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

of these processes are analogous to those that operate on terrestrial planets and in the Earth-Moon system.
Impacts by km-sized asteroidal and/or cometary objects have long been recognized as a dominant process
in sculpting the surfaces of most bodies in the solar system.
Less obvious and much less understood is the role played by smaller impactors, down to dustsize,
in modifying the surface composition and texture. Examples in the outer solar system include: the
neutral-colored material that darkens the C Ring and Cassini Division at Saturn; 68 and the dark material
which coats the leading side of Iapetus, thought to be derived from Phoebe or the other outer satellites. 69
More speculative are the long-term effects on the structure and lifetime of outer planet ring systems due
to ring-particle collisions and collisions of external impactors.
Important Questions
Some important questions concerning the role of surface modification via smaller impacts include
the following:
• What are the flux, size distribution, and chemical composition of the various populations of
impactors, from late-stage planetesimals 4 billion years ago to present-day interplanetary dust?
• What are the surface modification mechanisms for low-temperature smaller icy targets?
Future Directions for Investigations and Measurements
More sophisticated dust detectors carried by spacecraft such as Galileo, Ulysses and Cassini have
already refined—and in some cases revolutionized—knowledge of interplanetary dust, and much more
remains to be learned here. Near-infrared spectral studies of the Galilean and saturnian moons and rings
have led to new models for dust “contamination” of icy surfaces, but definitive identification of the
chemical species involved remains elusive and may require in situ sampling. Near-infrared spectral
studies of the rings and small moons in the ice-giant systems are needed to fully characterize the
differences of the dust populations in the more distant regions of the solar system. 70
GIANT PLANETS AS LABORATORIES FOR PROPERTIES AND PROCESSES ON EARTH
The planet that matters most to humankind is Earth. The planet’s health and ecologic stability are
of paramount importance to us all. Earth, however, is a notoriously difficult planet to understand. The
atmosphere interacts in a complex fashion with the lithosphere, hydrosphere, cryosphere, and biosphere
(surfaces that are, respectively, rocky, liquid, icy, or biologically active). Yet knowledge of this interplay
is critical for understanding the processes that determine conditions of habitability within the thin veneer
of Earth’s surface.
Giant planets, though larger than Earth, are in many respects simpler than Earth. The physics and
chemistry driving the processes in their thick outer atmospheres can be understood without reference to a
lithosphere, cryosphere, hydrosphere, or biosphere. The processes in giant-planet ring systems at times
resemble pure examples of Keplerian physics, with added interactions from collisions, resonances and
self-gravity. In a very real sense, the giant planets and their environs can serve as laboratories for the
fundamental physical processes that affect all planetary atmospheres and surfaces.
To work effectively in our planetary laboratory, we must understand our tools, and we must
frame proper experiments. Fundamental objectives associated with the goal of using the giant planets as
laboratories for properties and processes of direct relevance to Earth include the following:
• Investigate atmospheric dynamical processes in the giant planet laboratory,
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