Comet, Great

Figure 3. Morphometric documentation of individual comet lengths in UV-B untreated bovine passage I lens epithelial cells (n = 154). During culturing and harvesting cells were not differentiated into those from central and preequatorial region. About 10% of the cells show comets greatly deviating from the mean of 33,97 4-/-18,34 in one direction only. LEPZ = lens epithelial cells.

As flow velocities increase, chelant attack becomes substantially worse, with the flow pattern being reflected in the form of U-shaped depressions and long tails (comet tails). Thinning continues until boiler failure occurs through a rupture of the thinned metal surface. In areas of high stress and/or high turbulence, attack is greatly enhanced and may be very localized. 

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). 

We have learned a great deal about comets in the intervening centuries, but there still remain some unanswered questions. 

Fig. 3.9 Greatly simplified representation of the path taken by the material under study, beginning with nucleosynthesis and ending with laboratory analysis. Circumstellar dust (a component of the primeval presolar nebula) which was contained in asteroids or comets came to Earth in meteorites and was then available for exact study (Lugmair, 1999)...

Cometary activity occurring at great distance from the Sun (corresponding to temperatures <100 K) is probably controlled by ices more volatile than H20. For example, comet Hale-Bopp exhibited emission of highly volatile CO at great solar distances. Trapped CO was presumably released by crystallization of amorphous ice or sublimation of ice crystals at very low temperatures. 

It may be my fate to be a kind of comet, or flaming meteor in science, in the regions of which (like enough to a meteor) I made my appearance very lately, and very unexpectedly and therefore, like a meteor, it may be my destiny to move very swiftly, bum away with great heat and violence and become as suddenly extinct. 

The sedimentary evidence implies the existence of oceans. Although the initial deep volatile inventory of the planet would have been removed by the late great impact that formed the Moon, much of the water presently in Earth s oceans would have degassed from the hot mantle or infallen as comets soon after that great impact, and the ongoing volcanism would have added more. 

A contemporary and friend of Newton, Halley, observed a comet in 1682 and suspected others had observed it many times before. Using Newton s new mechanics (laws of motion and universal law of gravity) Halley calculated that the comet would reappear at Christmas, 1758. Although Halley was dead, the comet reappeared at that time and became known as Halley s comet. This was a great triumph for Newtonian mechanics. 

These events liave sporadically happened even in our history. The Meteor Crater of Arizona is tliought to liave been formed between 20,000 and -30,000 years ago [58], Explosion of Tunguska meteor in 1908 and collision of Comet Shoemaker-Levy 9 against Jupiter in 1994 are the representative great events in the history. 

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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