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

• Is there active cryovolcanism on Titan?
• What are the eruption mechanisms for Io’s lavas and plumes and their implications for
volcanic processes on early and modern Earth?
Future Directions for Investigations and Measurements
Key investigations and measurements into active tectonic and volcanic processes include 1)
exploration of Io’s dynamic volcanism in the temporal domain at high spatial resolution, over timescales
ranging from minutes (for the dynamics of active plumes), to weeks or decades (for the evolution of lava
flows and volcanic centers), 2) global maps of Titan’s surface morphology and surface compositional to
search for evidence for present-day geological activity, and 3) acquisition of higher-resolution thermal
and visible imaging of the active south pole of Enceladus, including temporal coverage, to elucidate
plume generation mechanisms. Other important objectives include search for activity on other satellites
such as Europa by looking for thermal anomalies, gas and dust plumes, or surface changes and collection
of additional in situ measurements of the composition of the endogenic materials lofted into the
atmospheres or plumes of these satellites.
What Processes Control the Chemistry and Dynamics of Satellite Atmospheres?
Satellite atmospheres are exceptionally varied (Table 8.1), and a great range of processes governs
their structures, chemistries and dynamics. Surface pressures range 12 orders of magnitude, from
picobars to 1.5 bar (~1.5 times Earth’s surface pressure). The thinnest atmospheres, including those of
Europa, Ganymede, and probably Callisto, are created by sputtering (i.e., ejection of particles from the
surface by plasma bombardment) and are dominated by oxygen molecules which are too sparse to interact
significantly with each other. 26 Io’s patchy atmosphere, dominated by sulfur dioxide, results from a
combination of volcanic supply and surface frost evaporation, 27 while Triton’s denser global molecular
nitrogen-dominated atmosphere is entirely supported by evaporation of surface frosts.
Ground-based observations have furthered understanding of the distribution of the atmospheresupporting
molecular nitrogen and methane frosts over the surface of Triton. 28 Ground-based and Hubble
Space Telescope observations have demonstrated that Io’s atmosphere is concentrated in the equatorial
regions and shows stable 10-fold variations in density with longitude. 29
By far the largest satellite atmosphere is Titan’s, dominated by nitrogen molecules, which dwarfs
Earth’s atmosphere, and originated from the outgassing of volatiles during its formation, continuing into
at least the recent past. Titan’s atmosphere experiences a range of dynamical and chemical processes. 30
(Figure 8.8). The second most abundant constituent, methane, exists as a gas, liquid and solid, and cycles
from the surface to the atmosphere, with clouds, rain and lakes. The temperature profile manifests
greenhouse warming and “anti-greenhouse” cooling. The dynamics of Titan’s atmosphere ranges in scale
from global circulation patterns to local methane storms. Titan’s atmospheric composition is affected
primarily by dissociation of methane and nitrogen by solar ultraviolet radiation and by magnetospheric
electrons, which leads to a complex chemistry that extends from the ionosphere down to the surface.
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
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