K. S. Noll
The high cosmic abundance of oxygen, the relatively high condensation temperature of water ice, and favorable thermochemical equilibrium behavior at low temperatures conspire to guarantee that water ice is a key component in the assembly of solid bodies in the solar system at and beyond Jupiter's distance from the sun. From the measured mass and radii of the regular satellites of Juptier and Saturn and models of satellite interior structure it can be determined that Ganymede, Callisto, and Titan are each approximately 40% water ice in bulk composition with the remainder predominately silicate rock though the internal distribution of these components is not fully known. Europa has a higher bulk density indicating that water ice on its surface is a relatively thin veneer. Saturn's satellites Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus have lower densities indicating bulk compositions with 60% or more water ice. <\P>
Infrared spectra, particularly in the 1-3 micrometer wavelength range, are the primary data from which the presence and physical state of water ice on the surface of these satellites can be deduced. Relatively strong absorption bands occur in water ice at 3.0, 2.0, and 1.5 um with weaker bands falling at 1.25 and 1.04 um. The identification of these bands in spectra of Europa and Ganymede was the first evidence for the presence of water ice on these bodies. These bands have since been observed in Callisto and in Saturn's moderate-sized satellites. Saturn's large satellite Titan represents a special case because of its dense atmosphere and haze that make spectroscopic observations of its surface extremely difficult. Despite these barriers, however, isolate spectrophotometric measurements of Titan's surface and evidence for inhomogenous reflactance of the surface suggest the possible presence of water ice on the surface of Titan.
All of these satellites are tidally locked to their parent planet and therefore rotate with a period equal to the period of their revolution. Variations in the reflectance as a function of orbital position (and therefore longitude) are observed for virtually all of these satellites indicating the presence of a non-ice component on the surfaces of these objects. Resolved images from spacecraft flybys and HST show numerous albedo features that similarly necessitate the presence of non-water-ice components on the surface. From cosmochemical considerations it can be argued that silicate minerals should be present on the surfaces of these objects. Infrared spectra at wavelengths of 3 um and longer suggest that, indeed, silicate minerals such as olivine may be present on the surfaces of Callisto and Ganymede. All the icy satellites exhibit a strong absorption at ultraviolet wavelengths that is not attributable to water ice. Several possible sources for this absorption have been proposed including silicate minerals and iron-bearing minerals, but not firm identification has been made.
Environmental factors play an important role in altering the composition of satellite surfaces. Jupiter's and Saturn's satellites (except Iapetus and sometimes Titan) orbit within their parent planet's magnetosphere so that their surfaces are subject to large fluxes of high energy ions trapped in the magnetic fields. The principle consequence of this irradiation appears to be the buildup of oxygen in the surface ice which is manifested in a number of observables including UV and IR absorption by sulfur dioxide ice on Europa and Callisto, molecular oxygen and ozone trapped in the surface ice of Ganymede, possible IR absorption by carbon dioxide ice in Jupiter's satellites, indirect evidence for molecular oxygen atmospheres around Europa and Ganymede, and UV absorption by ozone trapped in the surface ice of Rhea and Dione. Micrometeorite gardening of the surface may also play a role particularly for Iapetus and Callisto. The most conspicuous inhomgeneity in either satellite system, the hemispheric dichotomy present on Iapetus, remains unexplained despite many attempts to do so.
Contact Information:
Keith S. Noll
Space Telescope Science Institute
3700 San Martin Drive
Baltimore, MD 21218
Telephone: (410) 338-1828
email: noll@stsci.edu