Geology Reference
In-Depth Information
while the frost areas would re
ect more energy and retard
sublimation.
A particularly fascinating discovery is the curious ridge
that girdles much of Iapetus through Cassini Regio, nearly
coincident with the equator (
Fig. 9.43
). The ridge is some
1,200 km long, 20 km wide, and stands as high as 10 km. It
includes continuous segments >100 km long, separate
peaks, and parallel ridge-sets and might extend into the
bright terrain as a series of isolated peaks. Jaumann et al.
(
2009b
) note that the ridge might have resulted from a disk
of impact debris striking Iapetus, from tectonic deformation
related to
“
despinning
”
of Iapetus, or from internal con-
vection and up-welling; however, all of these ideas have
problems and the ridge
'
s origin remains a mystery!
9.7.6 Small satellites
“
Small
”
satellites range in size from 25 to 220 km in
diameter and include a wide variety of objects, as dis-
cussed by Carolyn Porco and her Cassini colleagues
(Porco et al.,
2007
). Phoebe (
Fig. 9.44
), the outermost
saturnian satellite, is in a retrograde orbit inclined to the
equatorial plane of Saturn. This unusual orbit suggests
that Phoebe was captured by Saturn. Estimates of its
mass were derived from Cassini
flybys, which give a
density of ~1.63 g/cm
3
and suggest that it has the highest
proportion of rocky material of the satellites discussed
above. Phoebe is roughly spherical (
Fig. 9.45
) and is the
darkest object in the saturnian system, its surface being
composed of carbonaceous material.
Figure 9.43. Iapetus viewed by Cassini showing the dark terrain of
Cassini Regio and the unusual ridge that coincides with the equator;
the ridge is 1,200 km long and as high as 10 km; the origin of the
ridge remains to be explained (NASA PIA06166).
Figure 9.44. A mosaic of images for Phoebe with key place names (from Roatsch et al.,
2009
).
