Vision and Voyages for Planetary Science in the - Solar System ...

8
Satellites:
Active Worlds and Extreme Environments
This chapter is devoted to the major satellites of the giant planets: those large enough to have
acquired a roughly spherical shape through self-gravity. There are 17 of these worlds (four at Jupiter,
seven at Saturn, five at Uranus, and one at Neptune), ranging in diameter from 5260 kilometers
(Ganymede) to 400 kilometers (Mimas) (Table 8.1, Figure 8.1). They are astonishingly diverse, with
surface ages spanning more than 4 orders of magnitude, and surface materials ranging from molten
silicate lava to nitrogen frost. This diversity makes the satellites exceptionally interesting scientifically,
illuminating the many evolutionary paths that planetary bodies can follow as a function of their size,
composition, and available energy sources, and allowing us to investigate and understand an exceptional
variety of planetary processes. However, this diversity also presents a challenge for any attempt to
prioritize exploration of these worlds, as we move from initial reconnaissance to focused in-depth studies.
The sizes, masses, and orbits of all the large satellites are now well known, and are key
constraints on the origin of the planetary systems to which they belong. Additional constraints come from
their detailed compositions, which we are just beginning to investigate. Several worlds have unique
stories to tell us about the evolution of habitable worlds, by illuminating tidal heating mechanisms,
providing planetary-scale laboratories for the evolution of organic compounds, and by harboring
potentially habitable subsurface environments. Many of these worlds feature active planetary processes
that are important for understanding both these bodies themselves, and worlds throughout the solar
system. These processes include silicate volcanism, ice tectonics, impacts, atmospheric escape,
chemistry, and dynamics, and magnetospheric processes.
While much can still be learned from ground-based and near-Earth telescopic observations,
particularly in the temporal domain, and from analysis of existing data, missions to these worlds are
required to produce new breakthroughs in our understanding. During the past decade, our understanding
of these worlds has been substantially expanded by the Cassini spacecraft and its Huygens probe that
descended to Titan in 2005. Data from Cassini continues to revise and expand what we know about
Saturn’s moons. In addition, continued analysis from past missions such as Galileo has produced
surprises as well as helping inform the planning for future missions.
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
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