, Volume 73, Issue 3, pp 237-258

Origin and bulk chemical composition of the Galilean satellites and the primitive atmosphere of Jupiter: A pre-Galileo analysis

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Abstract

A theory for the origin and bulk chemical composition of the Galilean satellites is presented — to coincide with the start of the 2-year orbital tour of this satellite system by the Galileo Orbiter. The theory is based on the author's modern Laplacian theory of solar system origin (Prentice 1978a). The nub of the work reported here is that the Jupiter system is indeed a miniature planetary system that formed by much the same physical and chemical processes that were responsible for the condensation of the sun's own family of planets. In particular, a phenomenon of supersonic turbulent convection which I claim caused the proto-solar cloud to rid excess spin angular momentum, by shedding a concentric family of orbiting gas rings at the present planetary orbits, may also have operated with similar effect within the proto-Jovian cloud.

Several predictions are made for the bulk chemical composition and physical structure of the icy Galilean satellites which, it is hoped, can be tested by the Galileo Orbiter. The mean density of Callisto is consistent with that of a chemically homogeneous body consisting of about 50% rock, 45% water ice, and 5% ammonia ice, incorporated as the hydrate NH3·H2O. Such a higher-than-solar mass abundance ratio of rock to ice arises naturally within the proto-Jovian cloud since (i) only 34% of the available H2O vapor within the gas ring shed by the proto-solar cloud at Jupiter's orbit was condensed in solid form, and (ii) gravitational sedimentation of solids onto the mean orbit of the proto-solar gas ring leads to an enhancement in the heavy element fraction of the captured primitive Jovian atmosphere. All in all, I predict Jupiter's primitive atmosphere to be enhanced by a factor ζen ≈ 2 in its rock mass fraction (including S) and by a factor ≈ 1.3 in its water content, relative to solar abundances. NH3 and CH44 are present in almost solar proportions.

Initially, Ganymede consisted of a chemically uniform mixture of rock and water ice in the proportions 0.524 : 0.476. The observed mean density of this satellite, however, lies midway between the mean densities expected for homogeneous and fully differentiated rock/ice bodies. The calculations presented here suggest that this body is about half-differentiated. I predict that the Galileo Orbiter will find the mean axial moment-of-inertia factor of Ganymede to be 0.35 ± 0.01.

The circum-Jovian gas ring from which Europa condensed had a temperature of 302 K and a mean orbit gas pressure of 2.8 bar. Initially, this satellite consisted of a uniform mix of hydrated rocks, of which brucite Mg(OH)2 was the principal constituent. The observed mean density of Europa coincides with that expected for this mix, provided that its 9.4% native H2O content is now fractionated from the rock and resides at the satellite surface, forming a frozen mantle some 155 km thick. Regretfully, the mean density of Io cannot be matched by the solid composition reported here. Perhaps this satellite has a molten interior.