L
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008
On a global scale, we confirm the depressed topography of the
south polar terrains, but have also discovered large scale
“dimples” in the topography of Enceladus (Schenk and
McKinnon, 2008). These include at least two broad depressions
roughly 100 km across and 1 to 1.5 km deep. These are located
near the equator and at roughly 40°N latitude. They are located
within older cratered terrains or on the contact between cratered
plains and very young ridged plains, indicating that there is no
correlation with geology. They could represent isostatic
warping of the surface over irregularities in the rocky core or
over downwelling/upwelling plumes in the icy mantle, or more
likely isostatic depressions over mass anomalies.
4. URANIAN & NEPTUNIAN SATELLITE
4.2 Uranian Satellites
Voyager 2 data from 1986, obtained during southern summer,
provided views of only 25-45% of the surface, almost all at
southern latitudes (Figure 7). Excellent stereo imaging was
obtained for Miranda and Ariel (Figure 8) and good stereo for
parts of Titania. These data are locally supplemented by
terminator photoclinometry. Oberon, Umbriel and Puck were
too far away for anything other than reconnaissance images.
DEMs of these satellites are largely unpublished but early
analyses documented the depths of craters on the three mapped
satellites (Schenk, 1991), and showed that some large craters on
Ariel are also viscously relaxed or flooded by lavas (Schenk
and McKinnon, 1998).
Figure 7. Global map of Ariel. Map is in simple cylindrical
projection. Note northern hemisphere obscured by darkness,
and use of Uranus-shine images north of the equator. Dark
swath extending northward may be extension of volcanic plains.
4.3 Triton
Geologically useful stereo DEMs of Triton based on Voyager
images cover only a modest fraction of the surface (<10%).
Although these data have poor resolving power (50-200 m
vertical), they were sufficient to demonstrate that the centres of
cantaloupe terrain ovoids are depressed a few hundred meters
(Schenk and Jackson, 1993). The poor performance of these
data products is due mainly to the small stereo angles (imposed
by the encounter geometry and distance), and by the inherently
low topography of the surface. Few features on the surface
have amplitudes greater than 250 m. Instead we rely on
photoclinometry, which covers a 10-15° swath parallel to the
Voyager terminator (Figure 9). These data have resolutions of
0.7 to 1.5 km and much higher vertical fidelity than stereo
products.
Figure 8. Topographic map of Ariel. Orthographic projection
of base map has been colour-coded to show topography (red
high, blue low). Topographic range displayed in colour bar is
-5 kilometres.
Figure 9. Topographic map of equatorial regions of Triton.
Orthographic mosiac (north to right) has been colour-coded to
show topography (red high, blue low). Total dynamic range is
~0.5 km. Data derived form photoclinometry only. Individual
geologic features are well represented, but long-wavelength
topography (likely of very low amplitude) is not regarded as
reliable. Large amplitude features along the left edge of the
DEM may be photoclinometric artefacts.
5. CONCLUSIONS
Cartographic and topographic mapping of the icy satellites of
the Outer Solar System is an ongoing task, especially for the
970
'y
¡rag
m
W; i
Mag