55 



variations and their occurrences among different components 

 strongly suggest that more than one source region and/or more than 

 one production mechanism must have been involved. 



Evidence that the clays in carbonaceous meteorites were pro- 

 duced in secondary aqueous alteration processes raises the possibility 

 that some of the organic matter might also have been produced at 

 the same time by alteration of preexisting compounds by water. Pos- 

 sibly, simple species (such as cyanide compounds), which occur 

 abundantly in the interstellar clouds (table IV- 1) and have been 

 observed in comets (table IV-2), could have been present and served 

 as precursors for some of the more complex molecules found in these 

 meteorites. 



From studies of interstellar dust and gas, comets, and meteo- 

 rites the initial conditions in the solar nebula and its subsequent 

 chemical evolution are being elucidated. Continuing investigations 

 into the cosmochemical origins of organic matter are crucial because 

 organic chemistry occurs throughout the cosmos, and the organic 

 matter that results constitutes a molecular and isotopic record of the 

 materials and processes involved in its formation. 



We have outlined a scenario by which the building blocks of 

 solar-system bodies may have developed from dusty and gaseous 

 starting materials. We might ask if there are other solar systems in 

 the universe. We feel that this possibility exists because of the fre- 

 quent occurrence of binary and multiple stars (which now seem to be 

 well over 50% of the star population), and the fact that the separa- 

 tions between most of these binary stars are comparable to the 

 dimensions of our solar system. The inference is that stellar conden- 

 sation tends to form more than one object; when the residual matter 

 is insufficient to form a second star, planets may occur instead. We 

 see this same tendency for multiplicity within our solar system. Only 

 Venus and Mercury are without satellites, and this lack may be 

 attributed to gravitational perturbations caused by the close proxim- 

 ity of these bodies to the Sun. 



COMPARATIVE PLANETOLOGY 



It is instructive to examine the Jupiter satellite system in this 

 context. The Voyager spacecrafts have confirmed and extended the 



