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

a major influence on satellite evolution. Tidal and other energy sources drive a wide range of geological
processes, whose history is recorded on the satellite surfaces.
Objectives associated with goal of understanding the formation and evolution of the giant planet
satellites include the following:
• What were conditions during satellite formation?
• What determines the abundance and composition of satellite volatiles?
• How are satellite thermal and orbital evolution and internal structure related?
• What is the diversity of geological activity and how has it changed over time?
Subsequent sections examine each of these objectives in turn, identifying important questions to
be addressed and future investigations and measurements that could provide answers.
What Were Conditions During Satellite Formation?
The properties of the existing regular satellite systems provide clues about the conditions in
which they formed. The regular satellites of Jupiter, Saturn, and Uranus orbit in the same planes as the
planets’ equators, suggesting that the moons likely formed in an accretion disk in the late stages of planet
formation. 1 Neptune has one large irregular satellite, Triton, in an inclined and retrograde orbit (opposite
from the direction of Neptune’s rotation). Triton may be a captured Kuiper belt object, and moons that
might have formed in a neptunian accretion disk were probably destroyed during the capture. Each of the
regular systems has unique characteristics. Jupiter has four large satellites (the Galilean satellites), the
inner two of which are essentially rocky bodies while the outer two moons are rich in ice. The saturnian
system has a single large satellite, while closer to Saturn there are much smaller, comparably sized icy
moons. The regular uranian satellites lie in the planet’s equatorial plane that is tilted by 97° to the ecliptic
(i.e., the plane of Earth’s orbit).
The outer planet satellites have also been modified by endogenic (e.g., internal differentiation and
tides) and exogenic (e.g., large impacts) processes that have strongly influenced what is seen today.
While the present orbital dynamical, physical and chemical states of the satellites preserve information
about their origins, this can be hidden or erased by processes taking place during the evolution of the
moons.
The Cassini mission has opened our eyes to the wonders of the saturnian satellites. Titan’s
surface is alive with fluvial and aeolian activity, 2 yet its interior is only partially differentiated, 3 has no
magnetic field, and probably no metallic core. On tiny Enceladus, water vapor plumes have been
discovered emanating from south polar fissures, warmed by an unusual amount of internal heat. 4 These
observations together with the satellite’s density have important implications for the interior of Enceladus
that in turn impose limitations on its formation and evolution. Iapetus is remarkably oblate for its size,
and its ancient surface features a unique equatorial belt of mountains, providing unique constraints on its
early history. Cassini observations of the other saturnian moons—Rhea, Dione, Mimas and Tethys—
have increased our knowledge of their surfaces, compositions, and bulk properties.
Measurements of volatile abundances are enabling the reconstruction of the planetesimal
conditions at the time of accretion of the satellites, but this is still far from understood. Titan’s dense
atmosphere makes it especially interesting: The dominance of molecular nitrogen and the absence of the
expected accompanying abundance of primordial argon is an important result that constrains origin. 5
Important Questions
Some important questions concerning the conditions during satellite formation include the
following:
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