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

The evident processes and properties of Saturn’s ring system provide essential clues as to how all
rings and other disks of material behave (including circumstellar disks and proto-planetary disks). Nongravitational
forces, including electromagnetism, drive the evolution of dusty rings such as Saturn’s E
ring (as well as Jupiter’s gossamer rings and probably Uranus’s dusty “zeta” ring), but much work is still
needed to clarify the processes involved. Cassini will continue tracking the orbits of “propeller”
structures through the end of its Solstice Mission in 2017. The direct detection of orbital migration
remains a major goal for Cassini in the Solstice Mission, either for nearby moons interacting
gravitationally with the rings or for the embedded and unseen “propeller” moonlets. Cassini will also
make further observations of the ring micro-structure. Cassini results also have focused renewed
attention on the origin and age of Saturn’s rings: the realization that the B ring may be much more
massive than previously thought has the potential to ease a primary constraint on the rings’ age, namely
pollution by interplanetary mass infall; a continuing goal of Cassini is to measure that flux.
Other Ring Systems
The most remarkable features of Neptune’s rings are the azimuthally confined arcs embedded in
the Adams ring. Although a resonance mechanism has been proposed for the confinement of these arcs,
post-Voyager observations at Keck have cast at least the details of this model into doubt. 48 The same
observations also reveal that the arcs are evolving on timescales as short as decades, consistent with our
emerging perspective of the dynamic nature of diffuse rings. Further close-range observations of both the
rings and their associated arcs will be necessary to resolve the outstanding questions regarding their
nature, origin and persistence.
The uranian ring system is massive, complex, and diverse, and much about it remains poorly
understood. It includes several narrow and sharp-edged dense rings whose confinement mechanism
remains unclear. The 2007 equinox of Uranus provided an unprecedented opportunity to study its ring
system. During the edge-on apparition (which last occurred in 1965), two new diffuse Uranian rings were
discovered in Hubble images in 2005. 49 They appear eerily similar to Saturn’s E and G rings in color and
planetocentric distance, 50 but have yet to be characterized in any detail. Equinoctial Keck observations of
Uranus also detected the diffuse dusty inner “zeta” ring for the first time since the 1986 Voyager flyby,
and revealed it has changed substantially since then. 51 The reason for this change is unknown; temporal
effects of Uranus’s extreme seasons on the rings may be contributing factors.
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