48 



planetesimal-like building blocks for some of the outer planets and 

 their satellites. Present knowledge places the origin of comets in the 

 outer regions of the primitive solar nebula, both in and beyond the 

 space now traversed by the giant planets (Jupiter, Saturn, Uranus, 

 and Neptune). Perturbation of their original orbits, by the formation 

 of these giant planets, is believed to have sent some protocometary 

 bodies into the inner solar system (to collide with the Sun and inner 

 planets — Mercury, Venus, Earth, and Mars) and others into orbits 

 extending great distances from the Sun (up to 50,000 astronomical 

 units (AU); 1 AU = 150X10 6 km, or the distance from the Sun to 

 the Earth). 



Comets consist of a nucleus, a coma, and a tail (see fig. IV- 1). 

 According to a current model, comet nuclei contain simple and com- 

 plex organic molecules, and meteorite-like dust and rock imbedded 

 in a matrix of frozen water, possibly solid carbon dioxide and other 

 ices. As comets approach the Sun, heating occurs and the ices vapor- 

 ize, ejecting volatile "parent" compounds (possibly water, carbon 

 dioxide, methane, acetylene, ammonia, hydrogen cyanide, etc.) and 

 entraining nonvolatile dust and rock from the nucleus. In the coma 

 that results, interactions of the gaseous parent compounds with solar 

 radiation can lead to physical and chemical processes that cause the 

 partial to complete breakdown of the so-called parent molecules to 

 "daughter products." The uncharged daughter products are observed 

 in the coma, whereas the positively charged ones are observed in the 

 tail. According to an alternative view, all the observed daughter prod- 

 ucts already existed "frozen" in the nucleus of the comet, and were 

 simply released directly into the coma by evaporation. In addition to 

 the species indicated in table IV-2, metallic elements (iron, silicon, 

 magnesium, calcium, nickel, sodium, chromium) have been detected 

 by means of spectroscopic analysis of comets that pass very close to 

 the Sun, and of meteor showers associated with comets. The relative 

 abundances of these elements suggest similarities between the chemi- 

 cal compositions of cometary dust and carbonaceous meteorites. 



The nucleus of a comet is thought to be small, typically 1 to 

 10 km diam, but no direct measurement of a nucleus has ever been 

 made. Nuclei appear as small points of light imbedded within the 

 bright and extensive coma of the comet. The mass of nuclei could 

 range from 10 1 5 to 10 1 8 g. The light from the visible coma and tail 

 is emitted by atoms and molecules that have interacted with solar 



