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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.