proportion and consequently the atmospheres should also be in solar propor- 

 tions. The situation for the second model is not so simple because various frac- 

 tionation processes can occur during nucleation, aggregation, and accretion. 

 Thus the initial elemental composition would not have been preserved. Calcula- 

 tions suggest that, as compared to solar composition, the heavier elements 

 (carbon, oxygen, nitrogen, etc.) should be enriched relative to the lighter hydro- 

 gen and helium. This model would favor the development of an atmosphere 

 richer in the biogenic elements and more conducive to the abiotic synthesis of 

 biogenic compounds and even, possibly, some form of life. 



The data that are available for Jupiter and Saturn show factors of 2 to 3 

 enrichment in carbon (primarily in the form of methane, CH 4 ) relative to solar 

 abundances and apparently a similar enrichment in nitrogen, although the latter 

 is complicated. Nitrogen occurs mostly as ammonia, NH 3 , that is not uniformly 

 distributed in the atmosphere because it freezes out and also reacts with hydro- 

 gen sulfide, H 2 S, another atmospheric gas. These data support the nucleation 

 model. However, oxygen (as water, H 2 0) is depleted in the Jovian atmosphere 

 by about a factor of 30. This could be because water condenses to the liquid or 

 solid state at the higher elevations: water vapor exists only lower in the atmo- 

 sphere where observation is difficult. Sulfur has not yet been detected, but 

 H 2 S is very photolabile (to form elemental sulfur) and readily reacts with NH 3 

 to form solid compounds. The abundance of phosphorus varies too much to be a 

 useful discriminator, perhaps because of the reaction with H 2 in the lower 

 atmosphere. 



Isotopic data are of poor quality and provide conflicting evidence: Low- 

 temperature equilibration of hydrogen and water in icy materials leads to an 

 enrichment of deuterium of up to five times solar, but observations of hydrogen 

 deuteride (HD) and deutero-methane (CH 3 D) suggest a nearly solar ratio. This 

 result implies that low-temperature H/D equilibration has not contributed to the 

 isotopic composition and therefore that aggregation, fractionation, and accretion 

 of an atmosphere characterized the formation of Jupiter. However, data on the 

 carbon isotopes suggest that 12 C/ 13 C is about 1.8 times solar, opposite what 

 should be observed if fractionation processes did take place. One hypothesis is 

 that the Jovian 12 C/ 13 C ratio is representative of that in the primordial nebula, 

 but if this is true why is the ratio different from that in the Sun? It may be that 

 the isotopic composition of the solid carbonaceous condensates in the outer 

 solar system differed from that of the bulk of the carbon that prevailed as gas. 



Obviously, higher-quality observations than those currently available are 

 necessary to more accurately define these isotopic ratios on Jupiter and Saturn 

 and to determine them on Uranus and Neptune. The relative isotopic composi- 

 tions of these four planets may be required to disentangle clues to the formation 

 processes from signatures of early solar nebula conditions. For H 2 , HD, CH 4 , 

 CH 3 D, and NH 3 , most of the observations have been made on molecular lines 

 that occur in the visible and near-infrared regions of the spectrum. Measurements 



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