The Origin And Nature Of Comets

Modern assumptions derived by geochronology and chemical data agree that approximately 4.6x109 years ago, a portion of a dense interstellar cloud collapsed to create the Sun and the solar nebula that, eventually, formed the planets, satellites, comets, and asteroids in our solar system (Stern, 2003). In this way, in the outermost regions of the solar system, where temperatures are very low, silicate and carbonaceous materials behave as condensing surfaces for atoms and molecules from the environment; cometesimals were, thus, formed basically from silicate particles covered by organic materials and ices (Kossacky et al., 1997). As a result, they acquired a covering icy grain mantle (Ehrenfreund et al., 2001). Molecules contained in the presolar nebula could have been conserved into comets soon after they formed (Hartman et al. 1985). In fact, comets and asteroids are considered relicts that can help us to understand the formation and origin of our solar system (Irvine, 1998).

The formation of comets includes not only nucleation, but also the slowly accretion of interstellar grains into icy bodies (Hudson and Moore, 1999). While some of these cometesimals could have been stored in the exterior of the planetary region, close to Neptune's orbit, others were thrown to outer regions, in the Oort Cloud (Fernández, 1999). According to this, there are at least two known big reservoirs for comets: the Oort Cloud and the Kuiper Belt (Amici et al., 2000). Neither of these is directly detectable, but they have been deduced from the orbital properties of comets (Gladman, 2005).

The study of the Oort Cloud has been considered of enormous significance in reference to understanding the formation of the solar system. This region is a spherical cloud that surrounds the planetary region of our solar system at a distance of 104-105 AU from the Sun. The name was given to honor the Dutch astronomer Jan Oort, who first proposed its existence in 1950 (Oort, 1951). Oort's estimations suggested that the population of cometary nuclei in this region could be of 1.9X1011. He also pointed out that these kinds of comets were formed in the planetary region and then ejected by planetary perturbations (Neslusan and Jakubík, 2005). This implies a change in temperature during cometesimal formation at ~100 K and during their ejection into the cloud at ~20 K (Delzeit and Blake, 2001); that is, the cold memory of comets is related to the period of their formation. Comets in the Oort Cloud are weakly bound to the Sun, and any small disturbance can change their orbits, causing their injection to the inner solar system. Comets here extend their orbit up to 10,000-20,000 AU from the Sun; they have long orbits and, as a consequence, are long-period comets (Harold et al., 2001; Stern, 2003). Comets Halley, Hyakutake, and Hale-Bopp are part of the Oort Cloud family.

A second population of comets has been detected; they have shorter periods and their orbits are within the elliptic. This observation suggests a second source region at distances of between 40 and 50 AU. It would be the Kuiper Belt, named after the Dutch astronomer Gerald Kuiper, who suggested the existence of such a reservoir beyond the orbit of Neptune. Since then, around sixty cometary bodies have been detected in planetary orbits.

Cometesimals remain in their orbits, either in the Oort Cloud or in the Kuiper Belt, until an alteration by gravitational effects occurs. This causes their orbits to alter in such a way that their perihelions move closer to the Sun, leading to passages through the inner solar system. These passages cause the vaporization of ice and the appearance of the object as a comet (Wood and Chang, 1985). In general, a comet can be considered an object of low density, formed at low temperatures, with an internal structure that may have been preserved since its origin (Davidsson and Gutiérrez, 2004).

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