Tethys (moon)
Giovanni Domenico Cassini stood before a large aerial telescope on the grounds of the Paris Observatory in 1684. He peered into the darkness and found four new moons orbiting Saturn that night. Two of these were Dione and Tethys, which he discovered together. The other two were Rhea and Iapetus, which he had spotted earlier between 1671 and 1672. Cassini named all four new moons Sidera Lodoicea to honor King Louis XIV of France. This French phrase translates to "the stars of Louis". By the end of the seventeenth century, astronomers began calling them Saturn I through Saturn V. William Herschel discovered Mimas and Enceladus in 1789. His discovery bumped the older five moons up two slots in the numbering scheme. John Herschel published Results of Astronomical Observations made at the Cape of Good Hope in 1847. He suggested naming the seven satellites after Titans and Giants from Greek mythology. Tethys took its name from the titaness Tethys, sister of Kronos. Denis Moskowitz later proposed a symbol combining the Greek theta with the crook of the Saturn symbol.
Tethys measures about 531 kilometers in radius. Its density is only 0.98 grams per cubic centimeter. This makes it the lowest-density major moon in the entire Solar System. Spectroscopy confirmed that water ice dominates its surface composition. A small fraction of rock exists within the body. An unidentified dark material also appears on the surface. The visual albedo reaches 1.229, making it one of the most reflective objects known. Only Enceladus shines brighter among Saturn's moons. Radar observations by Cassini revealed an icy regolith with porosity exceeding 95 percent. Thermal emission measurements showed the interior structure remains complex. No compound other than crystalline water ice has been unambiguously identified. Possible constituents include organics, ammonia, and carbon dioxide. Nanophase iron or hematite likely explains the dark spectral properties seen on Iapetus and Hyperion. If differentiated, the rocky core would not exceed 145 kilometers in radius. That core mass would be less than 6 percent of the total moon weight. Tidal and rotational forces shaped the triaxial ellipsoid form. The dimensions suggest a homogeneous interior rather than a layered one.
The western part of the leading hemisphere features Odysseus crater. Its diameter spans approximately 450 kilometers. This basin covers nearly two-fifths of Tethys itself. The floor conforms to the spherical shape due to viscous relaxation over geologic time. Rim crests rise about 5 kilometers above the mean satellite radius. A central pit sits 2 to 4 kilometers deep surrounded by massifs elevated 6 to 9 kilometers. Ithaca Chasma stretches more than 2,000 kilometers long. It measures roughly 100 kilometers wide and 3 kilometers deep. This valley occupies about 10 percent of the surface area. A pole of Ithaca Chasma lies only 20 degrees from the crater's center. Smooth plains cover a small portion of the trailing hemisphere. These plains extend about 60 degrees northeast from the exact antipode of Odysseus. Their sharp boundaries contrast with surrounding cratered terrain. Age determination based on Cassini images showed Ithaca Chasma is older than Odysseus. Scientists believe tidal heating during early orbital resonance caused internal liquid water to solidify. This expansion cracked the icy crust to accommodate extra volume.
Tethys orbits Saturn at a distance of about 295,000 kilometers. Its orbital eccentricity remains negligible while inclination stays near 1 degree. The moon locks into an inclination resonance with Mimas. Low gravity between these bodies prevents noticeable orbital eccentricity or tidal heating. Two co-orbital moons share Tethys's Lagrange points. Telesto orbits 60 degrees ahead while Calypso follows 60 degrees behind. Both satellites measure under 25 kilometers in diameter. Telesto has a mass around 4 times 10 to the power of 15 kilograms. Calypso weighs approximately 2 times 10 to the power of 15 kilograms. They were discovered by Voyager spacecraft in April and March 1980 respectively. Plasma co-rotating with Saturn strikes the trailing hemisphere constantly. Energetic electrons and ions bombard the surface from the magnetosphere. Temperature maps obtained by Cassini show this bluish region cooler at midday. The satellite takes on a Pac-Man-like appearance at mid-infrared wavelengths. These particles drift opposite to planetary rotation and impact areas close to the equator.
