Geology Reference
In-Depth Information
Figure 9.24. A visible image
(left) of Mimas compared with a
combined thermal map and
visible image (right) showing
the puzzling
pattern
of temperatures in which high
temperatures (>90 K) are shown
in white and average surface
temperatures (~77 K) are shown
in gray (NASA PIA12867).
“
Pac-man
”
severely fractured the terrain. Oeta Chasma, however, is
tangential to Hershel and would seem unrelated to this or
any other obvious impact structure seen on Mimas. In
addition to the large chasmata, small grooves and hints
of
fine fractures are seen, which are also attributable to
impacts.
A bizarre discovery for Mimas was made when the
Cassini infrared spectrometer mapped the surface temper-
atures. Highest temperatures were expected over the
terrain warmed by the Sun in the afternoon, but maps
showed a peculiar V-shaped pattern like the video game
“
eating a dot, in this case Herschel crater
(
Fig. 9.24
), in which the highest temperatures were
found in the morning. In addition to the
“
V,
”
a warmer
spot was mapped just to the west of Herschel
'
s rim. Awide
variety of explanations has been offered, including possi-
ble relationships to Hershel
Pac-man
”
is ejecta deposits, but none is
entirely satisfactory. This discovery demonstrates that the
Solar System continues to hold new and unanticipated
findings, especially when data from new instruments and
higher resolutions are obtained.
'
Figure 9.25. A Voyager 2 image of Tethys showing heavily cratered
terrain in the upper left of this view and the smooth plains to the
lower right. The central-peak impact crater toward the top of the
view is Telemachus, which is about 92 km in diameter (NASA
PIA01397).
9.7.2 Tethys
rugged topography and a high frequency of degraded
craters >20 km in diameter. The plains are more sparsely
cratered and are approximately centered on the trailing
hemisphere.
The two most striking features on Tethys are Odysseus,
an impact crater 445 km in diameter (about 40% of the
satellite
Tethys is 1,048 km in diameter, more than twice the size of
Enceladus, and also displays evidence of crustal deforma-
tion and resurfacing. Its density is only 0.97 g/cm
3
, mean-
ing that it is composed mostly of water-ice. Ridges,
grooves, and smooth plains are present but are not as
extensive as those on Enceladus. The general geology of
Tethys was mapped from Voyager images as older, heav-
ily cratered terrain and smooth plains (
Figs. 9.25
and
9.26
)
which represent resurfacing. The cratered terrains have
s diameter), and an enormous canyonland named
Ithaca Chasma, which wraps at least three-quarters of the
way around the globe. Odysseus is a shallow crater with a
'
