The Debate About The Pristine Nature Of Comets

For many years, comets were considered fully pristine and the most intact material that has been preserved in the solar system. It was thought that comets were held in stasis during their storage in the Oort Cloud or the Kuiper Belt during 4.6 billion years. Now, new results and understanding has changed that perception. Cometary ices have been at least partially processed (Ehrenfreund and Schutte, 2000). Nonetheless, it is widely held that, in fact, comets reflect, at some degree the composition of interstellar grains (Strazzulla, 1997; Ehrenfreund et al., 2001; Szopa et al., 2003).

There are essentially three phases during which alteration of comets can occur: formation, residence in the reservoirs, and, a very important one, passage through the inner solar system, (Kochan et al., 1998). Irradiation of those bodies starts since the comets are formed from interstellar grains; UV photons and cosmic ions had already interacted with those grains (Hudson and Moore, 1999; Strazzulla, 1999). Later, during their storage in the reservoirs mentioned, their surfaces were continuously irradiated by cosmic rays. All of this exposure alters the chemical and physical properties of those bodies (Hudson and Moore, 1999).

Comets have experienced ionizing particle flux due to cosmic rays and UV photon flux. Donn (1976) suggests that accumulated irradiation doses in the outer layers of comets could be enough to polymerize the simple volatile original ice. It has been also suggested (Strazzulla and Johnson, 1991) that comets exposed to background particle radiation in the Oort Cloud obtain another web of nonvolatile material which leads to the formation of a "crust." All of these irradiations alter the chemical and physical properties of cometary ices. synthesizing new molecules and destroying others. As a consequence, the observed molecules during the passages of comets near the Sun are the result of, among other things, the irradiation of ice material at low temperatures (Hudson and Moore, 1999). Many of the observed products in the comae of comets are not the result of direct sublimation of the ice in the nucleus, but evidence of a continuous transformation.

It has been recognized that many processes contribute to the evolution of material in those bodies (Strazzulla and Moroz, 2005). Energy sources include particles, ions, and photons (interstellar radiation field); ionizing radiation mainly includes cosmic rays (mostly H+ and He+), solar wind particles and magnetospheric particles, and gamma rays (Johnson, 1990). The processes that result from those interactions are thermal, collisional and radiation processing (Stern, 2003). Almost all of these processes have been modeled in the laboratory. Besides that, comets may undergo modifications such as stratification, resulting from the phase change of the icy component produced by solar heating (Henrique et al., 1999; Capria et al., 2003). Actually, the nucleus is a structure with stratified ice, not only in terms of density, but also in temperature and porosity (Ehrenfreund et al., 2002).

It is essential to note that neither the origin nor the degree of alteration of cometary material is well understood (Ehrenfreund and Charnley, 2000). This is because more information is needed in order to obtain a complete picture of comets.

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