The Oort Cloud

Already in 1932 E. Opik assumed that long-period comets originate in a cloud at very large distance from the inner solar system. Since the long-period comets have random orbits, such a cloud must be distributed spherically around the solar system. Oort, 1950 [255] postulated its existence from the volatile composition of comets and their orbits (see Fig. 5.2). He found a peak in the numbers of nearly isotropic distributed comets with apheUa of roughly 20 (X)0 AU. 

The cloud of cometary like bodies that encloses spherically the solar system at a distance of about 50 000 AU (about 0.75 light year) is named Oort cloud. Newer investigations have revealed that it is in fact composed of two parts  

The Oort cloud is the source of long period comets. Especially the objects in the outer cloud are only loosely bound to the Solar System and can be easily disturbed 

The mass of the outer cloud is assumed to be about 5 times the mass of the Earth, the number of objects larger than about 1 km is several 10. The majority of the objects consist of ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide. There are also indications, that rocky objects may exist there. 

To explain the origin of the Oort cloud, we must go back to the time of the formation of the solar system 4.6 billion years ago. The population of the Oort was originally 

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Binzel et al. (1991) give an account of the origin and the development of the asteroids, while Gehrels (1996) discusses the possibility that they may pose a threat to the Earth. The giant planets, and in particular Jupiter, caused a great proportion of the asteroids to be catapulted out of the solar system these can be found in a region well outside the solar system, which is named the Oort cloud after its discoverer, Jan Hendrik Oort (1900-1992). Hie diameter of the cloud has been estimated as around 100,000 AU (astronomic units one AU equals the distance between the Earth and the sun, i.e., 150 million kilometres), and it contains up to 1012 comets. Their total mass has been estimated to be around 50 times that of the Earth (Unsold and Baschek, 2001). 

The source of long-period comets is thought to be the Oort cloud (Weiss-mann, 1998). 

The latter group was probably responsible for the early bombardment of the protoplanets. Delsemme believes that the cometary nuclei of the members of the Jupiter family never experienced temperatures greater than 225 K. The values suggested for the others are Saturn family, 150K Uranus family, 75 K Neptune family, 50 K. During many million years, these comets got mixed together in the Oort cloud (which has a diameter of around 50,000 AU). 

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. 

Hale-Bopp Comet from the Oort Cloud with a short period, discovered in 1995 when it was 7AU from the Sun. 

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. 

Given that interstellar ices are the building blocks of comets and comets are thought to be an important source of the species that fell on primitive Earth, the structures of molecules in comets may be related to the origin of life. It is possible that organic materials formed in the solid ice phase of interstellar materials provided raw materials used for life originating solely on Earth. If so, the deep freeze of ice in the Oort cloud would have been an excellent place to store these, especially the unstable ones, awaiting delivery to a planet. 

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. 

Figure 2 Distribution of l/a in units of 10 AU the inverse of the semimajor axes (half length of the orbital ellipse) of 264 LP comets. The peak near zero contains dynamically new comets coming directly from the Oort cloud for the first time. Comets with larger values of l/a have previously entered the planetary region of solar system and have been perturbed into smaller lower-energy orbits (source Marsden, 1989b).

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... 

Stern S. A. and Weissman P. R. (2001) Rapid collisional evolution of comets during the formation of the Oort cloud. Nature 4m, 589-591. 

S4 [90,91] (Table 16.2). A typical comet composition is based on the relative amounts of the different gas species measured in the coma of comets from the Oort cloud, an immense spherical cloud surrounding the solar system and extending from approximately 5000 to 100,000 AU. About half of the short-period comets arc from the Kuiper Belt, a disk-shaped region containing many small icy bodies extending beyond Neptune s orbit from 30 to 50 AU, which are depleted in carbon-chain molecules. The other short-period comets have the typical composition of Oort cloud comets. So far, carbyne molecules detected in the coma of active comets are very short chains unlike the longer chains characteristic of natural carbyne crystals such as chaoite in the Murchison meteorite [20,67]. 

Astronomers have used the differing orbital paths of short- and long-term comets to hypothesize a possible source for each kind of comet. The first person to devise such a theory was the Dutch astronomer Jan Hendrick Oort (1900-92). Oort argued that, since long-term comets could appear from any point in the sky, their "home" must reside outside the solar system. Oort calculated that this "home" would consist of a spherical shell of debris located between 50,000 and 100,000 AU from Earth. This shell or cloud (now known as the Oort cloud) would be very stable, and individual pieces (comets) would be torn away only when the solar system passed close to a star, an interstellar cloud, or some other massive body. In such cases, a comet would be propelled out of the Oort cloud either toward the center of the solar system, becoming a comet visible to Earth, or away from the solar system, where it would be lost to interstellar space. 

When the universe was forming, most of the material became concentrated in the planets and moons. There were however many small, rocky objects called planetesimals that also formed from the gas and dust. These planetesimals include comets and asteroids. A large cloud of comets, known as the Oort cloud, exists beyond Pluto. A change in the gravitational pull of our galaxy may disturb the orbit of a comet causing it to fall toward the Sun. The ice in the comet turns into vapor, releasing dust from the body. Gas and dust then form the tail of the comet. 

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