Comet Oort Cloud

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. 

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

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

Oort Cloud A source of comets in the solar system at a distance of 50 000 AU. 

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. 

Compositional variations among comets, based on the production rates ("Q") of carbon-chain (C2 and C3) and NH molecules relative to water (OH). Filled symbols are Kuiper belt comets, and open symbols are Oort cloud comets. After A Hearn etal. (1995). 

Although comets are not expected to have experienced the thermal processing that asteroids have, some of the larger KBOs and Oort cloud objects may have been heated by decay of radionuclides. The relative proportions of rock and ice may determine the amount of heating, as radionuclides occur in the rock fraction. Comets and asteroids may have had similar impact histories, and many of these bodies may now be collisional fragments. 

Distinguish between comets, Kuiper Belt Objects (KBOs), and Oort cloud objects. 

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. 

In the 1950 s, three major concepts have brought d isive advances to the physics of comets Oort s model of a distant cometary cloud, Biermann s solar wind theory of tail formation and dynamics, and Whipple s icy conglomerate model of the cometary nucleus. The main ideas of these concepts shall be outlined briefly. 

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).

Originally it was thought that SP comets were Oort cloud comets that had been captured into short-period orbits following a close encounter with Jupiter. This process can occur and even explain why SP comets usually have prograde orbits with relatively low inchnations. However, Fernandez (1980) pointed out that this source could not produce the number of SP comets that are observed, and suggested that the SP comets were derived from a disk-hke distribution of bodies beyond Neptune. It was shown by numerical simulations that this trans-Neptunian distribution of comets could quantitatively supply the SP flux and explain their inclinations (Duncan et ok, 1988). 

Mumma, M. J., DiSanti, M. A., DeUo Russo N., Magee-Sauer K., Gibb E., and Novak R. (2002) The organic volatile composition of Oort-cloud comets evidence for chemical diversity in several Oort cloud comets. American Astronomical Society, DPS meeting 34. 

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

Ip and Fernandes [ 101] calculated that 6x lO to 6x lO s g of cometry material could liave been delivered to Earth at the time of the formation of the great Oort Cloud of comets. This amount is equivalent to 4-40 times the present mass of the oceans, assuming about 50% of the cometaiy mass is ice. Owen and Bar-Nun [102] examined tlie ability of amorphous ice formed at temperatures below lOOK to trap ambient gases. By comparison of the compositions of gases trapped by ice with tlie compositions of the interstellar medium, comets, and planetary atmospheres, Owen and Bar-Nun [102] concluded that icy comets delivered a considerable fraction of the volatiles to the imier planets. Owen [83] emphasized that Uie potential supply of cometaiy materials is more than adequate. 

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