaiy dust and comet SlPAVild 2 dust) and Hayabnsa (asteroid Itokawa regohth) missions shonld provide information primarily about chemical properties. The ground truth expected from space mission is thus quite far away in the future, with e g. the Rosetta rendezvous with comet 67P/Chuiyumov-Gerasimenko and landing on its nucleus in 2014. 

Measurements of four Stokes parameters over the coma in comet Halley by Dollfiis and Suchail [6]. There is evolution of dust physical properties with distance from the nucleus. 

The separation of the continuum from emission features is a difficult problem. There are numerous unidentified emissions [57] that can form a pseudocontinuum of gas-rich comets. It is not clear whether the continuum is reached in the continuum bands therefore, we can conclude that at least a part of the gas-rich comets have a low polarization at large phase angles and blue color mainly due to a low spectral and spatial resolution of observations. Thus, the existence of two taxonomical classes of comets with different physical properties of their dust particles still remains an open question. 

Lunine, J. I. (2004) Giant planets. In Treatise on Geochemistry, Vol. 1. Meteorites, Comets, and Planets, ed. Davis, A. M. Oxford Elsevier, pp. 623-636. This chapter nicely summarizes how the physical and chemical properties of the giant planets are determined and discusses models for the origin of these bodies. 

Water ice is a major component of comets and one that dominates cometary activity and physical evolution. The phase of the ice is of considerable interest, because different phases have quite different properties. At temperatures... 

Perhaps the most important connection between the two men was Halley s use of Newton s theory of gravitation to explain the motion of comets. Within a decade of the publication of the Principia, the scientific world had become convinced of the value of his theories in the explanation of the motion of the planets and of the Moon around the Earth. The same theories also explained a number of well-known physical phenomena that occur here on Earth. Scientists were not so certain, however, that Newton s theories had more general application, such as to the motions of stars, comets, meteors, and other astronomical objects. When Halley undertook to study the orbital properties of the comet of 1682, he was able to show that Newton s theories had much broader applications than had previously been appreciated. 

All planets from Mercury to Neptune and most of their satellites have been observed from Earth-based telescopes and at least once, some repeatedly, from spacecraft. Therefore, sufficient information was available to emphasize the physical principles in the discussions in Chapter 6. Trans-Neptunian objects and asteroids have been explored to a much lesser degree. Their small sizes, for many their large heliocentric distances, and their low surface temperatures prevented detailed exploration. Until recently, only a few samples of an enormous amount of objects have been investigated. Therefore, the treatment of these objects, grouped in this chapter, is primarily a summary of presently known properties. Section 7.1 discusses Pluto and its satellite Charon Section 7.2 is devoted to comets and Section 7.3 to asteroids. 

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