Presentation #200.04 in the session “Plenary Panel: Active Outer Solar System Satellites”.
I am often asked which is my favorite icy world, to which I reply “I love them all equally.” Each tells us something of relevance to how icy worlds sustain their activity, and even the inactive icy worlds provide us with context and contrast. Why is Enceladus active but Mimas not? Why did some worlds (e.g., Dione, Ariel, Ganymede) initiate widespread activity but then stall, while other comparable worlds (Tethys, Umbriel, Callisto) never started, resulting in several paired satellite dichotomies? Why has Triton maintained activity for ~4 Gya if orbit circularization was primordial? Why has Pluto developed very recent icy volcanism on the flanks of Sputnik Planitia in the absence of modern-day tidal forces? Post-Voyager mapping of icy moons and planets revealed geologic complexity that drives us to reexamine the simplistic “tides-drive-everything” systemology. Impact-driven hydrothermal processes on ice-rich Ceres point to revealing geochemical and organic processing on bodies within mixed-composition targets such as Ceres, Callisto, Ganymede. Glacial activity on Pluto and precipitation on Titan point to Earth-like processes occurring on bodies in different geochemical systems. Cassini mapping of the icy moons of Saturn revealed fracturing of Tethys, resurfacing of Dione, and multiple shifting episodes of resurfacing on Enceladus. Internal chemistry and dynamics are defining parameters and the closer we examine these systems the greater the complexity required. Obliquity and inter-moon tides add to the mix of possible heat sources. Internal heat driven by composition may be more relevant than post-Voyager consensus held, particularly in solar-orbiting bodies such as Pluto that lack tide-inducing primaries. But the chemistry of multiphase ice systems, especially under heat loads, is simply not understood. Our first task is to determine what ice phases are present in the interior and on the surface, and what volcanic products resulted from heating on bodies such as Dione, Ariel, Charon and Pluto. When did these events take place? The chronology of heating and fracturing events is poorly constrained and is needed to understand the dynamical events leading to their formation. We have much to do.