Observability of spectroscopically active compounds in the atmosphere of Jupiter

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The abundances of several hundred volatile compounds have been calculated at several different levels in the atmosphere of Jupiter. Complete chemical equilibrium has been assumed, and a solar-composition, adiabatic-equilibrium model of the atmospheric composition and structure is used throughout. The results which relate to upper limits on the abundances of spectroscopically active compounds in the upper atmosphere are, however, directly applicable to subadiabatic models. The principal results are that only H2, CH4, and NH3 are predicted to be observable with present techniques. These three species are in fact the only compounds conclusively identified in Jupiter spectra to date. A number of plausible previously suggested constituents of the upper atmosphere, such as SiH4, H2S, PH3, H2Se, C2H6, etc. are shown to be absent if equilibrium is attained. A number of different possible cloud-forming condensates are identified, mostly ammonium compounds. Silicon is shown to reside mainly in the gas phase, principally as SiH4, SiO, and Si, and a deep diffuse SiO2 cloud layer is predicted. Should Jupiter have a core, it must be mainly Fe, Ni, Cr, MgO, Al2O3, CaO, and Ti2O3, possibly in the form of an Fe alloy plus a mixture of MgO, spinel, sphene, and some more complex refractory silicates. The presence of a core is not an essential feature of this model.A simple solar-composition adiabatic model for the atmosphere of Jupiter is found to be completely compatible with present observational evidence. It is not, however, possible to conclude that this model is correct: a subadiabatic model or one in which the H2 abundance is varied by as much as a factor of 10 would agree with the observations equally well, except of course with regard to the H: C ratio.A probe to the 500° level of the atmosphere of Jupiter, where the total pressure is several hundred atmospheres, would permit verification or refutation of a number of features of this model. The most crucial experiment, besides a thermocouple or resistance thermometer and a pleasure gauge, would be a small mass spectrometer capable of measuring the abundance of atmospheric constituents up to at least mass number 40. Coupled with spectroscopic and magnetic-field experiments performed by a flyby vehicle, these measurements would be capable of defining a very specific and detailed model of the atmosphere of Jupiter.The general conclusions of this paper apply without substantial modification to Saturn, but not necessarily to Uranus or Neptune, since the latter cannot have solar composition.