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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
topographic features all point to an ice shell that is not thin or 
weak, but that can support topography. 
3. MID-SIZED SATURNIAN SATELLITE 
3.1 Cartography and Topography 
Cassini imaging as of October 2007 allows for global 
cartographic control and mapping (Figure 4) on the middle- 
sized icy satellites of Saturn down to resolutions of 400-500 m 
(over >75% of their surfaces). These include Phoebe, Iapetus, 
Rhea, Dione, Tethys, Enceladus, and Mimas. Global 
topographic maps have also been completed for Rhea and 
Dione and large parts of the other satellites at resolutions of 0.5 
to 1 kilometre (Figure 5). In addition, isolated high resolution 
DEMs have been produced from Cassini data on many of the 
satellites (Figure 6). 
Figure 4. Global map of Dione (as of April 2008). Base 
resolution of global product is 400 m. 
Figure 5. Global topographic map of Dione (as of April 2008). 
Global base map (Figure 4) has been colour-coded to show 
topography (red high, blue low). Total dynamic range is ~5 km. 
Topographic data are from stereogrammetry and 
photoclinometry. 
3.2 Geology and Geophysics 
Most of the icy Saturnian satellites are heavily cratered and 
impact effects dominate topography. Large degraded basins 
350-500 kilometres across (and not apparent in imaging) are 
revealed on Rhea and Dione. Topography also reveals radial 
gouges centred on several of these ancient impacts (Figure 5), 
as well as two crossing orthogonal sets of grooves or ridges on 
Rhea, indicating that this satellite was more active than the 
cratered surface might suggest. On Tethys, we see the 
topographic signature of smooth plains, despite the fact these 
plains are heavily cratered. Topography also suggests the 
presence of a circumferential ridge 350-400 km beyond the rim 
of the Odysseus impact basin, indicating that this impact may 
have globally modified the shape of Tethys. 
A major result of the Cassini topographic data is the extent of 
viscous relaxation on these satellites. Several large ancient 
basins on Tethys and Rhea are partially relaxed (Moore et al., 
2004; Schenk, 2006), but not some younger basins, such as 
Odysseus on Tethys (Moore et al., 2004). A surprise was the 
abundance of relaxed smaller craters (D~ 10-30 km) on Dione 
(Figure 7) discovered from Cassini in association with smooth 
plains, indicating that heat flow was significantly higher in the 
past. Modelling indicates that residual impact heat beneath the 
crater floor is also required to explain the anomalously high 
central peaks seen on Dione (Dombard et al., 2008), some of 
which rise 3 to 5 kilometres above the surrounding plains 
(Moore et al., 2004; Schenk, 2006)! Larger craters on Dione 
are highly relaxed, including Evander (Figure 6; Schenk, 2006). 
Extensive relaxation of craters also occurs on Enceladus and 
mapping and modelling is in progress (Schenk, 2006). 
Figure 7. Perspective view of the south polar terrains on 
Enceladus. Colour-coded DEM is based on photoclinometric 
analysis of the highest resolution (~10 m/pixel) image currently 
available of the surface. 
Figure 6. Perspective view of relaxed central peak craters 
(lower right) on Dione. Data from Figure 5. Central peaks are 
3 to 6 km high and project well above the ground plane. At 
bottom left are the concentric inner ring and rim scarp of the 
relaxed Evander impact basin (D-350 km). Vertical 
exaggeration is considerable.