Satellites of the Outer Planets Evolution of Ice-Rock Bodies

1. Homogeneous mixture of rock and ice.
2. At depth of 2 kbar pressure, a thin internal layer of dirt and water is formed.
3. Melting progresses, leaving only an inner core and thin crust remain undifferentiated.
4. Outer shell differentiates, leaving an outer layer of convecting water/ice over a muddy layer, which in turn lies on an undifferentiated core.
Note: the larger the body, the further it proceeds along this evolutionary path.

Overview of Jupiter's Galilean Satellites

I. Galilean satellites are the largest of Jupiter's moons, visible with low power telescopes and first described by Galileo. In order of distance from Jupiter: Io, Europa, Ganymede, and Calisto.

II. Bulk densities and radii. Note: bulk density decreases with distance from Jupiter, analagous to the behavior of planets' distance from the Sun, suggesting a process similar to a condensation sequence also exists for this "small solar system." This simply means that there must have been a temperature gradient established early in the history of condensation and acretion of these satellites such that temperatures decreased with distance from Jupiter. Origin of the temperature gradient could be due to internal heat source of Jupiter and/or decrease in tidal heating away from Jupiter

A. Io bulk density :3.53 gm/cm3, radius 1820 km.

B. Europa: bulk density 3.03 gm/cm3, radius 1552 km

C. Ganymede: bulk density 1.93 gm/cm3, radius 2635 km

D. Calisto: bulk density 1.83 gm/cm3, radius 2420 km.

III. Thermal structure. Blackbody radiation gives average surface temperatures of 100 deg K. Temperature profiles inferred from assumption of an outer conductive layer having a steep gradient up to a melting temperature for high pressure phases of ices, followed by a near adiabatic gradient in a convective zone below.

IV. Surface coloration of Galilean satellites (salt and pepper model): ices appear bright; soils dark. If enough heating occurred in formation of satellite, soils sink to center, making planet have an icy bright surface. Further heating evaporates water, leaving compounds such as sulfur at the surface. In this way, bulk and surface temperature variations effect the surface features and colaration of the Galilean satellites.

Specifics of Galilean Satelites

Io Interior must be rocky with little or no ice content. Red, yellow, and bright white surface.

I. Io Heat source: Tidal heating causes intense volcanism, ejecting sulfur ash. Io is strongly affected by Saturn's magnetic field, inducing currents in its near surface layers that might be sufficient to keep a shallow conductive layer molten (analagous to the plate decoupling zone on Earth). Orbital resonances of Galilean satelites and its orbital eccentricity act to make tidal bulge oscillate back and forth along line connecting centers of mass of Io and Jupiter, greatly increasing tidal friction.

II. Io interior structure: deep mantle of very FeO rich silicates, overlaid by a strongly heated, low viscosity layer of molten silicates with a silicate crust floating on it. Molten sulfur ocean above silicate crust. Sodium and sulfar atoms dislodged by solar radiation account for a yellow auroral glow around Io. Silicate and sulfur volcanism both likely, with an outer sulfur layer overlying a silicate crust. .

III. Io surface topography: Absence of impact craters suggests actrive surface.Volcanic collapse craters. The silicate curst that underlies an outer sulfur layer is the only layer strong enough to account for elevation differences of up to 9 km. Temperature of surface hotspots is consistent with liquid molten sulfur (386 deg K to 718 deg K.)

Europa Near absence of craters indicates active surface processes. Surface is nearly 90% ice but density consistent with rocky interior. Network of lines observed on surface and presence of water detected. Ice tectonics. Small magnetic
field thought to be generated by currents in a subsurface salty water layer. Currents in this water layer are induced
by the strong external magnetic field of Jupiter.

Ganymede Density consistent with a thin icy curst overlying a watery interior. Cratered surface with faults suggest early activity but currently inactive surface.

Calisto Carbonaceous soil mixed with water ice. Cratered surface. Difference in lack of early tectonic activity on Calisto compared to Ganymede due to either early hotter initial temperature on Ganymede or heating by large impact on Ganymede in its early history.

Larger Satellites of Saturn

In order from Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus. These range in radius from 200 km to 700 km., except Titan, which has a radius of 2575 km. Densities range from 1.2 to 1.4 gm/cm³. Titan is 1.9 gm/cm³.Compositions probably dirty ices, dominated by H₂O. Surface tectonic activity variable depending on early thermal history and tidal heating.