What are Comets

Comets are small bodies with diameters of a few kilometers (1-15), with masses of the order of 1015 to 1018 g (Lazcano-Araujo and OrĂ³, 1981) that can be detected only when they approach the Sun. When this happens, the surface volatiles sublimate, creating an atmosphere of dust and gas which is referred as the coma (Stern, 2003). Due to their small sizes, comets do not have much gravity, and, thus, their coma largely expand from their surfaces. Deep impact, Stardust and Giotto missions have made a great contribution for a better understanding of cometary nuclei.

There are severals models of comets. The early model of a "dirty snowball" proposed by Whipple in the 50s has been modified since that time. This model suggests that comets are made basically of water ice and rock particles (Ehrenfreund and Charnley, 2000). There are some others models which describe the structure and composition of comets; for example, there is the fractal and the primordial rubble pile proposed by Donn (1990). According to the first model, a comet is formed by small cometesimals (around 100 m) weakly bound to each other, forming a porous structure with internal void spaces. The second model is similar, but, in it, the cometesimals are closely packed together by evaporation and ice freezing (Weissman 1986).

Another model suggests that comets are made of icy, rocky, and organic components (Delsemme, 1983). Greenberg (1998) proposed that up to 30% of a comet's mass may be water ice, 26% silicates, 23% refractory material, and 9% carbonaceous molecules. Recent observations have shown that cometary ice is predominantly made of water; in fact, water-rich ice is an important component of dense molecular clouds, and is common throughout the solar system; this is followed by CO and CO2 and by small quantities of organic constituents. It is important to note that water is a common molecule in the interstellar medium and, in general terms, in astrophysical environments (Ehrenfreund and Charnley, 2000); this is corroborated in comets, where water ice constitutes the major component in which the others are diluted (Gerakines et al., 2001). The Deep Impact mission on Comet 9P/Tempel 1 revealed the presence of H2O, C2H6, and HCN (Disanti et al., 2007). Other authors, such as Mumma et al. (2005), quantified eight parent volatiles (H2O, C2H6, HCN, CO, CH3OH, H2CO, C2H2, and CH4) in the same comet (Tempel 1) using high-dispersion infrared spectroscopy. The results obtained suggest that the volatile ice in Tempel 1, and in most Oort Cloud comets, originated in a common region of the protoplanetary disk. There could still be a number of undetected products in comae due to their low concentrations and high vapor pressures (Cottin et al., 2001).

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