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

• How hydrostatic are the satellites?
There are also body-specific questions:
• Does Io have a magma ocean and what is the compositional range of its magmas?
• What is the origin of the topography of Io?
• What is the magnitude and spatial distribution of Io’s total heat flow?
• What are the thickness of Europa’s outer ice shell and the depth of its ocean?
• What is the magnitude of Europa’s tidal dissipation, and how is it partitioned between the
silicate interior and the ice shell?
• What is the relationship between Titan’s surface morphology and internal processes,
particularly for the history of the methane budget and lakes or seas and possible replenishment of methane
from the interior or subsurface?
• Does Titan have an internal liquid water ocean?
• What is the spatial distribution of Enceladus’s heat output, and how has it varied with time?
• Does Enceladus have an ocean or some other means of providing large tidal dissipation, and
to what extent is its behavior dictated by its formation conditions (e.g., presence or absence of a
differentiated core?)
• What does the diversity of the uranian moons tell us about the evolution of small to mediumsized
icy satellites? What drove such dramatic endogenic activity on Miranda and Ariel?
• What powers past or possible ongoing activity on Triton, which currently has negligible tidal
heating?
Future Directions for Investigations and Measurements
Many of the future investigations needed to understand formation arise here as well because of
the interplay of formation conditions and subsequent thermal evolution. A better understanding of the
internal structure and thermal evolution requires measurements of static gravitational fields and
topography to probe interior structure and of tidal variations in gravity and topography, as well as
electromagnetic induction in the satellites at multiple frequencies to search for oceans. The presence and
nature of intrinsic magnetic fields also constrain internal thermal evolution and initial conditions. Another
needed key investigation is subsurface sounding (e.g., radar), to investigate the structure of the upper
lithosphere. Heat flow can be sufficiently large to be detected through thermal infrared techniques. This
provides a powerful constraint on the satellite’s thermal state. Improved maps of composition and
geology of the satellite surfaces will constrain the extent and nature of transport of heat from the interior.
Critical to interpretations from these investigations are improved laboratory determinations of the
thermophysical and mechanical properties of relevant candidate materials to better constrain interior
processes.
What is the Diversity of Geological Activity and How has it Changed Over Time?
The surfaces of solar system bodies provide important clues to their history and evolution.
Collectively, outer planet satellites show the scars of almost every surface process, including impact
cratering, tectonic deformation, cryovolcanism, and aeolian and fluvial erosion. Many of these processes
are still mysterious. Icy-satellite tectonism is often extensional, 14 sometimes bringing interior materials
up to the surface, but strike-slip faulting is also observed. Compressive tectonism is less evident on icy
satellites; however, it is likely responsible for Io’s towering mountains. 15 Much of Europa’s surface is
disrupted by extensive and mysterious chaos regions. 16 Solid-state convection is likely to be an important
driver of icy satellite geology, but details are unclear. The large range of ages and processes provides a
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