The Kuiper Belt

The group of small solar system bodies, SSSBs, comprises asteroids, comets and smaller objects. The most part of the smaller objects is concentrated in belts. The orbits of the objects of the Main Asteroid Belt between Mars and Jupiter and the Kuiper Belt outside the orbit of Neptune are concentrated near the ecliptic (plane of the solar system), the objects of the Oort cloud are distributed spherically and extend as far as to 50 000 AU from the Sun. 

Comets and asteroids consist mainly of unchanged remnant debris from the solar system formation process some 4.6 billion years ago. This and the fact that many impacts of these objects on terrestrial planets especially in the early solar system occurred explain the interest to study them. 

The Kuiper belt extends from about the orbit of Neptune (30 AU) to about 55 AU and even more. The first Kuiper Belt Object, KBO, was discovered in 1992, presently several thousand KBOs are known. Their total number is estimated over 70 000 for objects over 100 km in diameter. It can be compared with the asteroid belt which occurs between the orbits of Mars and Jupiter but there are two important differences  

Hanslmeier, Water in the Universe, Astrophysics and Space Science Library 368, DOI 10.1007/978-90-481-9984-6 5, Springer Science+Business Media B.V. 2011 

The existence of two different populations follows not only from the difference in their inclination but also from the fact they differ in color. The authors conclude that the hot population was formed by a single close stellar encounter at a distance of 80 000 AU. These caused an increase in the eccentricities of the hot population objects and their perihelia moved to 35 AU. Because of the outward migration of planet Neptune (see e.g. Levison and MorbidelU, 2003 [200]) most of them were removed, less then 10% remained in stable orbits. This is consistent with the observations of the present distribution. Neptune s migration was also studied by an N-body simulation (Hahn, Malhotra, 2005 [152]). Neptune s migration can explain Neptune s 5 2 resonance and other resonances. Also Centaurs can be produced by such simulations. It is also estimated that the total number of KBOs having radii 50 km orbiting interior to Neptune s 2 1 resonance is A 1.7 x 10.  

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In August 2006, the International Astronomical Union redefined the term planet and decided that the former ninth planet in the solar system should be referred to as a dwarf planet with the number 134340. The dwarf planet Pluto and its moon, Charon, are the brightest heavenly bodies in the Kuiper belt (Young, 2000). The ratio of the mass of the planet to that of its moon is 11 1, so the two can almost be considered as a double planet system. They are, however, quite disparate in their composition while Pluto consists of about 75% rocky material and 25% ice, Charon probably contains only water ice with a small amount of rocky material. The ice on Pluto is probably made up mainly of N2 ice with some CH4 ice and traces of NH3 ice. The fact that Pluto and Charon are quite similar in some respects may indicate that they have a common origin. Brown and Calvin (2000), as well as others, were able to obtain separate spectra of the dwarf planet and its moon, although the distance between the two is only about 19,000 kilometres. Crystalline water and ammonia ice were identified on Charon it seems likely that ammonia hydrates are present. 

Short-period comets are thought to have originated in the Kuiper belt (Luu and Jewitt, 1996). 

It has recently been suggested that the comets also went through a number of subtle, but important, evolutionary processes in the Oort cloud and the Kuiper belt. Thus, their present nature is probably not the original one, as was previously thought (Stern, 2003). The assumption that the material which comets contain is in the same state as it was when the solar system was formed must be revised or modified. The evolutionary mechanisms to which they were subjected are likely to have changed their chemical composition. 

When, many, many million years in the future, our sun expands in its Anal phase to become a red giant, the habitable zone of our solar system will shift by 1-2 AU, to the region where Triton, Pluto/Charon and the Kuiper Belt are found. This zone is referred to as the delayed gratification habitable zone . All the heavenly bodies in this zone contain water and organic material, so that chemical and molecular... 

Comets Dirty snowballs with short and long orbits originating from the Kuiper Belt and Oort Cloud, respectively... 

In this chapter we will consider the cosmochemistry of ice-bearing planetesimals. We will focus first on comets, because more is known about their chemistry than of the compositions of objects still in the Kuiper belt and Oort cloud. We will then explore asteroids whose ices melted long ago, and we will briefly consider some larger icy bodies, now represented by satellites of the giant planets. The importance of ice-bearing planetesimals to cosmochemistry stems from their primitive compositions, which have remained largely unchanged because of hibernation in a frozen state. 

Although planetesimals that formed beyond the snowline are composed of relatively primitive materials (chondritic solids and ices), their compositions are variable. That should not be surprising, because objects now in the asteroid belt, the Kuiper belt, and the Oort cloud formed in different parts of the outer solar system and were assembled at different temperatures. In a systematic study of the spectra of 41 comets, A Heam el al. (1995) recognized two compositional groups, one depleted in carbon-chain (C2 and C3) compounds and the other undepleted (Fig. 12.18). NH compounds in the same comets show no discemable trend. The depleted group represents comets derived from the Kuiper belt, whereas the undepleted group consists of Oort cloud comets. 

Kuiper Belt a region in the outer Solar System beyond Neptune s orbit populated by small icy planetesimals or Kuiper Belt objects, and dwarf planets. Many short-period comets (possessing orbits of less than 200 years) are thrown into the Solar System from the Kuiper Belt. 

Comets are surviving members of a formerly vast distribution of solid bodies that formed in the cold regions of the solar nebula. Cometary bodies escaped incorporation into planets and ejection from the solar system and they have been stored in two distant reservoirs, the Oort cloud and the Kuiper Belt, for most of the age of the solar system. Observed comets appear to have formed between 5 AU and 55 AU. From a cosmochemical viewpoint, comets are particularly interesting bodies because they are preserved samples of the solar nebula s cold ice-bearing regions that occupied 99% of the areal extent of the solar nebula disk. All comets formed beyond the snow line of the nebula, where the conditions were... 

Comets are not stable near the Sun and they are short lived in regions of the solar system where they exhibit cometary activity. Active comets are derived from two major reservoirs where they can be stored in adequate long-term isolation from solar heating and planetary perturbations. These reservoirs are called the Oort cloud and the Kuiper Belt. It appears that virtually all comets with low-inclination orbits with orbital periods less than 30 yr are derived from the Kuiper Belt while others come from the Oort cloud. 

Like the Oort cloud, the Kuiper Belt was initially hypothetical but, due to its proximity, techniques were eventually developed so that the larger Kuiper Belt objects (KBOs) could be telescopically detected from Earth. In 1992 the first KBO was discovered by Jewitt and Luu (1993). It was a 23rd magnitude object with a diameter of —320 km at an average solar distance of 44 AU. By the end of 2002, over 700 KBOs had been discovered, over 500 since the beginning of 1999. The dramatic rise in detection was due to heroic... 

A remarkable aspect of the Kuiper Belt is the number of large bodies that it contains. Pluto is in 3 2 resonance with Neptune and it is a member of the bodies swept up by Neptune. Due to orbital... 

Choi Y., Cohen M., Merk R., and Prialnik D. (2002) Long-term evolution of objects in the Kuiper Belt zone-effects of insolation and radiogenic heating. Icarus 160, 300-312. 

Morbidelh A., Thomas F., and Moons M. (1995) The resonant structure of the Kuiper Belt and the dynamics of the first five trans-Neptunian objects. Icarus 118, 322-340. 

Flynn G. J. (1996) Sources of 10 micron interplanetary dust the contribution from the Kuiper belt. In Physics, Chemistry, and Dynamics of Interplanetary Dust (eds. B. A. S. Gustavson and M. S. Manner). ASP Conference Series, vol. 104, pp. 171-175. 

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