ICY MOONS OF JUPITER: WORLDS IN THEIR OWN RIGHT

Oxygen is the third most abundant element in the Solar System, and its condensed, solid form, water ice, is the dominant “crustal” mineral on most bodies far from the Sun, including three of the four major moons of Jupiter: Europa, Ganymede, and Callisto. The geological study of icy worlds began in earnest with the Voyager flybys of these moons in 1979, some 30 odd years ago. Ice is in most respects a perfectly normal geological material, albeit one with a low melting temperature, high homologous strength, and pronounced polymorphism. Unusually, it has the ability to float in its own magma (water), as well as form non-stochiometric compounds (clathrates) with guest molecules such as CO₂. These properties are key to understanding the richness and variety of geological expression on the 3 icy Galilean satellites. Key also are the influences of what lies beneath: rocky interiors or mantles, and in two cases, metallic cores; but exerting paramount control on all geological activity is an exogenically driven, dynamical phenomenon — tidal heating — which determines whether a body is active (Europa), dormant (Ganymede), or extinct (Callisto). In this talk I will review the lessons learned from 30 years of geological study of these 3 amazing moons, with emphasis on endogenic geological processes and evolution through geologic time. I will conclude with 10 great unanswered questions, 2 for Callisto, 3 for Ganymede, and 5 for that arguably most intriguing icy satellite of all, Europa. The overarching questions for Europa are whether it is presently geologically active (there is no doubt as to its vigorous activity over the past several 10s of Myr) and whether there is life (even if only primitive life) in the ocean underneath its floating ice shell.