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

orbits of Io and Europa), and potentially allows orbital measurement of surface composition via sputtered
products. Europa may lose around 2 centimeters of its surface to plasma sputtering every million years. 32
Implantation of exogenic species can be significant (for instance, sulfur of likely ionian origin is found on
Europa’s trailing side), and radiolytic processing generates reactive species such as molecular oxygen and
hydrogen peroxide in surface ices, which might, in the case of Europa or Enceladus, deliver chemical
energy to underlying bodies of liquid water in quantities sufficient to power biological activity. 33
FIGURE 8.9 Near-infrared maps from the Galileo mission of non-water-ice surface materials on Europa
(left, sulfate hydrates), Ganymede (middle, carbon dioxide), and Callisto (right, carbon dioxide) Red high
non-water-ice abundance regions for Europa and Callisto are centered on the trailing hemispheres,
consistent with expected magnetospheric effects. SOURCE: John Cooper, NASA Goddard Flight Space
Center.
Micrometeoroids play a crucial role in regolith generation and in redistributing radiolytic
products to the subsurface layers through impact gardening. Regolith thickness may be many meters.
Impacts may eject surface dust samples to altitudes where they can be analyzed by orbiting or flyby
spacecraft. Macroscopic impacts are the major landform generators on many satellites, and are powerful
probes of the structure and composition of the subsurface that they penetrate. Crater populations provide
information on relative ages of surface units and on the population of projectiles over time.
Solar radiation also alters planetary surfaces. Extreme ultraviolet photolysis (photon-driven
chemistry) modifies surface composition (though it is dominated by particle radiation on Jupiter’s
moons), and solar ultraviolet radiation has a major influence on the atmospheric chemistry of Titan.
Solar-driven frost sublimation is an important process in atmospheric support and the modification of
surface albedo and composition.
Recent studies based on Cassini data indicate that in Saturn’s magnetosphere, the loss of surface
material from plasma sputtering from the icy satellites is minimal (less than a few g/s for all of the
satellites). Cassini observations show that the loss rate of heavy ions from Titan due to solar and
magnetospheric effects is much larger than expected, and a mass as large as the mass to the present day
atmosphere may have been lost to space over the lifetime of Titan, a conclusion supported by evidence for
significant nitrogen-15/nitrogen-14 fractionation. 34 Analysis of Cassini data from Iapetus suggests that its
long-mysterious extreme albedo dichotomy results from a combination of exogenic processes (infall of
dark dust and the resulting sublimation and migration of water ice), while Enceladus’s plumes have
influenced the albedos and leading/trailing photometric asymmetries of the inner Saturn satellites.
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
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