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

• What causes the enormous differences in the ion to neutral ratios in these systems?
• What can our understanding of the giant planet magnetospheres tell us about the conditions to
be expected at extra-solar giant planets?
Future Directions for Investigations and Measurements
Despite concentrated observations of Jupiter and Saturn both in situ and from Earth-based
facilities, there remain many unanswered questions. These can be addressed in part by JEO
measurements in its initial orbital phase, including observations of auroral emission distribution and
associated properties from close polar orbital passes, as well as of upper atmosphere energetics. 36
Nearly nothing is known about the magnetic fields and magnetospheres of Uranus and Neptune
aside from the brief Voyager encounters more than two decades ago. Either a flyby spacecraft or an
orbiter could address some aspects of ice giant magnetospheric science, but an orbiter would
tremendously advance our understanding of ice giant magnetospheres. 37
Extrapolation of the solar system’s magnetospheres to those expected for exoplanets can be
addressed by gaining knowledge of comparative magnetospheres; these should include Earth and the four
giant planets, and scaling relations should be determined between magnetospheric size, density, strength
of interaction with the solar/stellar wind, and other parameters. Most giant planet missions could
contribute to answering these questions; our knowledge is most lacking for the ice giants.
Use Ring Systems as Laboratories for Planetary Formation Processes
Investigations of planetary rings can be closely linked to studies of circumstellar disks. Planetary
rings are accessible analogues in which general disk processes such as accretion, gap formation, selfgravity
wakes, spiral waves, and angular momentum transfer with embedded masses can be studied in
detail. The highest priority rings recommendation of the last decadal survey 38 was accomplished: to
operate and extend the Cassini orbiter mission at Saturn. 39,40 Progress has also come from Earth-based
observational and theoretical work as recommended by the last decadal survey and others. 41,42
Saturn’s Ring System
Cassini data, supported by numerical and theoretical models, have revealed a wealth of dynamical
structures in Saturn’s rings, including textures in the main rings produced by inter-particle interactions
and patterns generated by perturbations from distant and embedded satellites. Its observations of the
orbits of embedded “propeller” moons in Saturn’s rings reveal surprisingly robust orbital evolution on
approximately one-year timescales, possibly due to gravitational or collisional interactions with the disk. 43
Collective inter-particle interactions produce phenomena including what are now termed “self-gravity
wakes” (elongated, kilometer-scale structures formed by a constant process of clumping counter-balanced
by tidal shearing), radial oscillations in the denser parts of the rings that may be due to viscous overstability,
and “straw-like” textures seen in regions of intense collisional packing such as strong density
waves and confined ring edge.
Moons embedded within the rings are observed to produce gaps, though the origins of many other
gaps remain unknown. In Saturn’s F ring, Cassini images show evidence for active accretion triggered by
close approaches of the nearby moon Prometheus, 44 while recent accretion is inferred for other known
ring moons. 45,46,47 Data from Cassini’s spectrometers and other instruments are elucidating the
composition and thermal properties of the icy particles in Saturn’s rings, as well as the characteristics of
the regolith covering larger ring particles. These properties vary subtly between different regions of the
rings, for reasons that are currently not understood.
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