( , , or as Greek Τηθύς)
that was discovered by Giovanni Domenico Cassini
Tethys is named after the titan Tethys
of Greek mythology
. It is also designated
or S III
Cassini named the four moons he discovered in 1671–1684 (Tethys,
) Sidera Lodoicea
("the stars of Louis")
to honour king Louis XIV
found Tethys using a large aerial
telescope he set up on the grounds of the Paris
the end of the seventeenth century, astronomers fell into the habit
of referring to them and Titan
through Saturn V
Dione, Rhea, Titan, Iapetus). Once Mimas
were discovered in 1789, the
numbering scheme was extended to Saturn VII
bumping the older five moons up two slots. The discovery of
in 1848 changed the numbers
one last time, bumping Iapetus up to Saturn VIII
Henceforth, the numbering scheme would remain fixed.
The names of all seven satellites of Saturn then known come from
(son of William Herschel
, discoverer of Mimas and
Enceladus) in his 1847 publication Results of Astronomical
Observations made at the Cape of Good Hope
, wherein he
suggested the names of the Titans
sisters and brothers of Kronos
analogue of Saturn), be used.
The correct adjectival form of the moon's name is Tethyan
although other forms are also used.
A composite image of Tethys'
Tethys is an icy body similar in nature to Dione
density of Tethys is 0.97 g/cm³, indicating that it is composed
almost entirely of water-ice. The Tethyan surface is heavily
and contains numerous cracks
caused by faults in the ice. Its surface is one of the most
reflective (at visual wavelengths) in the solar system, with a
visual albedo of 1.229. This very high albedo is the result of the
sandblasting of particles from Saturn's E-ring, a faint ring
composed of small, water-ice particles generated by Enceladus'
south polar geysers.
There are two different types of terrain found on Tethys, one
composed of densely cratered regions and the other consisting of a
dark colored and lightly cratered belt that extends across the
moon. The light cratering of this second region indicates that
Tethys was once internally active, causing parts of the older
terrain to be resurfaced. The exact cause of the darkness of the
belt is unknown but a possible interpretation comes from recent
of Jupiter's moons Ganymede
, both of which exhibit
light polar caps that are made from bright ice deposits on
pole-facing slopes of craters. From a distance the caps appear
brighter due to the thousands of unresolved
ice patches in small craters
present there. The Tethyan surface may have been formed in a
similar manner, consisting of hazy polar caps of unresolved bright
ice patches with a darker zone in between.
western hemisphere of Tethys is dominated by a huge impact crater
called Odysseus, whose
400 km diameter is nearly 2/5 of that of Tethys itself.
The crater is now quite flat (or more precisely, it conforms to
Tethys' spherical shape), like the craters on Callisto, without the
high ring mountains and central peaks commonly seen on the Moon
is most likely due to the slumping of the weak Tethyan icy crust
over geologic time.
The second major feature seen on Tethys is a huge valley called
, 100 km wide and 3
to 5 km deep. It runs 2000 km long, approximately 3/4 of
the way around Tethys' circumference. It is thought that Ithaca
Chasma formed as Tethys' internal liquid water solidified, causing
the moon to expand and cracking the surface to accommodate the
extra volume within. The subsurface ocean may have resulted from a
2:3 orbital resonance
Dione and Tethys early in the solar system's history that led to
Tethys' interior. The ocean would have frozen after the moons
escaped from the resonance. Earlier craters that formed before
Tethys solidified were probably all erased by geological activity
before then. There is another theory about the formation of Ithaca
Chasma: when the impact that caused the great crater Odysseus
occurred, the shockwave traveled through Tethys and fractured the
icy, brittle surface on the other side. The Tethyan surface
temperature is -187°C.
The co-orbital moons Telesto
are located within Tethys'
and , 60 degrees
ahead and behind Tethys in its orbit respectively.
performed a close targeted flyby of Tethys on September 23, 2005 at
the distance 1500 km. Although Cassini will continue to
observe Tethys at moderate distances during its extended mission,
there are no plans for further close fly-bys.
Tethys in fiction
- In US dictionary transcription, .
- An Extract of the Journal Des Scavans.
of April 22 st. N. 1686. Giving an account of two
new Satellites of Saturn, discovered lately by
Mr. Cassini at the Royal Observatory at Paris.,
Philosophical Transactions 16 (1686-1692) pp. 79-85
- Fred William Price - The planet observer's
handbook - page 279
- As reported by William Lassell, Monthly Notices of the Royal Astronomical Society,
Vol. 8, No. 3, pp. 42–43 (January 14, 1848)