The trailing hemisphere gets increasingly red and dark as motion approaches its anti-apex. This darkening creates hemispheric albedo asymmetry across the moon. A bluish band spans 20 degrees south and north from the equator. It forms an elliptical shape narrowing toward the trailing hemisphere. Comparable bands exist only on Mimas among Saturnian moons. Energetic electrons greater than about 1 MeV cause this blue coloration. Dark particles from outer satellites deposit onto the trailing side while bright ice falls from the E-ring. The leading hemisphere reddens slightly without noticeable darkening. Impact of plasma from Saturn's magnetosphere contributes to the darkening process. Radar observations at 2.2 centimeters wavelength showed complex ice regolith structure. Far-infrared observations constrained bolometric bond albedo values. Plasma measurements demonstrated Tethys produces no new plasma in the magnetosphere. It remains geologically dead despite constant bombardment. The visual spectrum stays flat and featureless except for water ice absorption bands.
Tethys formed from an accretion disc or subnebula around Saturn. Water ice served as the primary solid material due to low temperatures. Volatile compounds like ammonia and carbon dioxide likely existed but remain poorly constrained. Conditions favored conversion of molecular nitrogen into ammonia and carbon monoxide into methane. This explains why Saturnian moons contain more water ice than Pluto or Triton. Oxygen freed from carbon monoxide reacted with hydrogen to form water. One theory suggests rings and inner moons accreted from tidally stripped ice-rich crusts. A Titan-like moon may have been swallowed by Saturn after losing its crust. Accretion lasted several thousand years before full formation occurred. Impacts during this period heated the outer layer to about 155 Kelvin. Thermal conduction ended formation while interior heating continued. Cooling near-surface layers contracted while interiors expanded. Extensional stresses reached estimates of 5.7 MPa causing surface cracking. Radioactive decay provided negligible heating since rock content remained minimal. Significant melting never occurred unless tidal forces intervened. Passage through orbital resonance with Dione might have generated internal heat. Current knowledge of evolution remains very limited despite extensive data collection.
Pioneer 11 flew past Saturn in 1979 reaching closest approach on September 1st. Distance measured 329,197 kilometers from Tethys center. Voyager 1 passed 415,670 kilometers away on the 12th of November 1980. Resolution did not exceed 15 kilometers per pixel. Voyager 2 came within 93,010 kilometers on the 26th of August 1981. Images achieved resolution as high as 2 kilometers. Ithaca Chasma was discovered first by Voyager 1 in 1980. Voyager 2 revealed it circled almost 270 degrees around the moon. Odysseus crater appeared later during that same mission. Cassini entered orbit around Saturn in 2004 for an extended study period. A targeted flyby occurred on the 24th of September 2005 at 1,503 kilometers distance. Non-targeted flybys happened throughout primary and equinox missions since then. Another close pass took place on the 14th of August 2010 during solstice mission. Penelope crater, measuring 207 kilometers wide, was imaged at 38,300 kilometers distance. High-resolution maps reached 0.29 kilometer resolution using Cassini data. Spatially resolved near-infrared spectra showed water ice mixed with dark material. Future missions remain uncertain though Titan Saturn System Mission offers one possibility.
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Common questions
When was the moon Tethys discovered and by whom?
Giovanni Domenico Cassini discovered the moon Tethys on the 1st of January 1684 while observing from the Paris Observatory. He found it together with Dione during a night when he used an aerial telescope to identify four new moons orbiting Saturn.
What is the physical composition and density of the moon Tethys?
The moon Tethys has a radius of about 531 kilometers and a density of only 0.98 grams per cubic centimeter, making it the lowest-density major moon in the entire Solar System. Spectroscopy confirms that water ice dominates its surface composition while radar observations reveal an icy regolith with porosity exceeding 95 percent.
How large are the geological features Odysseus crater and Ithaca Chasma on Tethys?
Odysseus crater spans approximately 450 kilometers in diameter and covers nearly two-fifths of the moon Tethys itself. The valley known as Ithaca Chasma stretches more than 2,000 kilometers long and measures roughly 100 kilometers wide across the surface.
Which other moons share orbital positions with the moon Tethys around Saturn?
Two co-orbital moons named Telesto and Calypso share Tethys's Lagrange points by orbiting 60 degrees ahead and 60 degrees behind respectively. These satellites measure under 25 kilometers in diameter and were discovered by Voyager spacecraft in April and March 1980.
When did space missions first observe the moon Tethys and what data did they collect?
Pioneer 11 flew past Saturn on the 1st of September 1979 reaching closest approach to Tethys at a distance of 329,197 kilometers from its center. Voyager 1 passed within 415,670 kilometers on the 12th of November 1980 while Voyager 2 came closer at 93,010 kilometers on the 26th of August 1981.