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

Important Questions
Some important questions concerning exogenic processes include the following:
• Is Io’s intense magnetospheric interaction responsible for its volatile depletion?
• How is the strong ionosphere of Triton generated?
• How do exogenic processes control the distribution of chemical species on satellite surfaces?
• How are potential Europa surface biomarkers from the ocean/surface exchange degraded by
the radiation environment?
• What do the crater populations on the satellites reveal about the satellites’ histories and
subsurface structure and about the populations of projectiles in the outer solar system and the evolution
thereof?
Future Directions for Investigations and Measurements
Important investigations and measurements into exogenic processes include improved mapping of
satellite surface composition to understand and separate the distributions of endogenic and exogenic
materials. As most of the exogenic materials are carried between the moons by plasma processes, in situ
measurements of the field and plasma environments are required to understand the relative roles of
exogenic and endogenic processes in defining the surface chemistries of the moons. These measurements
may also be able to discover active venting from satellites. Improved remote sensing of impact structures,
including topography and subsurface sounding (e.g., to reveal melt sheets and crustal thinning), will
enhance our understanding of impact processes and their effects on surface evolution. New laboratory
studies should be performed to understand the effects of irradiation on ices infused with exogenic and
endogenic materials. Data on bulk ices and not just thin films is important because energetic electrons and
photons often travel large distances before interacting with the contact material. More laboratory data are
also needed to understand how the spectral characteristics of the icy satellites are modified by ion induced
sputtering, electron irradiation, micrometeoroid bombardment and energetic photon bombardment in the
cold, low-pressure environments of the icy satellites.
How Do Satellites Influence Their Own Magnetospheres and Those of Their Parent Planets?
The magnetospheres of Jupiter, Saturn and Neptune (but not Uranus) derive a large fraction of
their plasma and neutral content from their embedded satellites. In Jupiter’s magnetosphere, Io’s
volcanoes deliver between 1-2 tons/s of material (mostly sulfur dioxide, sulfur, and oxygen) through Io’s
atmosphere to the magnetosphere, and changes in plasma density may be related to changes in volcanic
activity. 35 Saturn’s magnetosphere is dominated by material from Enceladus, as detailed below.
The plasma in Neptune’s magnetosphere appears to be dominated by N + ions derived mainly from
the atmosphere of its moon Triton. Escape of neutrals from Triton supplies a neutral torus with a peak
neutral density of ~400 cm -3 near the orbit of Triton. 36
Ganymede’s magnetosphere derives its plasma from its own sputter-generated atmosphere and
also captures plasma from the magnetosphere of Jupiter. The residence time of plasma is quite short, and
overall densities of charged particles are quite small. 37
Ground-based telescopic observations of Io’s torus and the associated fast neutral nebula continue
to improve understanding of how Io refills its torus and ultimately supplies plasma to Jupiter’s
magnetosphere. Continued analysis of Galileo and Cassini data have stressed the importance of Europa
as another important source of plasma in Jupiter’s magnetosphere, revealing that a neutral atomic- and
molecular-hydrogen torus is present near the orbit of Europa. 38
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
8-19