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

valuable window into solar system history, constraining thermal and compositional evolution and
allowing a better understanding of how planetary systems form and evolve.
The science return from the Cassini mission has been phenomenal. Multiple flybys of Titan have
confirmed the presence of numerous methane lakes on the surface—the only bodies of surface liquid on
any known world other than Earth—along with fluvial channels (Figure 8.3), and evidence for seasonal
variations. 17 Images of Enceladus reveal a long and complex geological history that continues to the
present day, and includes ridges that are morphologically similar to Europa’s ubiquitous double ridges. 18
Wispy features on Dione and Rhea’s trailing hemisphere have been revealed to be huge cliffs, evidence of
a tectonically active past. 19 Images of Iapetus show an ancient equatorial belt of mountains, a remnant of
Iapetus’s early evolution. Cassini images have shown unusual impact crater morphologies on Hyperion.
Continued analysis of Galileo images has constrained the population of primary and secondary impactors
in the outer solar system, 20 and provides continued new insights into the remarkable geology of Europa.
FIGURE 8.3 False color Cassini radar image of a methane sea near Titan’s north pole, fed by dendritic
drainage channels. The image is 360 kilometers from top to bottom. SOURCE: NASA/JPL/USGS.
Important Questions
Some important questions about the diversity of geological activity and how it has changed over
time include the following:
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
8-10