The fifth largest moon of Saturn. It was
discovered by Giovanni Cassini on March
21, 1684, and is also known as Saturn III.
| Voyager 2 photo of Tethys showing objects about
5 km (3 miles) in size. It was taken on Aug. 26, 1981,from a range
of 282,000 km (175,000 miles). A boundary between heavily cratered
regions (top right) and more lightly cratered areas (bottom right)
is similar to boundaries on the moons Dione and Rhea, indicating a
period of internal activity early in Tethys' history that partially
resurfaced the older terrain. The large crater in the upper right
lies almost on the huge trench system that girdles nearly three-fourths
of the circumference of the satellite. The trench itself is seen in
this image as a linear set of markings to the lower left of the crater.
The trench, several kilometers deep, is indicative of a cold, stiff
ice crust at the time of its formation. Formation of this trench system
could have resulted from the expansion of Tethys as its warm interior
| This is the highest-resolution view of Tethys obtained
by Voyager 2(Aug. 26) when the spacecraft was 120,000 km (74,500 miles)
from the moon. This image was taken 1× hours after the spacecraft
passed through the planet's ring plane. The smallest features visible
here are about 2.2 km (1.4 mi.) across.
Tethys appears to be composed primarily of water ice and is marked by a
huge, globe-encircling canyon and numerous impact craters. The giant canyon,
named Ithaca Chasma, is about 2,500 km (1,550 miles) long (three quarters
of the circumference of the moon), and has an average width of about 65
km (40 miles) and a depth of 3 to 5 km (about 2 to 3 miles). If Tethys were
once a ball of liquid water covered with a thin, solid crust, freezing of
a thick, watery mantle would have produced enough surface expansion to account
for the area of the trough. But it is unclear why all the expansion would
have taken place in a single band rather than in widely distributed faults.
Also prominent is a large crater, Odysseus, about 400 km (250 miles) across,
or more than one third of the diameter of Tethys. A line drawn through the
center of Odysseus would lie roughly at right-angles to Ithaca Chasma, suggesting
a possible link between the two features. The temperature on the surface
is -187°C (-305°F).
||1684, by Giovanni Cassini
||294,619 km (183,107 miles)
||1081 × 1062 × 1055 km
(672 × 660 × 656 miles)
||0.39 km/s (1400 km/h, 870 mph)