Origin, bulk chemical composition and physical structure of the Galilean satellites of Jupiter: A post-Galileo analysis

Authors
Citation
Ajr. Prentice, Origin, bulk chemical composition and physical structure of the Galilean satellites of Jupiter: A post-Galileo analysis, EARTH MOON, 87(1), 1999, pp. 11-55
Citations number
109
Language
INGLESE
art.tipo
Review
Categorie Soggetti
Space Sciences
Journal title
EARTH MOON AND PLANETS
ISSN journal
0167-9295 → ACNP
Volume
87
Issue
1
Year of publication
1999
Pages
11 - 55
Database
ISI
SICI code
0167-9295(1999)87:1<11:OBCCAP>2.0.ZU;2-K
Abstract
The origin of Jupiter and the Galilean satellite system is examined in the light of the new data that has been obtained by the NASA Galileo Project. I n particular, special attention is given to a theory of satellite origin wh ich was put forward at the start of the Galileo Mission and on the basis of which several predictions have now been proven successful (Prentice, 1996a -c). These predictions concern the chemical composition of Jupiter's atmosp here and the physical structure of the satellites. According to the propose d theory of satellite origin, each of the Galilean satellites formed by che mical condensation and gravitational accumulation of solid grains within a concentric family of orbiting gas rings. These rings were cast off equatori ally by the rotating proto-Jovian cloud (PJC) which contracted gravitationa lly to form Jupiter some 4 1/2 billion years ago. The PJC formed from the g as and grains left over from the gas ring that had been shed at Jupiter's o rbit by the contracting proto-solar cloud (PSC). Supersonic turbulent conve ction provides the means for shedding discrete gas rings. The temperatures T-n of the system of gas rings shed by the PSC and PJC var y with their respective mean orbital radii R-n (n = 0, 1, 2, ... ) accordin g as T-n proportional to R-n(-0.9). If the planet Mercury condenses at 1640 K, so accounting for the high density of that planet via a process of chem ical fractionation between iron and silicates, then T-n at Jupiter's orbit is 158 K. Only 35% of the water vapour condenses out. Thus fractionation be tween rock and ice, together with an enhancement in the abundance of solids relative to gas which takes place through gravitational sedimentation of s olids onto the mean orbit of the gas ring, ensures nearly equal proportions of rock and ice in each of Ganymede and Callisto. Io and Europa condense a bove the H2O ice point and consist solely of hydrated rock (h-rock). The Ga nymedan condensate consists of h-rock and H2O ice. For Callisto, NH3 ice ma kes up approximate to5% of the condensate mass next to h-rock (approximate to 50%) and H2O ice (similar to 45%). Detailed thermal and structural models for each of Europa, Ganymede and Cal listo are constructed on the basis of the above initial bulk chemical compo sitions. For Europa (E), a predicted 2-zone model consisting of a dehydrate d rock core of mass 0.912M(E) and a 150 km thick frozen mantle of salty H2O yields a moment-of-inertia coefficient which matches the Galileo Orbiter g ravity measurement. For Ganymede (G), a 3-zone model possessing an inner co re of solid FeS and mass similar to0.116M(G), and an outer H2O ice mantle o f mass similar to0.502M(G) is needed to explain the gravity data. Ganymede' s native magnetic field was formed by thermoremanent magnetization of Fe3O4 . A new Callisto (C) model is proposed consisting of a core of mass 0.826M( C) containing a uniform mixture of h-rock (60% by mass) and H2O and NH3 ice s, and capped by a mantle of pure ice. This model may have the capacity to yield a thin layer of liquid NH3. 2H(2)O at the core boundary, in line with Galileo's discovery of an induced magnetic field