That Saturn's little moon Enceladus has active plumes venting material from its south pole is incredibly exciting. Perhaps even more intriguing, however, is that the activity is so localized and that Enceladus' surface exhibits such a wide range of geologic terrains, potentially reflecting heterogeneity in its interior.
The youth of the South Polar Terrain is demonstrated not only by the current activity observed there but also the lack of impact craters.
Enceladus' Southern Hemisphere
assini Imaging Science Subsystem (ISS) mosaic of Enceladus' southern hemisphere. The enhanced color emphasizes fresh exposures of coarse-grained ice.
The impact cratering rate likely varies to some degree across the Saturnian system, but not to the dramatic extent illustrated over Enceladus' surface or by comparison of Enceladus and nearby Mimas, which is similar in size. Therefore, such differences in the number of craters indicate differences in the ages of the surfaces.
Map of Enceladus - December 2008
ap of Enceladus. The currently active south-polar region has few if any craters and cratering is sparse over some longitude ranges (~60°W to ~120°W, and ~240°W to ~330°W). However, the terrain at high northern latitudes and longitudes ~30°W and ~180°W have much higher crater densities.
Map of Mimas - June 2008
ap of Mimas' much more heavily cratered and older surface for comparison.
Much of Enceladus' surface has clearly been modified by tectonic activity extensive enough to completely erase older features such as impact craters.
Tectonics on Enceladus
ectonic ridges and fractures dominate these areas of Enceladus; only a few younger craters are superimposed.
In some places the beginnings of this process can be seen, where craters are being modified by fresh fractures.
mages of Enceladus showing craters cut by more recent fractures.
NASA / JPL / Space Science Institute
Craters and Cracks
In contrast, at high northern latitudes impact craters dominate much of the terrain. Although these areas are clearly much older than many of the others illustrated above, the impact craters are not entirely pristine. They are often much shallower than fresh craters and the floors of some of the largest are bowed upwards, indications that the surrounding ice is warm enough to have flowed over geologic timescales (similar to honey flowing in to fill an area that has been scooped out, but over a much longer timescale!).
Enceladus' north pole
iew from above Enceladus' north pole.
or comparison, the craters on Mimas have not been modified in this way.
Mimas Against the Rings
iew of Mimas (seen in front of Saturn's rings).
The extent to which Enceladus' craters have been modified provides information about the amount of interior heat in these areas: were there no source of heat at all, the craters would be much more pristine, like those of Mimas; were there significantly more heating the craters would have relaxed completely away, even under the very low gravity at Enceladus' surface.
These observations give us important clues about Enceladus and its interior. That the geologic activity varies so significantly across the surface, from currently active areas to others likely eons old, suggests that the heat reaching the surface from the interior also varies from place to place. However, even the oldest areas, appear to have undergone some subtle modification, demonstrating that they, too, are not without some heat from the interior. In order to understand Enceladus, it's essential to consider the moon in its entirety.
Zibi Turtle is a research scientist in the Planetary Exploration group at Johns Hopkins University's Applied Physics Lab. She is an associate of Cassini's imaging team and member of the Lunar Reconnaissance Orbiter Camera team. Her research interests include impact cratering and planetary geology, e.g., lake formation on Titan, crater formation and modification, and mountain building on volcanic Io. When not sitting in front of a computer, she enjoys racing with the Baltimore Rowing Club, taiko, and playing with her nieces.