56 



impressions gained from ground-based and Pioneer spacecraft obser- 

 vations. The four large satellites of Jupiter exhibit gradients in aver- 

 age density and other properties that are strikingly reminiscent of 

 the gradient observed among the planets. The inner satellites, Io and 

 Europa, have densities of 3.5 and 3.0, respectively, indicating a pre- 

 dominantly rocky composition. In contrast, the more distant satel- 

 lites Ganymede and Callisto have densities of 1.9 and 1.8, respec- 

 tively, suggesting compositions that include a high percentage of 

 carbon, nitrogen, and oxygen compounds and, presumably, are 

 dominated by water (ice). 



The surface appearances of these objects substantiate this inter- 

 pretation. The icy crusts of Ganymede and Callisto have evidently 

 been unable to support the topography associated with large-impact 

 craters, although crater densities in the smaller size range are close to 

 saturation. The surface of Europa appears to be covered with a 

 layer of water ice that has obliterated any trace of its history of early 

 bombardment. The few craters that do appear are comparable in the 

 number per unit area to that found on Earth. Io is so wreaked by 

 continuous volcanism that its entire surface must be reworked on a 

 time scale that is very short compared with 4.6 b.y. As a result of its 

 tidally induced volcanic activity, this satellite appears to be exten- 

 sively degassed, with the volcanoes possibly relying on sulfur dioxide 

 as a working fluid instead of water. 



While obviously much smaller than the Sun, Jupiter is appar- 

 ently large enough to have caused the same gradient in the properties 

 of its retinue of satellites as the Sun has caused in the planets 

 (fig. IV-3). This fractionation of material can be attributed to the 

 heat released during the formation of the central body. The resulting 

 similarity between these two systems (Jupiter's and the Sun's) 

 strengthens our intuitive feeling that the gross characteristics of the 

 solar system are probably representative of those found elsewhere; 

 with small dense inner planets possessing secondary degassed 

 atomospheres. The exploration of the Saturn system by 

 Voyager has introduced an important caveat: For this gradient 

 in properties to exist, the central body must be massive enough 

 to heat the surrounding space during the time the system forms, 

 and the planets or satellites must be large enough to represent 

 a homogeneous sample of the accreting material. Neither of these 

 conditions was met in the case of Saturn (fig. IV-4), with the result 



